Building a DB6NT 3.4GHz Transverter system Building a DB6NT 3.4GHz Transverter system Building a DB6NT 3.4GHz Transverter system Building a DB6NT 3.4GHz Transverter system

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By Peter Day ~ G3PHOBy Peter Day ~ G3PHOBy Peter Day ~ G3PHOBy Peter Day ~ G3PHO 3.4GHz(9cm)was the band on which I made my first microwave transmissions back in the very early 1970s. In those days the gear was based around a 60 milliwatt output klystron, the 726A/B. How times have changed! I have been wanting to get back on the 9cm band for many years but it took a visit to Germany to set me off on this latest construction project, a 15 watt portable transverter for 3.4GHz narrowband.

Photo above: the completed 3.4GHz transverter

The homemade aluminium case is designed to withstand rough treatment when used under portable conditions . Note the finned heatsink on the right hand side. This is for the 15 watt power ampli-fier. In September 2002, I visited the VHF/UHF COnvention in Weinheim, Germany. There I purchased a DB6NT 3.4GHz transverter kitset, direct from DB6NT himself as he had a stand at the event. It wasn't until I got home that I realised the construction manual was in German and I don't know more than half a dozen words of that lan-guage! However, all was not lost because I was well-acquainted with Michael Kuhne’s (DB6NT)excellent kits, having already built up 10GHz and 5.7GHz DB6NT tranverters. The following pages show you how I put the 9cm kit together and integrated it into a potent 15 watt output transverter.

Photo below: DB6NT transverter module The basic module is available in kit form from DB6NT for around £200 sterling. You can also buy a ready made version but you'll get more satisfaction if you build your own! The photo above shows the circuit board side with all components finally soldered into place. Just before I went to Weinheim I had also collected a surplus solid state 9cm PA from Mark, GM4ISM. He has been supplying these for some time now, along with re-ceiver sections and even antennas if anyone wants them. The PAs come from the now defunct Ionica telephone system which used the 3.4GHz band to relay telephone data from base to subscribers. These excellent amplifiers require no modification at all if one is happy with around 14 to 15 watts output. Mark and others have achieved around 22 watts RF output but have made some modifications, particularly to the biassing arrangements. I am quite happy to get 15 watts with no modifications as the gear is used from portable locations, where battery current consumption is an important factor.

Photo: The Ionica 15watt solid state power amplifier with the lid off The amplifier requires only around 1 milliwatt of drive at 3.4GHz for 15 watts out-put! The input is applied to the sma connector lower right while the output appears at the upper right connector, also sma. The 15 pin D connector on the left is multi-purpose, receiving all power requirements and outputting useful readings such as PA temperature and a power output indication. The DC supply requirements are: +10V (at approx 4 amps), +12V, -12V and +5V.

Tools and soldering techniques

This photograph (above) shows me at work at my test bench making the basic DB6NT 3.4GHz transverter module. The kitset comes in two flat, plastic, compartmental-ised boxes in which all the components are sorted and labelled. In fact, DB6NT scores 100% for presentation! A full construction manual is also provided but the constructor is expected to have some previous experience at soldering small sur-face mount devices (SMDs) onto pcbs, as well as sufficient knowledge about safe handling procedures in relation to static sensitive items such as the GaAsFETs and HEMTs that come with the kit.

The photo also serves to illustrate some basic needs in the way of tools and other aids. Note the headband binocular magnifier that I am wearing. I find this abso-lutely essential! The components are often very small and it needs a steady hand to solder them in place. The extra "dimension" provided by the magnifier makes all the difference. The soldering iron I am using here is a variable temperature con-trolled one marketed by Vann Draper for around £60. The temperature is accurately set by the adjustable control on the front of the white base unit to my right. It's usually set to 300 degrees Celcius but can readily altered to suit particular components. The other iron is a temperature controlled Weller TCP-D type but the temperature is fixed by the type of bit used at the time. I much prefer the Vann Draper iron, which has a 0.5mm diameter bit, ideal for delicate soldering work.

