TAPR PSR #144 Winter 2020
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
President’s CornerBy Steve Bible, N7HPR
HamSCI, the Ham Radio Science Citizen Investigation, is a platform for the publicity and promotion of projects that are consistent with the following objectives:• Advancescientificresearchandunderstandingthroughamateurradioactivities.• Encouragethedevelopmentofnewtechnologiestosupportthisresearch.• Provideeducationalopportunitiesfortheamateurcommunityandthegeneralpublic.HamSCI serves as a means for fostering collaborations between professional researchers and
amateurradiooperators.Itassistsindevelopingandmaintainingstandardsandagreementsbetweenallpeopleandorganizationsinvolved.HamSCIisnotanoperationsorfundingprogram,norisitasupervisoryorganization.HamSCIwasstartedbyham-scientists,whostudyupperatmosphericandspacephysics.These
scientists recognized that projects such as the Reverse Beacon Network, WSPRNet, PSKReporter, DX Cluster, ClubLog, and more are generating big data sets that could provide useful observations oftheEarth’sionosphereandrelatedsystems.Becauseofthis,HamSCI’sinitialfocusisonthesefieldsofresearch.SoundslikesomethingTAPRshouldbeinvolvedwith.Well,weare.TAPRhasahandinthePersonalSpaceWeatherStation(PSWS)projectthatultimatelyaimsto
create a small, multi-instrument system that can make ground-based measurements of the space environment.Theobservationsfromthisprojectwillnotonlybeusefultotheownerofthesystem,butalsoaggregatedintoacentraldatabaseforspacescienceandspaceweatherresearchpurporses.Initialworkfocusesonthedevelopmentofascientific-gradeHFradioreceiver,aswellasthe
necessarysoftwareandnetworkinfrastructure.TAPR’sisworkingonvariousaspectsoftheproject:• ScottyCowling,WA2DFI–TangerineSDRhardwaredesignandcheerleading.• JohnAckermann,N8UR–GPSandtiming• DaveWitten,KD0EAG–Magnetometerboard
President’s Corner 01NOTICE 02TAPR.ORG – The Next Generation 02TAPR Members to Attend 2020 HamSCI Workshop 03On the Air with YT7MPB 04Donate to TAPR 04DCC Wrap-Up 05AX.25 + FEC = FX.25 08TAPR Wear Available 12Automatic Tuner for Small Magnetic Loop Antennas 13Write Here! 29On the Net 29The Fine Print 30Our Membership App 31
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
TAPR.ORG – The Next GenerationBy John Ackermann, N8UR
We’re pleased to introduce you to the new TAPR.ORGwebsiteandon-linestore.We’vecompletely rebuilt the site using a modern content management system and e-commerce platform, hosted at a well-respected provider thatprovidesautomaticsecurityupdates.
We’ll be updating the front page with new posts regularly, so plan to stop by even if you’re not shopping!There’salotmorecontenttocomeaswe
continue to migrate content from the old site to the new, but for now the on-line store is fully functionalandloadedwithallcurrentTAPRproducts.Wehaveremovedsomeveryoldproductsfrom the store; if you need one of those, please contact us and we’ll let you know if we still have it available.We’realsointegratingourmembershipmanagementintothenewsystem.Allnewmemberships
and renewals are available as products within the store, and there will be special members-only featuresonthesite.Finally,we’llbechangingthewaythePacket Status Register (PSR)isdistributedandwillbe
discontinuingthecurrentmailinglists.BeforethenextPSR is published, we’ll send a note to those listsexplainingthechanges,andhowtoaccessPSRgoingforward.Ofcourse,theremaybeafewglitchesaswegetthenewsiterolling.Ifyouhaveanyproblems,
or suggestions, please let us know!###
• TomMcDermott,NE5G–RFboard,designdocumentation and proofreading• DaveLarsen,KV0S–TangerineSDR
webmastering,weeklyTeamspeaksessionmoderating and programming• BillEngelke,AB4EJ–Buildingtheweb-
based agitator• NathanielFrissell,W2NAF–Scienceteam
and HamSCI meeting leaderTAPRpeepswillbeattendingtheHanSCI
WorkshopattheUniversityofScranton,March20-21,whichyoucanreadaboutonpage3.
Hope you can join us there!73, SteveBible,N7HPR,PresidentTAPR
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NOTICEMembership-onlyaccessofPSR will begin
withthenextissueofthenewsletter.Inotherwords,onlyTAPRmemberswillbeabletoviewthenewsletterwhenitispublished.(Youcanfinda membership application on page 31 or visit our membership webpage.)
Past issues of PSR will still be available on TAPR’swebsite and membership-only issues willappeartherewhentheyarenolongercurrent.
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TAPR PSR #144 Winter 2020PAGE 3
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
TAPR Members to Attend 2020 HamSCI WorkshopMarch 20-21, 2020, University of Scranton
By Nathaniel A. Frissell, W2NAF
TAPRmemberswillattendthe3rdAnnualHam Radio Science Citizen Investigation (HamSCI)workshopMarch20-21,2020attheUniversity of Scranton in Scranton, Pennsylvania as part of a team meeting for the HamSCI PersonalSpaceWeatherStationproject.HamSCIis a collective of hams and professional space science researchers working together for the mutualbenefitofthehamradiohobbyandtheadvancement of the understanding of ionospheric andspacescience.StartingJanuary1,2020,TAPRformally
became a participant in the National Science Foundation(NSF)-fundedprojecttodevelopaPersonalSpaceWeatherStation(PSWS)incollaborationwithHamSCIscientists.TAPR’smain role is the development of a new software definedradio(SDR)receiverwithspecificationsthat meet the needs of scientists to remotely sensetheionosphere.ThiscollaborationleddirectlytoTAPR’slatestproject,theTangerineSDR.Additionally,TAPRmembersare contributing to the development of PSWS ground magnetometer, PSWS software and databasesandothercriticalPSWSsystems.
At the HamSCI workshop, all aspects of
thePSWSwillbediscussed.LeadPrincipalInvestigator(PI)fromtheUniversityofScranton,Dr.NathanielFrissell,W2NAF,willpresenttheoverarching PSWS goals and objectives, as well asprovideanoverallstatusreportontheproject.TangerineSDRchiefengineer,ScottyCowling,WA2DFI,willpresentthelatestdesignsand
statusfortheTangerineSDRdataengine(DE).TomMcDermott,N5EG,willdiscusshisdesignandprogressfortheRFboardmodules,andJohnAckermann, N8UR, will present on the precision timingmodules.DavidWitten,KD0EAG,hasbeen leading the development of the ground magnetometer(GMAG)hardware,andplansto
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
provideandupdateandhardwaredemonstration,alongwithGMAGPIfromNewJerseyInstituteofTechnologyDr.HyominKim,KC2MCR,andbeta-testerFrankieBonte,KE8GTT.BillEngelke,AB4EJ,oftheUniversity of Alabama is a lead of the control software and database team, andwillprovideupdatesonthesesystems.Finally,theCaseWesternReserveUniversityteamledbyKristinaCollins,KD8OXT,Dr.DavidKazdan,AD8Y,andJohnGibbons,N8OBJwilldiscussthelowcostversionofthePSWS.AndDr.PhilErickson,W1PJE,ofMITHaystackObservatorywillpresentmethodsofionosphericremotesensingusingPSWScapabilities,includingFT8basedtechniquesandsignalsreceivedfromWWV.
In addition to serving as a team meeting for the PSWS project, the 2020 HamSCI workshop also seeks to strengthen ties between the HamSCI and auroralsciencecommunitieswiththeoverarchingmeetingtheme“TheAuroralConnection.”
Saturday morning will feature tutorial talks on optical aurora and citizen science, natural radio emissions from the aurora and using the auroral modepropagationasahamradiooperator.ThesetalkswillbegivenbyNASAScientistandAurorasaurusleadandfounder,Dr.ElizabethMacDonald,DartmouthradiophysicistDr.JimLaBelle,andhamradiooperatorDavidHallidy,K2DH,respectively.TheFridaynightbanquetwillfeature,TimDuffy,K3LR,theworld-
famouscontesterandCOOandgeneralmanagerofDXEngineering.Contributed talks, demos, and posters will also be given by members of the hamradioandsciencecommunities,aswellasthegeneralpublic.Tolearnmore about the 2020 HamSCI workshop, submit an abstract, or register for themeeting,visithttp://hamsci.org/hamsci2020.
