Parts List

Upper El emn t

Lower ElemntMatching Stub

Phasing Stub

Insulator Section

1/4"-20 Nuts

114" Lock Washers

Insulated Spacer

Hose Clamps

Description

1" x 48-3/4" 001" x 78-3/4" 001" x 23" 00114"-20 x 22" Threaded Rod

3/4" x 14-18" PVC Pipe

12 (or 16 if double nutted)

12

3/4" PVC Femle Coupling

2 1-114" Stainless Hose Clams

1

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,"Number 5 on your Fedback card

Collinear S/8-Wave OmniAntenna for 2 Meters

Commercia/antenna performance at a home-brew price!

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Ready to try your 'hand at buildi'ng anomnidirectional gainantennå? This may¡-e just the project for you! Using ordinaryhand tools. you can construct this antenna inone eyening from common hardware storemateriaK'

dB or Not dB?What does all this gain slUff mean.

really? An electronic amplifier has an abso-lute limit 10 the àmount of power it can 'pro-duce. regardless of the input leyel. Accord-ingly. amplifiers are often rated in wafts-anabsolute term. Antennas on the 'other hand.haye no maximum theoretical output pow-er-\\'hat you get out of them depends on\\'hat you put into them. Therefore. antennaperformance is rated in relaii \'e. rather than

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18 73 Amateur Radio Today. June. 1995

by John Conklin WD00

absolute. terms. Enter the decibel (dB). Adecibel is one tenth of a beL. named forAlexander Graham Bell (hence the little dand capital B). Originally established to

express changes in sound leyels. the decibel, is a term of relative power. A change of i. . dB in power level is just barely detectable

by the human ear., The correlation between the dB and pow-

er ratio is:dB = io log (output powerlinput power)A gain of 3 dB corresponds to a doubling

of power. Thus. an"'antenna with a gain of 3

dB will have the same effect on your signalstrength as if you had doubledoutput power.As an added boniis. the gain of an antennaapplies to recei\'ed signals as welL.

Where does all this extra power come

Figure 1. From dil1olelo deluxe. See ihis seeiio/l (fihe lexi.

from? According to the first b\\' of ¡hem10-úyn:mics (conservation of energy). you can'tget something for nothing. To cre::te gain inany giyen direction. the power must be takenfrom some other direction. In the case of abeam. most of the RF is concentrated toward'the front of the array and sacrificed at. thesides and rear. An omnidirection::l antenna.on the other hand. obtains its gain by reduc-ing ¡he amount of RF that is radiateú up-wards. Look at it this way: An omni antennahas :i raúiation pattern shaped like a dough-nut. In orúer to increase its gain. the dough-nui merely needs to be tlattened. thus puttingmore signal out instead of up.

G:iin must be expressed in relation tosome st::ndard for it to h:i\'e any meaning. Inanienn:i, work. these values are usu:il!y ren-

ç....

¡

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c:L.

RS-

RM

lRS-

RS.

VS.

RS

'ICS-

¡J;:r;.'¡J !n¡''If L-'","',

Siu('(' Jr1 ~~Hihirì\: radiahtr 1~ a purcl;- ih~~o-

n."ik"l aiit~niia. ;JIJ !1c'a:-ur~m..I\S in thi- ¡¡ni-ck "n~ ex¡irè~\..d in dBd. Im:iiknially. manynLu'juf~Lhn'.:r"" n~gh..t"l h~ Üh.'h.id(' an~ 't~H1'"

J;nd rtêfëh:lL\.:' Hi the-Ü ¡J~"b:eni""Hl:?~

Th;.~r(" 15- ric'i 'íAJY ni h.~¡hng v;hl"'tht'r~hcg;.Ün h.. I;Y\ ¿l .;í ~~

;1 dummy Iti,it! or i~iitl\\pic--..\('ii the \;,n('l-able dipole has 2,1 dB gain over b,\tlipk~lt\ Wí~è It' iake ,i,ht,t'¡¡~t'¡j daimo, wiih a

,)f ~~"Jh

l- ri)m Di¡ml*" to Dtluw

\11 ,issnrlmem 1'1 ,iiumimum "md jurJ-wal',~ ,:an doulile )CHill' efki.1 he radiatedP1l\\tl. He!\!', !tow.