Other essentials include precision tweezers. Once you have had a minute surface mount component fly across your workshop after your cheap tweezers slipped you will understand what I mean about "precision"! Such items can often be found in surplus medical equipment shops and even at amateur radio rallies and "hamfests". Ideally you might use silver solder for microwave work but I have only done this at frequencies above 10GHz. Below that I use normal cored solder of about 0.5mm diameter and with a low melting point rating. I find the average desk type workbench rather low for sustained electronic con-struction work so I lift the work a little higher by using a small makeshift "platform" (literally two bits of wood with a 25cm x 25cm square of hardboard nailed to them!) on the workbench. This brings the project to a comfortable height where I don't get an aching neck after hours of working on it! The DB6NT kitset comes complete with a prefabricated, double-sided printed circuit board of very high quality. All connections to the groundplane side of the board from the circuit side are done via plated through holes, ensuring high stability.

The construction manual (in English with export kits) should be carefully fol-lowed. Do not attempt to do your own thing! If you follow the instructions and solder carefully, your basic transverter module should work first time and should be completed in a few days. In fact my first DB6NT module, for 5.7GHz, was com-pleted in just two days. This latest one took a little longer, five days, because I spent less time each day on it. The method I use to solder the various SMD components, such as chip resistors and capacitors and SMD transistors, involves the use of small lengths of O.5mm diame-ter solder that I cut off in dozens from a reel of solder before construction be-gins. These 1 to 2mm lengths are kept in a surplus 35mm film cassette container when not in use. I find this is a much better way of obtaining a good, neat sol-dered joint. To solder in a component I lightly "tin" one end of it with the iron. Then, using one of the 1mm lengths of solder, I flow a very small amount of solder onto one of the connection pads on the pcb. I then place the component on the pcb and quickly "tack" it to the presoldered connection pad, making sure the component lies absolutely flat on the pcb. Next, I lay one of the 1 to 2mm pieces of solder right up against the other end of the component, which component is gently held down with the precision tweezers I used to pick it up. The tip of the soldering iron is quickly moved along the pcb stripline towards the unsoldered end of the SMD. The small 1-2mm piece of solder quickly melts and the iron is removed, leav-ing a very neat joint. The other end is then reflowed to ensure a good contact. All this takes longer to describe than to actually perform! The main thing is to avoid having to pull a length of solder from a reel when fitting the components. This can result in too much solder being applied to the job. The DB6NT transverter kitset modules are housed in tinplate boxes, into which the printed circuit board is soldered BEFORE any pcb components are mounted. Before that though, the tinplate box needs to have its seams soldered and the four sma connectors and feedthrough capacitors fixed to the end wall. This requires careful measurement and accurate drilling since the pcb is soldered up to the centre spig-gots of the sma connectors. It's essential to mount the board at the height stated in the manual or spurious resonances may be encouraged and/or some of the taller components, such as the silver-plated "pill box" filters, might even project higher than the edge of the box, preventing the lid from fitting! The photograph below shows the "groundplane" side of the pcb, mounted in the tinplate box. You can see that some relatively large components are fitted here .. two "pillbox" cavity filters and several heli-cal filters, as well as a couple of standard voltage regulators, trimpots, the Lo-cal Oscillator crystal (in its heater) and a large input attenuator resistor (50 ohm). All these need a hotter iron temperature setting than the surface mount com-ponents on the other side of the board. The sma connectors are screwed to the end plate of the box. Fitting the pillbox filters is perhaps the trickiest job of the lot. It is one of the first jobs to be done after the the pcb has been soldered in to the tinplate box. There are small locating points marked on the pcb and the filters MUST be ac-curately located to them as each filter has two short probes coming up through the board from the circuit side. The filter must be symmetrically located with respect to them. The probes are fitted and measured acurately BEFORE the filters are fi-nally soldered into place. To make sure the silver-plated filters are soldered correctly I heat them up first on a kitchen hotplate! When they are too hot to touch I quickly apply 0.8mm diame-ter low melting point solder all around their bases to pre-tin them. Then, one by one, with the adjusting screws fully inserted in the top of each filter I lift them with a small part of pliers and I press the the cavities onto the pcb, quickly wrapping a ring of solder around their bases. My soldering iron, with a broader bit than that used for the SMDs and set at 420 degrees Celcius(its highest temperature), is then applied to the base of the filter to encourage the solder ring to flow. All this time it is essential to keep the filter absolutely firmly in place, for if you move it out of position you may have to use a small flame