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On the Air with YT7MPB
Miroslav‘Misko’Skoric,YT7MPB,hasrecentlystartedlisteningforpactorcontactsat20m,40m,and80mbands.IfanybodycanhearYT7MPB-8node(basedonLinBPQsoftware)callingCQPACTORorhisforwardingpartners,newconnectsarewelcomed.Aswell,possibleforwardingoverpactor1-4and/orrobustpacket.YT7MPB-8isusuallyactivefrom1330UTCto1730UTC.DuetoheavyQRNinthelocalareaandnot-so-perfectantennas,withoutanyautomatictuningfordirectionandSWR,activityisinfrequentfornow.FromFebruary3to20,MiskoYT7MPBwillvisitIndia.Ifareciprocallicenseis
grantedontime,possibleVHFpacket,APRSexperimentswillbeperformed.###
Donate to TAPRTAPRisnowparticipatingintheAmazonSmileprogram!When you shop using the AmazonSmile program, Amazon makes
adonationtoTAPRequalto0.5%ofthepriceofyoureligibleAmazonSmilepurchases.AmazonSmileisthesameAmazonyouknow.Sameproducts,same
prices,sameservice.BookmarktheTAPRAmazonSmileProgramlink:
https://smile.amazon.com/ch/86-0455870Thatlinktakesyoutoaspecialloginportalwhereyouenteryour
normal Amazon credentials and get redirected at the same Amazon home pageexcepttherewillnowbeanoticethatyouaresupportingTAPR.OtherwaystodonatetoTAPR:
http://www.tapr.org/tapr_donate.html###
TAPR PSR #144 Winter 2020PAGE 5
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
DCC Wrap-UpBy Stana Horzepa, WA1LOU
The2019ARRL-TAPRDigitalCommunicationsConference(DCC)ishistory,butyoucanreadthepublishedDCCpapers(listedbelow)ontheTAPRwebsiteundertheLibrary>DigitalCommunicationsConferencePapersmenu.
DCC Papers
PSAT2DTMFExperimentAPRStt–Touchtone®DigitalCommunicationsUsinganyRadioforDataExchangebyBobBruninga,WB4APRExtendingD-STARwithCodec2byAntonyChazapisIPV6forAmateurRadiobyDanielEstévez,EA4GPZ/MØHXMSynchronizationinFT8byMikeHasselbeck,WB2FKOWSPRinaneducationalProjectbyAnthonyLeCren,F4GOHPortableAudioFrequency-ShiftKeyingSensorsusingaHamshield
minibyNolanPearce,KE8JCTbyStephenS.Hamilton,KJ5HY,andKateJDuncan,KB2ZOOAnFPGALearningExperience:SPIInterfacetoMax10FPGAby
GregoryRaven,KF5NModulation–DemodulationSoftwareRadiobyAlexSchwarz,
VE7DXWHowtoKillPacket-Radio&APRS?CometoSerbia!(Part2)by
Miroslav“Misko”Skoric,YT7MPBGPSWatchTechnologybyDarrylSmith,VK2TDS
DCC Prize Winners
As usual, a wide array of prizes were awarded to DCC attendees at
theSaturdayeveningbanquet.Theprizeswenttothefollowingluckywinners:KenwoodTH-D74:Kathy,WB0UDNIcomID51APlus2:MaxYarterMFJ-1270X-TNC-XPacketController:Brian,N8WRLMFJ-1270DGX-Digi:Brian,N8WRLRowetel.comSM1000FreeDVAdaptor:Al,WA1VHDARRLgiftcertificates:George,K9TRV,Jason,KC5HWB,John,
WA8TON,Jonathon,KC3EFX,Mike,KG4DSG,Richard,K3PLRDXEngineeringgiftcertificate:John,W9DDDGreatScottGadgetsGreatFETOne:Greg,KF5NHamRadioDeluxesoftwareCD:Bob,AC8YVHROgiftcertificates:Corey,KB9JHU,Matt,WS8UTAPRgiftcertificates:Brannon,KF5WVW,Dan,AD0CQ,Garrett,
KD2SAK,Mark,N2OJO,Roland,W9HPXTAPRGPSKit(1995):Chris,KF7TUPWestMountainRadiocertificate:Dave,KD0EAG
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WA2DFI photo
TAPR PSR #144 Winter 2020PAGE 6
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
DCC Photos by K9TRV & WA2DFITop: N7HPR, K9TRV, N8WRL, WB0UDN, N7HPR, K0PFXMiddle: WB9QZB, K3PLR, AC8ESBottom: KA9Q, KB9JHU, AC8YV, W9DDD, WA2DFI, KF7TUP, Max Yarter, N7HPR
TAPR PSR #144 Winter 2020PAGE 7
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
Digital Communications Conference
Make your reservations now for three days of learning and enjoyment at the Charlotte Airport Hotel. The Digital Communications Conference schedule includes technical and introductory forums, demonstrations, a Saturday evening banquet and an in-depth Sunday seminar. This conference is for everyone with an interest in digital communications— beginner to expert.
September 11-13 Charlotte, North Carolina
2020 ARRL/TAPR
Call Tucson Amateur Packet Radio at: 972-413-8277, or go online to www.tapr.org/dcc
Charlotte Airport Hotel
Untitled-1 1 01/09/2020 12:04:10 PM
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AX.25 + FEC = FX.25By John Langner, WB2OSZ
Whatcanyoudoifyourradiosignalisn’tquitestrongenoughtogetthroughreliably?Movetohigherground?Getabetterantenna?Morepower? Send data very very slowly with narrow bandwidth?Sometimesthosearenotoptions.Anotherwaytoimprove
communication reliability is to add redundant information so the messagewillstillgetthroughevenifpartsofitaremissing.
In the data communications world, adding additional information, sothereceivingendcanfixuperrors,isknownasForwardErrorCorrection(FEC).Thisisverycommon.Itisusedinallsortsofradiocommunication,QRcodes,DSL,anddiskdrives.ItisevenusedwithCD’sandDVD’stoworkarounderrorscausedbyscratches.
AX.25
TheAX.25frame,usedbyAPRSandAmateurPacketRadio,isnotverytolerantoflowqualityradiolinks.A16bit“FrameCheckSequence”(FCS)iscomputedfromtheearlierparts.ThereceivingendrecalculatestheFCSanddiscardstheframeifthereisamismatch.
Allittakesisasinglebadbittoruintheentireframe.Traditionalconnected mode packet has automatic acknowledgment of received framesandrequeststoresendanythataremissing.APRSdoesnotusethisfeaturesotheinformationisjustgoneandnobodyknows.
FX.25
Backaround2006,theStensatGroupcameupwithacleverwaytoaddForwardErrorCorrectiontoAX.25whilemaintainingcomplete
backwardcompatibilitywithexistingequipment.Atransmissionstartswithcarefullychosen64bitpatternstoidentifytheformatofthedatatofollow.DependingontheCorrelationTag,therearecertainnumbersofbytes
forthe“data”and“check”parts.TheReed-Solomonalgorithmisusedtofixanyerrors.Thenumberofbytesthatcanberepairedisonehalfthenumberofcheckbytes.
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
ThesendercanchooseanappropriatecorrelationtagdependingonamountofspacerequiredfortheAX.25frameandthedesiredstrengthoferrorcorrection.Thereceiverneedstobeabletoprocessanyofthem.FX.25hasbeenusedforseveralAmateurRadioSatellitesandallofthe
receptionsoftwarehasbeenorientedtowardthatspecializedapplication.NowthatFX.25hasbeenintegratedintoageneralpurposeTNC,manynewpossibilitiesarenowopen.Herearetheresultsofsomeexperimentstoseehowwellitactually
works.