\~ )0l1 rr,¡!iahly k¡W\L ;i half-\,;l\èb ~tntêr. -rtú;.

b \.. ht'rit ''U~T:i~1l j~ Jìhi ~h~- t,,~~~h-

.1::2,,~ î:- k~\\Ì¡~~L ihu~ pnl"'iJint: ~. iÜl~t~ h-~\;_imped;lin~ point Ii'r i:.mii,:c:tiiig 5:!.ol1mcoa\. Ai t!i(' enlÍs of the hall.wave antenna,iii,i the '~'rp",Íl(' ,iwation l'x¡,t,-thc ì.ur-ftn~

~% '\,,r~

Som~ ",n "i m.iidimg ..k\ ¡¡e lìll'¡ beu,cd ll onkr 10 oy..rctllle the impedance

mbinaidi if mi al1lelll,i b hI be end-fed. The

lluani,"r...wJ\.~ \"-it1,,(td ~H.Ih.. :j ('(tntiniJ~)u~,lý",";,¡n,oit'''fe n1:th"+á~hi. ~ìt't, i~~. rieT~

Think .~ ~f itf(~rar"i thi,;,1"- ~l Íi~~!~j¿J in h~lH" Tllt 1~\','r)' kiw al th..: do".."iI end "I' the matchingslll1 (c..nln 01' the dipole I. and v..ry hii,h at

¡he npen end .'Ith.. ,mh tend, "I' the dip.\k j,t-~n¡nt",~t th~"" ,1n~~~n:na ~h~" 1:'ritn tnJ an~J th~~f:tÜ~H1(t rtt."~H ~ht th'1'~,td (;l'IJlhz." ("an t~.? '1111-pI;. m,w¡l1~ the feed point up ¡.lr down ¡hesiuh. A, an added advantage. th(' dosed i~nd

or ih.. matdmlg ,iúh may l~ i,iuunded. thus

;:!nJ

';H',. h"'r .."qn¡~ 'i:nn. If th¿"

~.uitcnn;.i p. In'i.rta'O,,~d io t\\T!ilengths. the all':lUia "il exhihii ,inly ,light

!05 dBdl gain. This is because the currents~\¡"I1)! ,";idi ,~i,~m"'m ar.. . mt nr ph",;: ;uid ,'al1--L~! (~~h.:h i"lthi.r ~JH! t Figur~ i n HiJv~'(:'.';t,":r. if

~"I'l 1h-- g;"','1 h.JH~,\t ~l~¡'o: ¿+..~lYtènt:. fòJ

dOI(-' ::~aHi \.\'!H r.. l.9 JBd h(,~~~lU~!t

the cUll\~nh reiiihwce. rather than t"liù'Lcadi ,,¡her. in order hi achieve this phasing.. ¡h.. ,igl1a! mUS! tr;i:d :m c,tr;i half W;.\..-

leH~th hef¡¡wc- :ir~1\ .;~t 'the ~~í.d11d tit"nk~ntTht ;"tift'" ~, a.

";i§ 1t1~H thf ~hh~',, ~irt

and i.J~l~ei) "'pi.'k\:d if"" L RF ~l1n,,"rii~

aloiil! the stub are then ('qual in intensity hutilppnsitc in polarity. .:ausing 11K' i:urrciis W

c;.iW,,:t-l ~li!d pre." c¥iiint- ~hi.~ :-tuh it,;L~lf frn1n L~-t,JL.Ælin::,

.\mc'mu I'S ÙH'Ü1C-r ~i.-)",--i\h~j t.o, .~ dRd

£OJ ihc r,(!\\"t~(;n ~itnlt~llt~"Thì~ IS :ii:c"mplis!ie¡j hy kngtiwniiig ¡he ra-diating eleini'ms !,i SiX \\ ,II clengih ;indslwrkning the ph",ingstuh hy an equal

arni.\U;U 1 Fit!un: îDL Th;: ;.d~hid len~ih ~~f

2(1 73 o,Arnaiøt.ir. RadJ"o "'t()fjPiY" Junê.. ~Q1;;;- ..i".,' ~X

~nHcn-n:% ~.. ,_rut

n:m mi ih" added lengih 1;0 ~i1¡;iL ;IlJ ihcsection b lohor!. th,' nidiaiinn is in'iigiiilícanl.Further kiigthening or ihi, elciiwiits \\illC;~USt rfh'llC l'JHct:Har.iün. and the ~;Hn \\ iH