torch to free it later! This method usually results in quite a neat installation. The one shown to the left is NOT my best! Other components such as the helical filters also have to be soldered to the groundplane using a hot iron. In this case it is essential to solder quickly as you could melt the formers inside the coil hous-ing if you applied your iron for too long. I usually cool the component down with a damp cloth, AFTER I am certain the actual solder joint has set. Once all the components have been soldered into place, I

gently apply some alcohol ("Methylated Spirit") with an old toothbrush to both sides of the board to clean away any flux deposits. The module is then left over-night near a warm radiator to complete dry out. The screws are taken out of the cavity filters during this time.

Alignment of the transverter module

The construction and alignment of the DB6NT transverter is adequately described in the construction manual. DB6NT says you can do the alignment with the minimum of test gear and tools ...a +12V DC supply with 0.6A current limiting, an analogue test meter set to the 0-10V DC range and a dummy load. In practice I also found it useful to listen for the crystal oscillator on its fun-damental frequency... my IC706Mk2 easily found it, on 135.667MHz. I could also check it more accurately with my Racal Dina frequency counter, but the IC706 at least was a quick and easy way to verify that the circuit was working! DB6NT provides four test points for alignment. Aim to tune the bandpass filters for the voltages (at the frequencies stated) as shown in the manual. I found these voltages, when adjusted for optimum output, to be a little different to the book but my other test gear, in particular my HP spectrum analyser, was much more use than the voltmeter method as I could then easily tune each stage for optimum output consistent with lowest amplitude of spurious signals. If you have a spectrum ana-lyser then use it! When the module was finally adjusted I got a healthy 260 milliwatts output at 3.4GHz. This was later reduced by means of an attenuator to drive the 15 watt PA. However the "barefoot" transverter would still make some interesting contacts with just the quarter watt of RF output. When adjusting the receiver cavity (pillbox) resonator be sure to set the tuning screw to the first position where there is a noise peak... there is a second peak when the screw is turned further into the filter. Do not chose this one ... the first peak, ie with the adjustment screw furthest out, is correct. I then listened for a marker signal generated by my Adret synthesiser and diode multiplier, set to produce a signal on 3400.100MHz. I could not miss it... it "pinned" the S meter on the IC202S IF transceiver which I use with my all transverters! A method of gener-ating accurate marker signals in the amateur microwave bands is a most useful

"tool" and is highly recommended. You can find more information on this and other test gear elsewhere on this website. The transverter may be used "barefoot", without any more amplification. If you de-cide on this then you need only mount it and the microwave coaxial relay in a suitable box (an diecast aluminium box for example). In my case I decided to add a high power RF amplifier on the transmit side. This required a much bigger enclo-sure being made! The next section shows you how this was done.

Combining the DB6NT transverter with the 15W PA

I found I could not obtain a reasonably cheap, ready-made enclosure for this project so I set about making my own. I was lucky enough to have purchased several large sheets of 3mm thick aluminium at an amateur radio "flea market" a few years ago and so I decided to make an enclosure based on a design used for my 5.7GHz system.