Bit Error Rate
“BitErrorRate”(BER)isacommonmeasurementofhowwelladigitalcommunicationchannelperforms.Itissimplydefinedasthenumberofincorrectbitsdividedbythetotalnumberofbits.Errorscan be caused by many factors such as natural random noise, man-madeinterference,multi-path,fading,lowqualitymodems,incorrectadjustments(e.g.transmitaudiolevel),anddistortionaddedbytheradios(e.g.limitedbandwidth,pre-emphasis&de-emphasis).Itiseasytocomputetheimpactofdifferentbiterrorrates.Ifthe
probability of any bit getting corrupted is B then the probability of it gettingthroughcorrectlyis(1-B).TheprobabilityofNbitsgettingthoughcorrectlyis(1-B)N.Let’scomputetheprobabilityofan80byte(640bit)framegettingthroughwithdifferenterrorrates.
Nowwewilltryitexperimentally.GenerateathousandAPRSpacketsandmeasurehowmanyarereceived.Thereceivingendhasanoptiontointroduce a given Bit Error Rate, after the demodulator, using a random numbergenerator.
Theexperimentalresultsarequitesimilartothetheoreticalexpectation.Lossofonly1bitin1000(i.e.BER=10-3)causesalmosthalfoftheframestobelost.
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
FX.25 Improvement for APRS
Thesametestwillbeperformedwith1000framesagain,butthistimeusingFX.25.Thesenderhasachoiceof16,32,or64checkbytes.Onceagain, we introduce a controlled Bit Error Rate at the receiving end and countthenumberofframesreceivedcorrectly.
FX.25keepsgoingstronglongafterregularAX.25iscompletelyuseless.
FX.25 Improvement for traditional Packet Radio
ConnectedmodeusesthesameAX.25framesbutinadifferentway.Rather than a broadcast to everyone, it is used as a link between two specificstations.ThesendingTNCassignssequencenumberstotheinformationframes.Thereceivingendacknowledgeswhathasbeenreceivedandasksforfill-insofmissingpieces.
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ConnectedmodeAX.25packetradioisoftenused,withapplicationslikeOutpostPMandWinLinkExpress,foremergencycommunications.Temporaryantennasandlossofinfrastructuremakereliablecommunicationsmoredifficult.Anythingthatcanextendrangeandimprovereliabilitycouldhaveasignificantimpactonpeople’slives.Thistestuses100frameseachwith128informationbytes.Thetotalamountofapplicationdatatransferred,dividedbytheelapsed
time will give us an effective throughput rate, measured in bytes per secondhere.Astheerrorrateincreases,moreretriesarerequired,andthethroughputdecreases.Atsomepoint,itjustgivesupbeforereachingtheend.Wegivethisascoreof0.Undergoodconditions,FX.25isslowerbecauseithasmoreoverhead.
Whenconditionsdeteriorate,FX.25showlittleornodegradationlongafterAX.25iscompletelyuseless.OnecouldimagineasystemwheretheFX.25transmissionisenabled
ordisabledbasedontheerrorratedetectedonthereceivingend.Itwouldnothavetobesymmetrical.MaximumthroughputmightbeachievedbyalowpowerstationtransmittingFX.25andahighpowerstationsendingregularAX.25.
FX.25 Quick Start Guide
Ifyougotthisfar,youareprobablywondering,“HowcanIexperimentwiththis?”The“DireWolf”softwareTNCnowhasFX.25builtinsoitiseasyto
usewithallyourfavoriteAPRSandPacketRadioapplications.Someofyoumightnotbefamiliarwiththissoftware.DireWolfisamodernsoftwarereplacementfortheold1980’sstyle
TNCthatusedspecialhardware.Itusesthecomputers“soundcard”foraudioinandout.Themodemisimplementedwithdigitalsignalprocessing(DSP)software.Withoutanyadditionalsoftware,itcanperformasanAPRSGPS
Tracker,Digipeater,InternetGateway(IGate),andanAPRSttgateway.ItcanalsobeusedasavirtualTNCforotherapplications.ItisfreeopensourcethatrunsonWindows,Linux(including
RaspberryPi),MacOSX,andBSDunix.It’snowveryeasytogetstartedwithFX.25:
Reception
FX.25receptionisalwaysenabledsoyoudon’tneedtodoanythingspecial.WatchingforthespecialcorrelationtagsequencesaddsaninsignificantamountofadditionaloverheadwhenonlyregularAX.25isinuse.
Transmission
Fortransmit,simplyspecify–X(uppercase)onthecommandlinefollowedby16,32,or64,forthedesirednumberofcheckbytes.Thespecificcorrelationtagwillbepickedautomaticallybasedontheframesize.Currentlythissingleoptionappliestoallradiochannels.
At the time this is being written, the new functionality is available only inthedevelopment(“dev”)branch.Nottoofarinthefuture,itshouldbeinrelease1.6.That’saboutallyouneedtoknowtogetstarted.
Summary
FX.25hasbeenusedformorethanadecadewithsomeAmateurRadio
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satellites.Receivingimplementationswereorientedtowardthatveryspecializedapplication.NowthatFX.25iseasytouse,integratedintoageneralpurposeTNC,manyotherAX.25basedapplicationscangainthebenefitswhileretainingcompletecompatibilitywitholderequipment.
Recommended Reading
Wikipedia
https://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction
FX.25 FEC extension to AX.25
http://www.stensat.org/docs/FX-25_01_06.pdf
Dire Wolf documentation
https://github.com/wb2osz/direwolf/tree/dev/doc
Most relevant related documents:
• AX.25+FEC=FX.25(similartothisbutlonger)• AX.25Throughput:Whyis9600bpsPacketRadioonlytwiceasfast
as 1200?• A Better APRS Packet Demodulator, part 1, 1200 baud
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PersonalizedLand’sEndclothingwiththeTAPRlogoandyournameandcallsignarenowavailablefromtheTAPRStoreathttp://business.landsend.com/store/tapr/SelectfromtheMen’sorWomen’scatalog.(Tomakeshoppingeasier,
thereare“TAPRRecommendedShirts”intheMen’scatalogincludingtwostylesofpoloshirts,eachavailablewithorwithoutpockets.)Thelogoisavailableinthreecolors--red,blue,andwhite.Thename/
callsignmonogramthreadwillmatchthelogocolor.(Werecommendthatyouusethewhitelogowithdarkcoloredshirts.)Pricesareveryreasonable,forexample,afteraddingthelogoand
monogram,ameshpocketshirtis$39.85plusshippingandsalestaxwhereapplicable.Processingtimeis5-7days.
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TAPR WearAvailable
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
Automatic Tuner for Small Magnetic Loop Antennas©
By Andrew Cornwall, VE1COR / KB1RSE
Accordingtothe2010UnitedStatesCensus80.7%ofthepopulationlivesin‘urban’regions.Urbanresidentsaccountforover90%ofthepopulationinsevenstates.Atrendtowardurbanizationhashappenedthroughoutthecountry.Thereisconsiderableleewayinthedefinitionofanurbanarea,yettheimplicationisthatanincreasingnumberofU.S.hamsliveinapartmentsorinhouseswithlotstoosmallforafullsizeHFantenna.ManyU.S.hamswithsufficientlandresideinneighbourhoodssubjecttohomeownersassociationrulesthatdonotallowfullsizeHFantennas1.Increasingurbanizationisalsooccurringinothercountries.AccordingtoUnitedNationsdatafor2018,55%oftheworld’spopulation now lives in urban areas2.Theimplicationforhamsisthereisagrowingneedforasmallfootprint,compact,capableHFantenna.Suchanantennamightbeanautotunedsmallmagneticloop.Thisarticleisaboutmakingaremoteautomatictunerforsmall
magneticloopantennas.Thearticleexplainsinsomedetailhowtheautotunerfunctionsandisused.Implicitinthisdescriptionishowtomakeone.Idonotprovideapartslisthereandcataloguenumbers.Fortunately,circuitcomponentsarestandardelectronicsasidentifiedinthecircuitdiagram.