:mh"im:; !'n:

(d oll~trctiQn

Figure:: iIusirat..s tht. dimensions andlayout of the alHtl1lì;l, (\mSlnltiioii ¡,

JrJ ;Hltli.~~ '\îTirnc.'n, ~Lind

h-nna i~ ì'n-inì :/.r' ~d~hin1inÜn1l HihinF.althnu¡;Ì1 any tÌ¡amett'r lnH11 If:" Iü l~'should work fini:.

SHirt hy cuiiin!! the matdiing stuh l 23" I.the hyvù.~t radi:§tin~: eh:-rn-crH i 7X-3/"r' ~~~1i1J

radi.:i~inQ èh:-1'tcrn -lK".,~r,.r' ~ tf.~J r-~". a j 1 -thi.~ h0ttOUi ¿~nd ç¡f ilk" üpi"'''~r r:lihai..

¡fig ekmcnl, and Ilk' lOp end of the lower ra-diating ekmem. This wil allow tli" tubing10 damp nrml~ :1wimd the Í1hulawr.

l"c\L driB thi,_~ rncfunÜng h,_:h~"" in th-:

~,iuh

t~'h,¡' tt~p a1f the

kt\\-(~r "ìèn'l'::TIl. P(J~~Ü1nn~!U~ 4S~J/-r' dc1\\n

L\~

ln,m the lop ,if ihe hw..:r radiaiing dè'mcnt

and tape Ilitn1 IOg~llicr. This will keep themlined up \\hile drilling the" mminting holc~l\l;¡le marks on ¡he" Sil\-

3nt1 :::' lÖp Üv., siu~in;J ~~";t ~b-tTr1~ Í\~arefu1

'" ,m,h the whing ¡ ,mJ dri!l

Iwih píi:l",~;; at the same iime. Mollnt¡miicliing stuh In ilK~ lower radiating dement\i~-ith 31 l fl" \\';l.~.her~. nui-~ anti

h",rh~ need ~Jc4ts that dr;; threadedth(.~ \\ a\ t:ii ~he hcad l

11l'C ii1~nLilcr 1:"' ~z rü',¿r~la~ rndhih¡;ÜI1,ibh: at pl""lics :,ul'Ply hoii:oö ì or

wooden dowel waterproofed with eitherurethane or spar varnish. The immlatm,hüuld h'".: ~H !èj'tt 9" hlr(\~'"

~ 'H ,J dt arnèh~r

that a !.im¡o fï¡ msjt!e the tlbing,Slide the upper and lower d,'ments over theinsulalm'_ !raving 1/2" expfN'J between sec,.litHl~~

,¿-'\,R., driB a ~hi~,"t rnourning hük~

n~ ('jch ch.,~rnt'nt. The huh.:s HìUS! b\~2'. apart. TIh:' "tub á:!

made from :: 22" length of 10-24 Ilife~dedrod. Bend the l"etlWr of the flitl over ¡¡ 2" -di-,under pip.: to a ~l1oo¡h hent! Theil

lI~:r;~R u.l-unn -

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L. ~~UHJti TO

Fi'iU.'fl" ., defdlÉ,ih.

fasten the plnsing siiib iii the antenna with1O-2.l hardware. Stai¡¡k,~ Sled hflSC damps~ìfe lhCd tÜ cnnncCi the t-'\,~~.r\ £(1 th~ n13ichini,stuh. i.nJ tht' i'Yh.î ."""~, ,¡ax î~ ,¿atëû

RT\" s~::¿!t:TI~ í,~r ii~~tcni~.:a! Lipt'_

Adju1;tment

This antenna delivers g.ood pcrfiirmanccand ha.. ,i respt-,.table SWR ('liTh' over ih.:tnt¡r~ ~ rl1ett'Y h.ünd., T'lHHnp: h;,

Wt i'-td % \~..h-êrt' ¡~

t~larnpeij to tht" ¿ifHf?rnLH ~Hhèr up df d~:t'v\n hi,,('em\' ihe nest match, II

BibliographyJ(JX~ .lHRL ,-tnlfiur£l ft, '¡,'r.".¡(¡'el AU,i!L UUiidh,-'oLD(':\1;\a. IrlFtr, '\nu'mw .\f'cÍ'ill,.