A small magnetic loop antenna is relatively compact, easily portable, and does not have radials; it is directional, has some gain, and is suitableforuseoutdoors,and(withinsafeantennaproximitypowerlimits)indoorsoronabalcony3.Theantennaconsistsoftwoloops,onewithintheother,interconnectedbyinductance.Themuchsmallerloop,calledthecouplingloop,connectstoatransmitter/receiver.Thelarger,transmittingloop(italsoreceives),aboutametreindiameterforpopularHFbands,isconnectedinparallelwithavariablecapacitorcomprisinganLCcircuit.Tuninghappens,withmaximumantennaoutput,whenthe
variablecapacitoristurneduntiltheresonatefrequencyoftheLCloopmatchesthetransmittingfrequency.Atresonancethevariablecapacitorissubjecttoveryhighvoltages,uptoabout900voltswhentheantennaistransmitting5watts,and4,200voltsat100watts.Thecouplingloopdoesnotexperiencehighvoltages.Tuningforoptimumtransmittingconcurrentlyoptimizestheantennaforreceiving.TherearenumerousInternetsitescoveringsmallmagneticloop
antennadesignandconstruction.Further,thereareseveralsmallmagneticloopantennadesigncalculators.ThemostaccuratecalculatorformypurposesisKI6GD-GlennSperry’sMagneticLoopAntennaCalculator,LoopCalc.exe,whichcanbedownloadedfrom http://www.iw5edi.com/software/magnetic-loop-calculator.Picture1showsthetypeofsmallmagneticloopantennaIuse.The
autotunerisintheboxbelowtheantenna.FormyantennasIemploygoodqualitycoaxialcabletomakethetransmitterandcouplingloops.Itoperatesonthe40,30,and20metrebands.Otherbandsmay be accessed by changing the transmitting loop size and/or the rangeofthevariablecapacitor.Theantenna’svariablecapacitorisahighvoltageairvariablecapacitorwhichcanrotatecontinually360degrees4.Betteramateurradiosmallmagneticloopantennashaveatransmitting loop made of copper pipe or tubing, which is superior because of lower electrical resistance resulting in better performance andlowerQ.(Theautotunerdescribedhereshouldworkwellwithsuchatransmittingloop.)Alsoacostlyvacuumvariablecapacitor5 may be employed that can withstand much higher voltages, allowing forgreatertransmissionpower.Nevertheless,coaxbasedsmallmagnetic loop antennas with air variable capacitors are fairly popular andquiteeffective.Theyarealsoeasytoconstruct.
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Picture 1. Small Magnetic Loop Antenna with Autotuner
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TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
Table1providesthecharacteristicsoftheantennaIusedforthisarticle.Exceptforportableoperationsmallmagneticloopantennasare
rarelyfoundinhams’antennarepertoires.Theproblemistheantenna’sextremelyhighQmakingitdifficulttoaccuratelytune.Graph1illustrates the minute turning range of a traditional air variable capacitor toachieveanywherenearmaximumoutput.Theonlysignificanttransmitted signal occurs within a two degree portion of the 180 degree (half)rotationofavariablecapacitor,andsuccessfultuningiswithinless than a one-half degree range6.(Atleastthisisthesituationwithmysmallmagneticloopantenna.)
Thereisanotherinherentdifficultywithsmallmagneticloopantennas.Iftheantennaisnotequippedwithremotetuningtheradiooperatorhas to be able to reach the antenna’s variable capacitor to tune by hand, limitingwheretheantennacanbesetupandpossiblyexposingtheoperatortosignificantRFenergy.TherearecommercialandDIYdevicesthatmakeremotetuningpossible.Theseoftenutilizeasteppermotoror geared down DC motor at the antenna to turn the variable capacitor forminimumSWRatthetransmitter.Onetypehasahandheldmotorcontrol.AnotherisalogicbasedtunerthatautomaticallysearchesforminimumSWR.
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I wanted to make a remote automatic tuner that would be precise, easy touse,andinexpensive.OnceIsettledontheuniquestrategyforremoteautomatictuning,thedesignoftheautotunerwasobviousandextremelysimple.Itisalsoinexpensivecostingabout$50,notincludingmetalboxandpartsforthesmallmagneticloopantenna.WithmydesignIconfidentlyautotuneasmallmagneticloopantennaremotelywithinafractionofadBofbestpossible.Thereisalimitationtotheautotuner’sperformance,however,andthisisnotedlater.
I have constructed several autotuners out of varied parts for the stepper motor, stepper motor driver, and small magnetic loop antenna air variable capacitors.Theyalldidanacceptablejob.Forguidance,however,specificmodelsofsteppermotorandvariablecapacitorandtheirsuppliersarementionedinthearticle.Aprintedcircuitboardisavailablewhichfacilitatesassemblingthecircuit.
Unique Tuning Objective
Usually antenna tuning strives to achieve the lowest SWR measured atthetransmitterortheantennafeedpoint.Myautotuner’sobjective,however, is to achieve the highest possible output at the antenna from the powerdeliveredtoit.Maximumtransmittedpowermightnotoccuratthe antenna’s lowest SWR, because the impedance of an optimally tuned smallmagneticloopantennamaynotbe50ohms,typicaltransmitteroutputandtransmissionlineimpedances.TuningtheantennaforlowestSWRdoesnotguaranteeabsolutelyhighestantennaradiation.Fortunately,inmyexperience,thehighestradiatedpowerofasmallmagneticloopantennaisconsistentwithacceptablylowSWR.Ifthiswere not the situation high SWR at the antenna can be managed by addingatraditionalantennatunerprobablyatthetransceiver. Picture 2. Autotuner - Sampling Antenna Below Variable Capacitor
Protector
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Picture 2 shows the autotuner and variable capacitor mounted together onaplatformthatclampstothemastofthesmallmagneticloopantenna.ThevariablecapacitorishousedinawhiteshieldmadefromPVCpipe,andtheautotunerisinthemetalbox.(Theextraholesintheboxrelatetoaformerautotuner.)Picture3showstheplacementofpartsinsidethebox,andFigure1showsthecircuitoftheautotuner.Thecircuithasfivesections: sampling antenna, wide band receiver, microcontroller, stepper motordriver,andsteppermotor.Theyaredescribedbelow.
Sampling Antenna
Theperceivedsensitivityofthesamplingantennaincreaseswithfrequency.Thisismostlybecausesmallmagneticloopantennasofagivensizearemoreefficientathigherfrequencies,asshowninTable1fortheprototypeantenna.Thesamplingantennahasoneimportantlimitation.Itcanpickup
a signal from a nearby relatively high power transmitter and mislead the autotuner7.AlthoughinfrequentIhavehadtheembarrassmentof demonstrating the autotuner at a transmission laden hamfest with unsatisfactoryresults.Computermonitoringoftheautomatictuningprocess or autotuner manual mode, described later, can reveal where outsideRFislikelytothrowoffautotuning.Also,extraneousRFinterference can be reduced by using a shorter sampling antenna in conjunctionwithhighertuningpower,e.g.5watts.
Receiver
Thereceiverisverywideband,consistingoffour1N34ARFdiodesinabridgerectifier.A0.002uFcapacitorsuppressesnon-DCoutputfromthereceiver.Indeed,thecombinationofsamplingantennaandreceivercompriseafieldstrengthmeter.Linkingthereceivertothemicrocontrollerisa4N35optocoupler
which provides a non-inverting path, signal isolation, and limits the voltagetothemicrocontrollerinputto5volts,itsmaximumsafelevel.OutputfromthebridgerectifierisconnectedtotheLEDinsidetheoptocoupler.Theoptocouplerfunctionsinthelinearmode,wherecurrent passing through its output transistor varies in conjunction with theamountofcurrentgoingtotheinternalLED.ToomuchcurrentattheLED results in transistor saturation - this is the optocoupler’s switching
Picture 3. Inside the Autotuner Box
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Figure 1. Autotuner Circuit
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mode.Theoptocoupler’slinearmodesensitivity(beforesaturation)varies with the value of the output transistor emitter resistor, switchable betweenR1andR2inthecircuitdiagram.Voltageacrosstheemitterresistorisappliedtopin18ofthemicrocontroller.Ahighvaluefortheresistorresultsinhighsensitivitybutalsoearlysaturation.Conversely,alow value decreases the optocoupler’s sensitivity but allows for a greater rangeforlinearoperation.InthecircuitR1is3.0K-ohmfor‘HighSensitivity’andR2is1K-ohmfor‘LowSensitivity’,resultinginascalerangeof3:1.Bychangingthesensitivitysettingtheautotunermayoperate with a tuning signal that is neither too high nor too low, covering severalbandsgiventhelengthandpositionofthesamplingantenna.Thereistheopportunityforfurtherautotunersensitivityadjustment
atthetwopointgapatX3inthecircuitdiagram.Placingaresisterherereduces the current going from the diode bridge to the optocoupler LED.Aresistormaybeneedediftuningwattageisrelativelyhighandcannotbeaccommodatedbyshorteningormovingthesamplingantenna.Otherwisetheendsofthegapareconnected.