19Ki.Honeycutt. Richard A.. Po¡mlar ElectrO/iin,Augu~i 199~. ì' /15.

Parts ListUp,perLower elernentMatchina stubP/(t;:SifHJ stob

...:.~ 4B~3;4' ~-jh.irmn.u-rn Wbing

3;4' x 73.3/4" aluminum tulling3(4")( 23" alunilmim lublng

r~su~-atc~

.l

P~asnc. F;t-.rg1,as

-: 0-24 r~.).ß-

10..24 )t 22'- ttrr~,;adeó gatvamzeo lf,,"'ÓiiO'Ne' 'or:e" .;.11an';¿~ër ii:' t( h¡trng

26ß:)

'ZH) '.\o'asr~fS3,:1F ~ '3~1-'2' Ö-t:Jt5

3/16' nuts3f1ó" lock washcirs":' st.ainfess stf;,"€J! hcse cJanips

the hatchback of the vehicle to be opened with the antennainstaled, Fig 37.

A Yi-inch galvanized iron pipe supports the antenna sothe radiating portion of the J is above the vehicle roof line.This pipe goes into a bakelite insulator block, visible inFig 37. The insulator block also holds the bottom of the stub.This block was first driled and then split with a band saw,as shown in Fig 38. After splitting, the two portions areweatherproofed with varnish and rejoined with 10-32 stain-less hardware. The corners of the insulator are cut to clearthe L sections at the shorted end of the stub.

The quarter-wave matching section is made of v.-inch .type L copper tubing (5/16-inch ID, 3/8-inch OD). The shortat the bottom. of the stub is made from two copper L-shapedsectons and a short length of v.-inch tubing. Dril a lI8-inchhole in the bottom of this piece of tubing to drain any waterthat may enter or condense in the stub.

A 5/16-inch diameter brass rod, 1 Yi to 2 inches long,is parially threaded with a 5/16 x 24 thread to accept aLarsen whip connector. This rod is then sweated into one ofthe legs of the quarer-wave matching section. A 4Oinch whipis then inserted into the Larsen connector.

The antenna is fed with 52-ohm coaxal line and a coaxal4:1 half-wave balun. This balun is described in Chapter 26.As with any VHF antenna, use high quality coax for the

Fig 37- The J antenna, ready for use. Note the bakeliteinsulator and the method of feed. Tie wraps are used toattch the balun to the mounting block and to hold thecoax to the support pipe. Clamps made of flashing copperare used to connect the balun to the J antenna just abovethe insulating block. The ends of the balun should beweatherproofed.

16-22 Chapter 16

#10 DRILL .. I II i : II "--T(4 PLACES) I I I I i I I

. :¡; i ¡I: .._~l 1.75i I I ' ,'I I 0.375 3.5

_ _ _ _ _ -- 0,375 OIAI

0.125 DRILL - -, :: - -T f

j í' ~ -- 1.25-i -i '4-1 0.75O'T- pi I ilis DRILL

Fig 38-Details of 27/32 DRILLthe insulated 1.2mounting block.The material isbakelite.

ALL DIMENSIONS IN INCHES

CUT ALONG THIS LINE AFTER DRILLING

IJ--0l$0-~-TBOTTOM VIEW

balun. Sea all open cable ends and the rear 'of the SO-239

connector on the mount with RTV sealant.Adjustment is not complicated. Set the whip so that its

tip is 41 inches above the open end of the stub, and adjustthe balun position for lowest SWR. Then adjust the heightof the whip for the lowest SWR at the center frequency youdesire. Fig 39 shows the measured SWR of the antenna afteradjustments are completed.