Even when the autotuner is turned off the receiver via the sampling antennaisexposedtothefullpowerbeingtransmittedbythesmallmagneticloopantenna.Thereisthepossibilitythattransmittinghighpower will cause the current handling capacity of the 1N34A bridge rectifierdiodes,50mA,andthe4N35optocouplerinputLED,60mA,tobeexceeded,destroyingthem.ThecircuitprovidesforanoptionalSPST,normallyopen,relaytodisconnectthesamplingantennawhentheautotunerisnottuningornotinmanualmode.Itislikely,however,thattransmitting high power will arc the variable capacitor before creating sufficientcurrenttodamagethereceiverdiodesoroptocouplerLED.Aprocedure for estimating how much full transmitting power the autotuner
can tolerate is described in a short article that may be downloaded from mywebsite.Ifarelayisnotinstalledthentherewouldbenoconnectionsat X1 on pin 17 of the microcontroller, and the gap in the sampling antennaconnection,atX2,wouldbeclosed.Theautotunerprogramassumesthatarelayispresent,ifnotthereisnoeffect.
Microcontroller
ThebrainsoftheautotunerisaninexpensivePicaxe18M2+microcontroller.Picaxemicrocontrollersareentirelyself-containedandsimpletoimplement.Supportcomponentsconsistofonlytworesistors.Thebuiltinoscillatorcanrunupto32MHz.Mostpinsareprogrammable for input or output, with many having the input option of10bitanaloguetodigitalconversion.ThereisaresidentBASICinterpreterhavingawiderangeofbuilt-infunctions,including5and10bitintegermathematics.ProgramcreationinBASICanduploadingittothe18M2+isdonebymeansofthePicaxeDevelopmentProgram;versionsofwhicharedownloadablefreetoworkwithWindows,Mac,andLinux.Forprogrammingandserialdatacommunicationsthe18M2+connects to a computer via a serial to USB port adaptor cable made for thePicaxe.APicaxechipcanbereprogrammedmorethan100,000times-necessaryforexperimentation.ThecircuitdiagramshowsthedigitalI/Opinassignmentsofthe
18M2+connectingtotheA4988steppermotorcontrollermodule,threetuningstatusLEDs,5voltstatusbuzzer,andoptional5voltsamplingantennarelay.TheLEDs,buzzer,andrelaymusteachdrawlessthanthe20mA,theperpinI/Ocurrentlimitofthe18M2+.Inthecircuitdiagramthere is also an optional capability at pin 4, with pair of connections at X4 and a 10 K ohm pull-up resistor, for adding a turn-limit sensor to
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accommodatean180degreeturnvariablecapacitor;amodifiedprogramisneededforthis.
Stepper Motor Driver
TheautotuneremploysanA4988modularsteppermotordriverthatpowersandcontrolsatwophasebipolarsteppermotor.InputvoltagetotheA4988canrangefrom8to30volts;myautotunerpowersourceisapproximately12volts.TheA4988canprovideuptooneamp(twoampswithexternalheatsink)toeachofthetwophasecoilsofthesteppermotor.TheA4988hasanon-boardadjustablecurrentlimitingcontroltotailorpowertotherequirementsofthesteppermotor.Drivingasteppermotorcanbecomplex,andtheA4988module
handlesthistask.Specificpinsonthemodulereceiveinstructionsfromthemicrocontroller.Movingastepoccurswhenthe‘Step’pinreceivesapulse.Otherpinsaresethighorlowtocontrolsteppermotordirection,fractionalstepsize,andmotorpoweronoroff.
Stepper Motor
Theautotunersteppermotoristwophasebipolartypethatiscommonlyavailable.Thesteppermotorturnsthesmallmagneticloopantenna’svariablecapacitor.Asthenameimplies,asteppermotormovesindiscreteincrements.Themotorcanstart,stop,andgoforwardandbackward.ThesteppermotorreceivespowerfromtheA4988driverthroughfourwires,twoforeachphasecoil(inthecircuitdiagramthesearelabelledW1andW2).ForthisprojectIexperimentedwithsteppermotorshavingfull-step
turningincrementsof1.8degrees,0.9degrees,andagearedsteppermotorwiththeequivalentofabout0.25degrees.Thetuningincrement
ofthesteppermotorcanbefurtherreducedbyhalf-stepping,quarter-stepping,eighth-stepping,andsixteenth-steppingwithpotentiallyreducedmotortorque.Ihavehadthemostsuccesswith0.9degreesteppermotors.Theremainderofthisarticlegenerallyassumestheuseofa0.9degreesteppermotor,buta1.8degreesteppermotormaybeeasilyaccommodatedbyasmallchangeintheprogram.Thepresentautotunerhasa0.9degreeNEMA17cubesteppermotor,‘Steperonline’model17HM15-0904S,shownmountedtotheboxinPicture3.ThisandcomparablesteppermotorsmaybeobtainedfromAmazon.comorEbay,andotheron-linesources..Twophasebipolarsteppermotorshavethreepowerinput
specifications:voltage,ampsperphase,andresistanceperphase.Forexamplethepresentautotuner’ssteppermotorspecificationsare5.4voltsperphase,0.9ampsratedcurrentperphase,and6.0ohmsphaseresistance.Myexperienceisthatvoltageisnotanissuewhenamperageisatorbelowthesteppermotorphasecoilrating.Theturningforceprovidedbythesteppermotorisrelatedtothe
amperageitdraws.ThevariablecapacitorsI’veusedarefairlyeasytoturnbyhand(i.e.abletoturnthebareshaftwithfingerswithoutaknob)andcanberotated(evenwithpartialstepping)byasteppermotordrawing0.9amps.
Although the stepper motor turns in very small increments the chances ofthesteppermotorpositioningthevariablecapacitoratexactlymaximumradiatedpowerisstatisticallyhitormiss.Nearresonancethereare substantial changes in radiated power per increment of the stepper motor,asdisplayedinGraph1.Whilelandingthevariablecapacitorexactlyonmaximumradiatedpowerisnevercertain,usingpartial
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stepping improves the odds that near miss tuning yields an acceptable result.Sixteenth-stepping,with3200incrementsperone-halfturnofthevariablecapacitor,isemployedbythepresentautotuner.Thesuccessofthisstrategyisevidentinthetestresultsshownattheendofthisarticle.However, with earlier autotuners I have had reasonably satisfactory tuningresultswithevenquartersteptuning.
Microcontroller Automatic Tuner Logic
Theautotunerhastwomodesofoperation,‘manual’and‘autotuning’.In both modes the microcontroller can send data through its serial port (convertedtoUSBbythePicaxeconnectingcable)toaconnectedcomputer8.Forthemanualmodetobeusefulacomputermustbeconnectedto
theautotuner.Manualmodeisengagedbylettingtheautotunerrunwithout any tuning power to the antenna during the reconnaissance sweep,describedbelow.Afternotfindinganysignaluponwhichtotargettuning,theautotunerrevertstomanualmode.Powertothesteppermotoristurnedoffandthevariablecapacitorcanbeturnedbyhand.Thenmeasurementsofanyradiatedpoweraresenttothecomputeraboutthreetimespersecond.Thisdataisusefulforunderstandingthenatureofthesmallmagneticloopantenna.Withoutatransmittedtuningsignal,manualmodecanbeusedtoassessambientRFenergyfromnearbytransmitters.