THE SUPER-J MARITIME ANTENNAThis 144-MHz vertica antenna doesn't have stringent

. grounding requirements and can be made from eay to findpars. The material in this section was prepared by Steve

2.5

2.0

ci~'" 1.5

1.0144 145 146

FREOUENCY (MHz)

147 148

Fig 39-Measured SWR of the mobile J antenna.

Cerwin, W A5FRF, who developed the Super-J for use on hisboat.

Antennas for maritime use must overcome difficultiesthat other kinds of mobile antennas normally do not

encounter. For instance, the transom of a boat is the logicalplace to mount an antenna. But the transoms of many boatsare composed mostly of fiberglass, and they ride somedistance out of the water-from several inches to a few feet,dependig on the size of the vessel. Becuse the next best thgto a ground plane (the water surface) is more th an appreca-ble fraction of a wavelength away at 144 MHz, none of thepopular gain-producing antenna design requing a counter-

poise are suitable. Also, since a water surface does a goodjob of assuming the eah's lowest mean elevation (at leaston a ca day), anythig that ca be done to get the radiating

par of the antenna up in the air is helpfuL.One anwer is the venerable J-pole, with an exta in-phae

half-wave secon added on top. . . the Super-J antenna. Thetwo vertical half waves fed in phase give outstanding

omndiectional performance for a portable antenna. Alo,the "J" feed arrangement provides the desired insensitivityto height above ground (or water) plus added overal antennaheight. Best of all: a \I-wave CB whip provides enoughmaterial to build the whole drven element of the antenna,with a few inches to spare. The antenna has enough bandwidthto cover the entire 144-MHz band, and affords a meaure oflightning protection by being a "grounded" design.

Antenna OperationThe antenna is represented schematically in Fig 40. The

classic J-pole antenna is the lower portion shown betweenpoints A and C. The half-wave section between points BandC does most of the radiating. The added half-wave sectionof the Super-J version is shown between points C and E. Theside-by-side quarer-wave elements between points A and Bcomprise the J feed arrangement.

At first glance, counterproductive currents in the Jsection between points A and B may seem a waste of elementmaterial, but it is through this arrangement that the antennais able to perform well in the absence of a good ground. The

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¡'L/4Fig 40-Schematic representationof the Super-J maritime antenna.The radiating section is two halfwaves in phase.

L/4

-l A

two halves of the J feed arrangement, side by side, providea loading mechanism regardless of whether or not a groundplane is present.

The radiation resistance of any antenna fluctuates as afunction of height above ground, but the magnitude of thiseffect is small compared to the wildly changing impedanceencountered when the distance from a ground plane elementto its counterpoise is varied. Also, the J section adds \iwavelength of antenna height, reducing the effect of ground-height varations even further. Reducing ground-height

sensitivity is parcularly useflÙ in martime operation on thosedays when the water is rough.

The gain afforded by doubling the aperture of a J-polewith the extra half-wave section can be realzed only if theadded section is excited in phase with the half-wave elementB-C. This is accomplished in the Super-J in a conventionalmaner, through the use of the quarter-wave phasing stubshown between C and D.

Construction and AdjustmentThe completed Super-J is shown in Fig 41. Detais of the

individual pars are given in Fig 42. The driven element canbe liberated from a quarter-wave CB whip antenna and cutto the dimensions shown. All other metal stock can be

obtained from metal supply houses or machine shops. Metalmay even be scrounged for little or nothing as scraps orremnants, as were the parts for the antenna shown here.

The center insulator andthe two J stub spacers are

made of Yi-inch fiberglassand stainless steel stock, andthe end caps are bonded to theinsulator sectons with epoxy.If you don't have access to alathe to make the end caps, asimpler one-piece insulatordesign of wood or fiberglasscould be used. However, keepin mind that good electricalconnections must be main-tained at all joints, and

strength is a consideration forthe center insulator.

The quarer-wave phas-ing stub is made of liS-inchstainless steel tubing, Fig 43.The line comprising this stubis bent in a semicircular arc tonarow the vertca profie andto kee the weight distributionbalanced. This makes for anattractive appearance andkeeps the antenna from lean-ing to one side.