If a computer is connected while in autotuning mode, autotuning information on levels of antenna radiated power and variable capacitor positioniscommunicatedaftereachincrementofthesteppermotor.(Insignificantmeasurements,lessthan‘10’,arenotsenttothecomputer.)Thisinformationisimportantforunderstandingautotuningandfor
experimenting.AutotuningdataisthebasisofGraph1.Whenautomatictuninginthefieldcomputerdataarenotusuallyneeded,andtheautotunerishappytoworkwithoutacomputerconnection.Thetaskofthemicrocontrollerprograminautotuningmodeisto
maximizethesmallmagneticloopantenna’sradiatedpoweratagivenfrequency.Whilealowleveltuningsignalisbeingtransmittedtotheantenna and the stepper motor incrementally turns the variable capacitor, themicrocontrollertrackspowerbeingradiateduntilthemaximumisachieved.Thisdoesnotseemcomplicated,butI’vetriedseveralautotunealgorithmswithvariedsuccess.ForthepresentcombinationofautotunerandsmallmagneticloopantennaIusethe‘directmethod’-myterminology.
Direct method autotuning starts with a reconnaissance sweep of the entireturningrange(i.e.afullturn)ofthevariablecapacitortoidentifyaprobablemaximumsignallevel.Nextoneormoreactivetuningsweepscommence where the objective of the autotuner is to turn the variable capacitoruntilasignallevelisproducedthatisclosetoorexceedsthereconnaissancemaximum.Becausethereconnaissancesweepcouldhavebeenveryluckyinencounteringthemaximum,itsvaluemaynotbematchedduringactivetuning.Theinitialactivetuningobjective,therefore, is to turn the variable capacitor until a signal level is achieved thatisatleast95%ofthereconnaissancemaximum.Occasionallya95%valueisnotencounteredwhereuponanewactivetuningsweepisattemptedusinga90%threshold.Beforegivingupathirdandforthactivetuningsweepwillbeattemptedusing85%and80%thresholds.If tuning is not achieved after the fourth tuning sweep there is likely somethingwrongthatdoesnotinvolveautotuneroperation.Perhapsthere is a loose antenna connection, or there may be a high power
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ham rig nearby adding its transmitted signal to the autotuner sampling antenna.Theautotunerhastodealwithvariationsintransmittedtuningpower
when the transmitter’s foldback circuit responds to large changes inSWR.EncounteringhighSWRmaycausethefoldbackfunctiontoreducetransmittertuningpower.AstuningapproachesantennaresonanceSWRdeclinessignificantlyandthefoldbackfunctionallowstuningpowertoincreasetothespecifiedlevel.Togivethefoldbackcircuit time to catch up to declining SWR, the stepper motor slows downturningthevariablecapacitor.Inthepresentprogram,tospeedup the tuning process, the stepper motor turns at half-step until antenna resonanceisapproachedthenslowsdowntosixteenthstep.Theautotunervisuallyandaudiblycommunicatesitsoperatingstatus
bymeansofabuzzerandthreeLEDsonthebox.Table2outlinestheBuzzerandLEDindications.
Mechanical and Shielding Considerations
I suggest two mechanical shortcuts that I found helpful with prototype construction.First,evidentinPicture2,isfasteningthetwoendsofthetransmitting loop to the variable capacitor by 1/4 inch stainless steel bolts extendingfromthevariablecapacitorprotectivehousing,bymeansofsmallhoseclamps.Thecoaxbraidattheendsofthetransmittinglooparestrippedforaninch,orso,andtinnedtomakeafirmerconnection.Althoughnoteleganttheconnectionisphysicallyandelectricallysecure.Also,thetransmittingloopiseasytosetupandtakedown.Thesecondshortcut,inPicture4,isusingtwoshortlengthsof1/4
inchPVC,reinforced,flexiblehosetoattachthesteppermotortothevariablecapacitorbymeansofa1/4inchaluminumroddriveshaft.
Picture 4. Connection Between Variable Capacitor and Stepper Motor
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Thehoseistightlyclinchedtotheshaft’softhe stepper motor, variable capacitor, and rod byfoursmallhoseclamps.Theshaftsofthestepper motor, variable capacitor, and ends of the rod are roughened to enhance holding power.Ateachendoftherodthereisabouta3 mm gap inside the hose to electrically isolate thesteppermotorfromthevariablecapacitor.Ifthe variable capacitor turns relatively easily and the four hose clamps remain tight there is no slippagenortwistingevidentwiththislinkage.Furthertheflexiblehosecanaccommodatethe somewhat different diameters of the shafts of the variable capacitor, aluminium rod, and stepper motor; and overcome minor misalignment.Althoughmoreexpensivethanthejuryrigged
approach described above, stepper motor (CNC)shaftcouplersworkwelltoconnectthesteppermotorandvariablecapacitor.Acouplermustbespecifiedtoconformtothediametersoftheshaftsitisconnecting.Ifelectricallyconductive(i.e.madeofmetal)thentwo couplers are needed, one at the stepper motor and the other at the variable capacitor with a non-conductive drive shaft in between to electrically isolate the variable capacitor from theautotuner.Ihaveusedone-quarterinch,stiff
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plasticrodforadriveshaft.Theautotuner’scircuitrymustelectricallyfloatinsidethemetal
box.Thereisnoelectricalnorpowerconnectionbetweenthecircuit’scomponentsandthebox.ThemetalboxshieldstheautotunerelectronicsfromthepotentiallysignificantRFfieldsurroundingthesmallmagneticloopantenna.Shieldingalsooccursatthetransmissioncablefromthesamplingantennatotheautotunerreceiver.(InitiallyIusedRG-174coax,buttheouterbraidbecamepartofthesamplingantenna.)Iusea transmission cable made of shielded audio cable with the two inner wirescarryingthesignalfromthetwolegsofthesamplingantenna.Thecable’sshieldisgroundedtothemetalbox.(Thesteppermotorismountedtotheboxbutthemotor’scoilsareinsulatedfromit.)Themethodofwiringtheelectronics and stepper motor is not critical,
althoughaprintedcircuitsimplifiesassembly.
Autotuner Platform - Rigidity Rules
A lesson I’ve learned in building four autotuners with different variable capacitor and autotuner mounting strategies is the variable capacitor and autotunerhavetobemutuallymountedonaveryrigidstructure.Ifthereisanyflexingatorquewillbuildupduringtuningbetweenthevariablecapacitorand stepper motor which results in the variable capacitor continuing to turn slightlywhenautotuningiscompleted.PreviouslyI’vefinagledthisproblembyprogramming the microcontroller to turn the stepper motor backwards a small amountasautotuningfinishes.Theprototypeautotunernowemploysa3/4inchplywood(16”x6”).Therigiditythisprovidesallowsboththesteppermotorandvariablecapacitortostoptheinstanttuningiscomplete.
Variable Capacitor Considerations
Small magnetic loop antenna air variable capacitors able to withstand highvoltagecanbeexpensivetopurchasenew.Thisisanissuewith
smallmagneticloopantennasregardlessoftuningmechanism.Myprimarysourceforhighvoltagevariablecapacitorshasbeenhamfleamarkets.The27-127pFvariablecapacitorusedwiththesmallmagneticloopantennaforthisarticleisafleamarketfind;platespacingsuggestsitmaybeabletowithstand90watts.PreviouslyIextensivelyusedanavailable10-260pFhighvoltagevariablecapacitor,doublethecapacitancerangewarrantedforthedesiredbands.Thismadetuningtwiceasdemanding(i.e.foreachsteppermotorincrementthechangeincapacitanceisdouble).DespitetheaddedchallengetheautotunerconsistentlytunedwithinonedB(80%)ofoptimum.Imentionmyvariablecapacitorexperiencetogivehearttosmallmagneticloopantenna builders who face a compromise when using less than ideal variablecapacitors.RecentlyIsuccessfullytriedavariablecapacitorfromRFParts
(www.rfparts.com),model73-175-23,14pF-145pF,3.2Kv($60US).BasedonLoopCalc.execalculations,the3.2Kvvoltageratingallowsmysmallmagneticloopantennatohandleabout65wattson40meters,and50wattson30metersand20meters.Ihadtoreducethevariablecapacitor’sturningresistancebyslightlylooseningthescrew(withlockingnut)holdingtherotorshaftrearpivotpoint.Onceturningresistancewaslessened,the‘Steperonline’model17HM15-0904Ssteppermotor,specifiedpreviously,readilyturnedthevariablecapacitor.