The bottom shorting barand base mounting plate areof \i -inch staess steel plate,shown in Fig 44. The J stub ismade of 3/16-inch stainlesssteel rod stock. The RF con-necor may be mounted on the

shorting bar as shown, andconnected to the adjustable Fig 41-Andy andslider with a short section of the assembledcoaxal cable. RTV sealant Super-J antenna.

Mobile and Maritime Antennas 16-23

Fig 42-Details of parts used in the construction of the144-MHz Super-J. Not to scale.

T-Ii:

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1/4-WAVE PHASING STUBFOLDED AROUND ANTENNA1/S-INCH SS TUBING

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16-24

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Chapter 16

i.---~ 2. ------i 'o V." THROUGH 0

4-PLACES

o3/S X 24 NFTAPPED HOLE

o!4- 2 1/2"-I ..

BASE MOUNTING PLATE1/4-INCH STAINLESS PLATE

TOP AND BOTTOMEND CAPS AREIDENTICAL

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DRIVEN ELEMENT OBTAINEDFROM 1/ 4-WA VE CB WHIP

1/4". ... rl-

1/4" 1/4"-. ;.. -. l.. I I I I-i" ..3/B" Q,._OLiJT.

B-32 TAPPED HOLE.I.

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4- TAPPEDFOR FED-NE CONNECTON----.

, r 8-32~i..i:.ft :..T APPEC HOLE

~TOP J-STUS SPACER INSULA TOR

END CAPS IDENTICALASSEMSLE WITH EPOXY

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ENDVIEW

D'.ER'.~1/8"

~ B-32 T APPEC HOLEB.. " PLACES

J STUS3/16 SS ROD

T-1/2"-L

!. 1 1/4"--. .I .3/S"

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...SLIDING FEEDER INSUL .TOR

1/4"-. il-. .1/4"i _~ ~- TOP VIEW-i: i

:~.~_~ Fei il---- 1 1/4" ---------.! .

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SASE SHORTING SAR1/4-INCH STAINLESS PLATE

16-25Mobile and Maritime Antennas

Fig 43-A closeup look at the 1A-À phasing section of theSuper-J. The insulator fitting is made of stainless steelend caps and fiberglass rod.

should be used at the cable ends to keep out moisture. Theall-stanless construction looks nice and weathers well inmartime mobile applications.

The antenna should work well over the whole 144-MHzband if cut to the diensions shown. The only tung requiedis adjustment of the sliding feed point for minimum SWR inthe center of the band segment you use most. Settng the slider'2-13/16 inches above the top of the shorting bar gave the bestmatch for this antenna and may be used for a starting point.

PerformanceInitial tests of the Super-J were performed in portable

use and were satisfactory, if not exciting. Fig 45 shows theSupe-J mounted on a wooden mas at a portble site. Simplexcommunication with a station 40 miles away with a lO-wattmobile rig was full quieting both ways. Stations were workedthrough distant repeaters that were thought inaccessible fromthis location.

Comparative tests between the Super-J and a commercial5/8-wave antenna mounted on the car showed the Super-Jto give superior performance, even when the Super-J waslowered to the same height as the ca roof. The mast shownin Fig 45 was made from two 8-foot lengths of 1 x 2-inchpine. (The two mast sections and the Super-J can be easilytransported in most vehicles.)

The Super-J offers a gain of about 6 dB over a quarter-wave whip and around 3 dB over a 5/8-wave antenna. Actualperformance, especially under less-than-idea or variableground conditions, is substantialy better than other verticalantennas operated under the same conditions. The freedomfrom ground-plane radials, proves to be a real benefit inmartime mobile operation, especially for those passengersin the back of the boat with sensitive ribs!

16-26 Chapter 16

Fig 44- The bottom shortingbar and base mounting plate Fig 45- The Super-J in port-assmbly. able use at a field site.

A TOP-lOADED 144-MHz MOBilE ANTENNAEarlier in this chapter, the merits of various loading

schemes for shortened whip antennas were discussed. Quitenaturaly, one might be considerig HF mobile operation forthe application of those techniques. But the principles maybe applied at any frequency. Fig 46 shows a l44MHz antennathat is both top and center loaded. This antenna is suitablefor both mobile and portable operation, being intended foruse on a handheld transceiver. This antenna was devised by

Don Johnson, W6AAQ, and Bruce Brown, W6TWW.A combination of top and center loadig offers improved

efficiency over continuously loaded antennas such as the"stubby" pictured at the beginning of this chapter. Thisantenna also offers low construction cost. The only materialsneeded are a length of stiff wire and a scrap of circuit-boardmaterial, in addition to the appropriate connector.