Operating the Autotuner
A small magnetic loop antenna is attached to a transceiver with ordinary50ohmcoaxtransmissioncable.ItendtoinsertanRFisolatorinthetransmissionlineneartheantenna.A12voltDCpowersource(asmallgelcellwilldo)withaswitchattheoperator’sposition,connects
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totheautotunerbytwoconductorhookupwire.ThecoaxandDCpowerwiremaybeaslongasneededtoachieveasatisfactoryantennaposition.If computer connection to the autotuner is desired it is done by means ofthePicaxeserialtoUSBadaptercable(atthecomputer)andlengthsofstereoheadphoneextensioncableshaving1/8inchfemaleandmaleendconnectors.I’vechainedtogetherover30ft.ofheadphoneextensioncables.Ifthereisatraditionaltunerbetweenthetransceiverandthesmall magnetic loop antenna, it should be in by-pass mode while the autotuner is actively tuning and in manual mode9.Myautotuneronaveragedrawssomewhatoveroneampwhentuning;
while‘on’atothertimes(e.g.manualmode)thedrawislessthan20ma.Autotuningcommenceswhen12voltpowerisswitchedon.The
autotuner beeps once as a reminder to commence transmitting a small, constanttuningsignal,whichmaybeanAMorCWcarrier,ortonemodulatedSSB.Whiletuningprogressesthetransceiver’sSWRindicatordipsandrisesseveraltimes.Thisisnormal.AttheendoftuningtheSWRmetermostlikelywillberelativelylow.Thetimetocompleteautotuningwithoneactivetuningsweepisabout50seconds.Subsequentactivetuningsweeps,ifneeded,addanother20to30secondseach.Thetuningsignalshouldnotinterferewithotherhams,becauseoftheantenna’shighQitiseffectivelytransmittingonlyafewseconds.Oncetuningisdone,thereisafinalbeep,thesteppermotorstops turning, and the red and blue LEDs light up steadily indicating that tuningwassuccessful.Table2referstotheLEDandbuzzerindicationsfornon-successfultuningoutcomes.Afterwards12voltpowertotheautotunercanbeturnedoff.Turningtheautotuneroffbeforetuningisfinisheddoesnoharm.
Veryinfrequentlytheautotunermissesacceptabletuningwithinfouractivetuningsweeps.Whenmiss-tuningoccurs,tuneagain.Repeatedfailureconfirmsthatthereissomethingwrongoperationallywiththeautotuner, the small magnetic loop antenna, or the antenna’s connection withthetransceiver.Inmanualmodethesetupisthesameasautotuningexceptacomputer
mustbeattachedtotheautotuner.Manualmodeisactivatedbyturningon12voltpowerbutnottransmittinganysignaltotheantenna.Afterareconnaissance sweep without any radiated signal the autotuner goes into manual mode and starts providing continual radiated signal data to the computer.ManualmodestatusisindicatedbytheyellowLEDturnedon.Twelvevoltpoweriskeptonaslongasmanualmodeoperationisdesired.Nopowergoestothesteppermotorandtheantenna’svariablecapacitorcanbeturnedbyhand.
Autotuning Results
ToillustratethecapabilityoftheautotunerforthisarticleIconductedatotaloffifteentrails.ThesmallmagneticloopantennawassetupinthedrivewayofmyhomeinNovaScotia.AnMFJ-915RFisolatorwasinsertedinthecoaxialcableneartheantenna.Myoperatingpositionwas25feetawayinthegaragewhichhadACpower,andaYaesuFT-897transceiverandalaptopcomputeronamakeshifttable.Theafternoonwassunnywithambienttemperatureof68deg.F.Table3presentsthetuningresultsoffifteentrials.Therewerefive
consecutivetrailsinthreegroupsforfrequenciesinthe40,30,and20metrebands.Thecriteriaforthefrequencieswerethattheywerenotspecialuse,notforCWcode,andnotbusy.TheYaesuFT-897transmittedaCWtoneat5watts,thelowestpossibleoutput10.
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Threeparameterswererecordedforeachtrial:SWR,number of tuning sweeps, and percentage of highest signalachieved.SWRwasmeasuredbyanMFJ-813QRP,SWRmeter.Thenumberoftuningsweepsisthecountof360degreeactivetuningturnstheautotunertookto(nearly)matchthehighestsignallevelfoundinthattrial’sreconnaissancesweep.Asistypicalonlyonetuningsweepwasneeded.“PercentofHighestSignalfortheFrequencyGroup”
compares, in percentage terms, the signal level measured at the conclusion of each trial with the highest signal occurring amongallfivetrialsinthefrequencygroup.Thehighestgroup signal level is that encountered collectively among the fivetrialreconnaissancemaximumsandthefiveautotunedresults.Thetheoryisthatthehighestamongthesetenvaluesreasonablyestimatesthemaximumsignalpossibleforthefrequencygroup.AlsoshowninparenthesisinTable3isthepercentageof‘local’maximumpowerachievedforjustthattrail.TheresultsinTable3areselfevident.Theindividualtrial
signal values are very close to the probable highest signal valuefortheirfrequencygroup.Thelowestare92%forthesecondtrialinthe40metrebandand92%forthefourthtrailtheinthe20metreband;thesetrailsareboldedinthetable.Toputthispercentageintoperspective,92%isonly0.36dB(lessthanone-halfdB)belowthehighestprobablesignalvalue.Regardingeachtrail’slocalmaximum,theautotunedpercentageis96%orhigher.Theseautotuningresults
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aretypical.Tuninganyantennatothisdegreewouldbeconsideredasuccess.TherelativelylowSWRsareencouragingbyanystandard,buttheyrelatetothisparticularantenna.Itiscomforting,nevertheless,toknow that after autotuning with this small magnetic loop antenna no further signal treatment is needed to protect the transceiver from high SWR.
Where to Obtain Autotuner Picaxe Programs and Additional Information
Iamaddingtomywebsiteprogramfilesandadditionalinformationtocomplementthedescriptioninthisarticle.Therearetwoprogramfiles.Oneisacircuittesterthatverifiesallfunctionsofthecircuitareoperatingcorrectly.Thiscanberunbeforethecircuitisinstalledintheautotuner.Thesecondprogramcontrolstheautotuner.PicaxeprogramsmaybeopenedinthePicaxeDevelopmentProgramoranytexteditortolookat(andedit)thecode.PicaxeBASICiseasytofollowandthecodetendstobeselfdocumenting.Otherinformationonthewebsiteincludesa circuit parts list, assembly tips involving the printed circuit board, how to estimate highest tolerable transmitting power, and some project history.Ileavemanydetailsofautotunerassemblytothereader.Myplatform,
3/4”plywoodboardabout6”x18”,issuitableforthevariablecapacitorsandautotunerboxesthatIhaveused.Theshortcutsdescribedabovehave facilitated construction, and the way of mounting the autotuner onthemastofasmallmagneticloopantennahasworkedwell.Otherthanarequirementforrigiditybetweenthesteppermotorandvariablecapacitor,thereisnothingvitalaboutmyapproaches.Whattheyaccomplishcanbeachievedinotherways.Indeedmostaspectsofthe
autotunerarestillamenabletoexperimentation.
Availability of a Printed Circuit Board
TosimplifyassemblingtheautotunercircuitIhaveaprintedcircuitboard,showninPicture3.TheboardwasdesignedtoprofessionalstandardsbyEdThompson,thenastudentattheInstituteofTechnologyCampusoftheNSCC(NovaScotiaCommunityCollege).Ed’sabilityand diligence are commendable, and he recommended changes that significantlyimprovedtheprospectiveboard.