ConstructionThe entire whip section with above-center loading coil

--

-T\

ii

i

Fig 46- This 144-MHz antennauses a combination of top andcenter loading. It offers low con-struction cost and improved effi-ciency over continuously loadedantennas.

is made of one continuous length of material. An l8-inchlength of brazng rod or no. 14 Copperweld wie is suitable.

In the antenna pictured in Fig 46, the top loadig diskwas cut from a scrap of circuit-board material, but flashingcopper or sheet brass stock could be used intead. Alumnumis not recommended.

The diensions of the antenna are given in Fig 47. Firstwind the center loadig coil. Use a Yi-inch bolt, wood dowel,or other cylindrica object for a coil form. Begi widig ata point 3 inches from one end of the wire, and wrap the wiretightly around the coil form. Wind 5 Yi tu, with just enoughspace between turs so they don't touch.

Remove the coil from the form. Next, determie thelength necessar to insert the wire into the connector you'llbe using. Cut the long end of the wire to this length plus4 inches, meaured from the center of the coil. Solder the wieto the center pin and assemble the connector. A tight-fittingsleeve made of Teflon or Plexiglas rod may be used to supportand insulate the antenna wie inside the shell. An alternativeis to fil the shell with epoxy cement, and alow the cementto set while the wie is held centered in the shell.

The top loadig disk may be cicuar, cut with a hole saw.A circular disk is not requied, however-it may be of anyshape. Just remember that with a larger disk, less coilinductance will be required, and vice versa. Drill a hole atthe center of the disk for mounting it to the wie. For a morerugged antenna, reinforce the hole with a brass eyelet. Solderthe disk in place at the top of the antenna, and constructionis completed.

Tune-UpAdjustment. consists of spreading the coil turns for the

correc amount of inductce. Do this at the center frequency

~2"1

13"

L1/2"1/

Fig 47-Dimensions for thetop-loaded 144-MHz antenna.Se text regarding coil length.

5-112TURNS

4"

1

of the range you'll normally be using. Optimum inductanceis determined with the aid of a field-strength meter at adistance of 10 or 15 feet.

, Attach the antenna to a handheld transceiver operatigon low power, and take a field-strength reading. With thetranmitter tued off, spread the coil tur slightly, and thentake another reading. By experient, spread or compress thecoil turs for the maxmum field-strength readig. Very litteadjustment should be required. There is one precaution,however. You must keep your body, ars, legs, and head inthe same relative position for each field-strength measure-ment. It is suggested that the transceiver be placed on anonmet table and operated at ar's length for these checks.

Once the maxum field-strength reading is obtained,adjustments are completed. With this antenna in operation,you'll likely find it possible to accs repeters that are difficutto reach with other shortened antennas. W6AAQ reports thatin distant areas his antenna even outperforms a 5/8-À vertca.

VHF QUARTER-WAVELENGTH VERTICALIdealy, a VHF vertica antenna should be instaled over

a perfectly flat reflector to assure uniform omndieconalradiation. Ths suggests that the center of the automobile roofis the best place to mount it for mobile use. Alternatively,the flat porton of the trunk deck can be used; but wi resultin a diectional pattern because of car-body obstruction.

Fig 48 ilustrates how a Milen high-voltage connectorcan be used as a roof mount for a VHF whip. The hole inthe roof can be made over the dome light, thus providingaccessibilty through the upholstery. RG-59 and the Y--wavematching secton, L (Fig 48C), can be routed between the carroof and the ceilng upholstery and brought into the trunkcomparment, or down to the dashboard of the car. Insteadof a Mien connecor, some operators instal an SO-239 coaxconnector on the roof for mounting the whip. The methodis simiar to that shown in Fig 48.

It has been established that in general, Y--À verticalantennas for mobile repeter work are not as effecve as 5/8-À

Mobile and Maritime Antennas 16-27