I am planning to have a small supply of printed circuit boards available forsalebyFebruary2020,whenthisarticleisscheduledtobepublishedinthe‘PacketStatusRegister’.Howtoordertheboardwillbepostedonmywebsite.Ifdemandexceedstheinitialsupplyofboards,Iwillattempttosourcemore.
Acknowledgements
AnearlierarticleofminedescribingtheexperimentalnatureofthisprojectwaspublishedintheMay-June2018editionoftheRadioAmateursofCanadamagazine,TCA(TheCanadianAmateur).IwishtoacknowledgetheassistanceIreceivedfromtheTCAeditorandreviewersoftheearlierarticle.Manyoftheirsuggestionshelpedmyapproachtotheprojectandwritingaboutit.SincethenIhaverefinedtheautotunertomakeagooddeviceevenbetter.WiththepresentarticleIwaspleasedtohavetheadviceoftheTAPRreviewerBruceRaymond,ND8I.Anyandallproblemsassociatedwiththeautotuner,however,areentirelymyfault.Thereremainstheneedtodemonstratetheautotuner’scapabilities
with a wide range of small magnetic loop antennas used by other
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hams.Also,theautotuningapproachdescribedinthisarticleshouldbeapplicableforotherkindsofantennas..Therearealwaysopportunitiesforexperimenting.
About the Author
Andrew Cornwall resides in Nova Scotia, Canada where he has Basic withHonoursamateurradiocertification.Hewasfirstcertifiedin1992andretainsthecallsignVE1COR.HespendsconsiderabletimeintheUnitedStatesandhashadaGenerallicensesince2008withcallsignKB1RSE.Heisretiredfromcareersineconomicsanalysisandininformationtechnologysystemgroupmanagement.AndrewCornwall,VE1CORandKB1RSEEnfield,NovaScotiaCanadaEmail: [email protected]: www.acornwall.ca
Footnotes
1.Thewebsitewww.hoa-usa.comclaimsthatthereare351,000suchhomeownersassociationsintheUnitedStates.CommonlyhomeownersassociationsbanfullsizeHFantennas.2.UnitedNationsDepartmentofEconomicandSocialAffairs,2018
RevisionofWorldUrbanizationProspects.3.Ihavenotusedasmallmagneticloopantennaonabalcony,but
others have, for instance: www.qsl.net/kp4md/balconyloop.htm4.Theautotunercircuitdesignallowsforvariablecapacitorsthathave
an180degreeturninglimit.Thisrequirestheadditionofaturnlimitsensorandamodifiedmicrocontrollerprogram.
5.Theadjustingmechanismofavacuumvariablecapacitorisnotsuitableformyautotunermechanism.6.Someofthepeakinessofthesmalltuningrangeisduetothevery
highSWRoutsideofthevicinityofantennaresonance.Thisresultsintransmission line loss and also causes the transmitter’s foldback function toreducepoweroutput.7.Thereisthepossibilityofmeasuringtheoutputsignaloftheantenna
otherthanasamplingantenna,whichwouldnotbesubjecttoextraneoustransmissions.Theautotunercircuitandlogicwouldnotbealtered.8.Measurementsofsignalstrengtharerelativeandrangefrom10
toabout1000(anythingbelow10ismeaninglessandnotreported).Avalueof1000correspondstonearly5voltsfromtheoptocouplertothemicrocontroller.9.Thedatasignalfromtheautotunertoacomputercanbegarbled
whenafairlyhightuningsignal(sometimesaslowas5watts)isbeingtransmitted and the small magnetic loop antenna is close to the computer, ortheradiotransmissionlineisintheproximityofthedatacable.Thesolution is to move the antenna, separate the transmission and data cables,and/orlowerthepowerofthetuningsignal.10Runningsequentialtrialsisnotassimpleasitmightseem.Even
withthetuningsignalreducedto5watts,froma100wattcapabletransceiver, holding the key down repeatedly for about a minute heats up thetransmitternotably.Thiseventuallyreducestuningpowerbyasmallamount.Whileautotuningisaccurateforthetrail,theresultsarenotcomparabletoothertrails.Thesolutionforconsistencyistoallowtimeforheattodissipatebetweentrials.
###
TAPR PSR #144 Winter 2020PAGE 29
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
Write Here!Your PSR editor is working
onthenextissueofPSR and hopes to find a few good writers, particularly ham radio operators working on the digital side of our hobby, who would like to write about their activities and have them published here in PSR.Youdon’thavetobeHiramPercyMaximtocontributeto
PSR and you don’t have to use Microsoft Word to compose your thoughts.YourPSReditorcanhandlejustaboutanytextandgraphic
format, so don’t be afraid to submit whatever you have to [email protected] --- she can handle it! ThedeadlineforthenextissueofPSR is April 1, so write early
andwriteoften.###
FacebookAsyoumayknow,TAPRhasaFacebookpage,
www.facebook.com/TAPRDigitalHam.However,IalsocreatedaTAPRFacebookGroup,
www.facebook.com/groups/TAPRDigital/.IfyouhaveaFacebookaccount,“Like”theTAPRFacebookpage
andjointheTAPRFacebookGroup.If you join the group click on the Events link and indicate you’re
Goingtotheevents.
On Twitter, TooAccesstheTAPRTwitteraccountat
www.twitter.com/taprdigital.
Also on YouTubeTAPRnowhasitsownchannelonYouTube:the
TAPRDigitalVideosChannel: www.youtube.com/user/TAPRDigitalVideo.
At this time, there are a slew of videos on our channel including manyfromtheTAPR-ARRLDigitalCommunicationsConference(DCC)thatyoumayviewatnocost,sohaveatit!
###
On the NetBy Mark Thompson, WB9QZB
TAPR PSR #144 Winter 2020PAGE 30
TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.
Submission GuidelinesTAPRisalwaysinterestedinreceivinginformationandarticlesforpublication.Ifyou have an idea for an article you would like to see, or you or someone you know is doingsomethingthatwouldinterestTAPR,pleasecontacttheeditor([email protected])sothatyourworkcanbesharedwiththeAmateurRadiocommunity.Ifyoufeeluncomfortable or otherwise unable to write an article yourself, please contact the editor forassistance.PreferredformatforarticlesisplainASCIItext(OpenOfficeorMicrosoft Wordisacceptable).PreferredgraphicformatsarePS/EPS/TIFF(diagrams,blackandwhitephotographs),orTIFF/JPEG/GIF(colorphotographs).Pleasesubmitgraphicsataminimumof300DPI.
Production / Distribution PSRisexportedasAdobeAcrobatanddistributedelectronicallyatwww.tapr.orgPSR Editor:StanaHorzepa,WA1LOUE-mail [email protected]
PSR#144Winter2020,ISSN:1052–3626PublishedbyTAPRPhone(972)413–8277E-mail [email protected] www.tapr.orgFacebookwww.facebook.com/TAPRDigitalHamTwitterwww.twitter.com/taprdigitalTAPROfficeHours:MondaytoFriday,9amto5pm EasternTime
TAPR Officers President: Steve Bible, N7HPR, [email protected] VicePresident:ScottyCowling,WA2DFI,2018,[email protected] Secretary:StanaHorzepa,WA1LOU,[email protected] Treasurer:TomHolmes,N8ZM,[email protected]
TAPR Board of DirectorsBoardMember,CallSign,TermExpires,e-mailaddress John Ackermann, N8UR, 2022, [email protected] Steve Bible, N7HPR, 2020, [email protected] GeorgeByrkit,K9TRV,2021,[email protected] ScottyCowling,WA2DFI,2021,[email protected] StanaHorzepa,WA1LOU,2020,[email protected] JohnKoster,W9DDD,2021,[email protected] Dave Larsen, KV0S, 2022, [email protected] Bruce Raymond, ND8I, 2022, [email protected] DarrylSmith,VK2TDS,2020,[email protected] TAPRisanot–for–profitscientificresearchanddevelopmentcorporation[Section501(c)(3)oftheUStaxcode].ContributionsaredeductibletotheextentallowedbyUStaxlaws.TAPRischarteredintheStateofArizonaforthepurposeofdesigninganddeveloping new systems for digital radio communication in the Amateur Radio Service, andfordisseminatinginformationrequired,during,andobtainedfromsuchresearch.
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