This photo (left) shows the various modules, and other hardware making up the complete system, laid out on a sheet of aluminium that was to form the base plate of the enclosure. Each mod-ule was laid in its approximate final position and the sheet of aluminium was then trimmed to a suitable size. The lid of the box was made to a similar size, but 6mm wider and longer on both edges so that it would rest on the edges of the four box sides. The sides of the enclosure were cut to be just a few millimetres higher than the PA, shown here on the right hand side of the photo above.

Shown on the right is the box, almost completed. It measures 31cm x 26cm x 12.5cm. The sides, baseplate and lid are all held together by a simple framework made of aluminium angle stock with sides 15mmx15mm. All the holes needed for various switches, the me-ter, connectors, etc, were pre-drilled before the box was assembled.

This is the finished product... note the heatsink for the Power Amplifier. It is held by the same bolts that clamp the PA to the side of the enclosure. A continuous layer of thermal transfer compound was applied to the flat face of the heatsink and to the PA base before they were finally fixed to the box.

The whole enclosure was given two coats of silver hammer finish paint and the front panel labelling done with a Brother P-Touch 300 labeller, using clear tape. This labeller is easy to use and gives very acceptable results. The labels are extremely hard wearing.

The 15 watt system The actual transverter system will now described in more detail on the following pages. The various modules seen in the enclosure on the previous page are all essential parts of the 15 watt portable transverter. However they may be used individually in other transverter sys-tems, with little or no modification. To construct this 15 watt output transverter required several extra modules such as power supplies and an interface unit for the IC202S I.F or "prime mover". The block diagram on the following page shows the basic system while the photo below shows the various modules making up the complete system. The 144MHz IF feeds (via the BNC socket in the lower right corner of the photo) the IC202 interface module, which grounds the PTT feedthrough on the DB6NT transverter when the IC202S is put in the Transmit mode. The Relay power supply is for the microwave coaxial changeover relay shown in blue near the sma output connector. The PA power supply module provides all the voltages required by the 15W PA and is switched on for TX only by the Sequencer, which is, in turn, switched on by the "+12V on TX" output from the DB6NT transverter module.

The IC202S interface was built on perforated board (Veroboard) using the copper strips on the underside to complete the electrical cir-cuitry. 144MHz I.F is applied to the BNC con-nector on the left and the 144MHz RF passes on to the DB6NT module via the sma socket on the upper right. The uppermost feedthrough capaci-tor is connected to the PTT feedthrough on the DB6NT module, while the lower feedthrough re-ceives the +12DC supply for this unit. The re-lay, lower right, keys the DB6NT transverter from receive to transmit when the PTT switch on the IC202 microphone is pressed. The whole mod-ule is housed in a small diecast box for effi-cient RF shielding purposes. POWER SUPPLIES and SEQUENCER There were two power supplies required for this project .. the relay power supply and the PA power supply. The sequencer was purchased as a ready assembled pcb direct from DB6NT. The very reasonable price of this item makes home construction hardly worthwhile! The ANTENNA CHANGEOVER RELAY POWER SUPPLY

The simple unit shown to the left provides around 22 to 24V DC, easily enough to operate Transco or Dynatech type microwave 24 -28VDC coaxial relays. It consists of an NE555 oscil-lator which drives a complimentary pair of transistors(TIP31/TIP32)into a diode pump (2 x 1N4001). A small piece of veroboard was used to mount all the components except for the two transistors, which are securely bolted to the wall of the small diecast aluminium box which acts as a heatsink. One of the transistors is fitted with an insulating washer as its flange is above ground potential. I'm indebted to Dave Robinson, G4FRE (WW2R)for this nice little cir-cuit.

The PA Power Supply was also made on Vero-board and housed in an aluminium box with all input and output connections made via feedthrough capacitors. The supply provides nominal +10VDC, +12VDC, +5VDC and -12VDC. The latter is produced by a DC-to-DC voltage inverter IC, the NMS1212, shown here on the left side of this photo. This was the only IC I could find that would produce enough current for the -12V PA bias. The +10V sup-ply is produced by an LT1083CP regulator which is fed from the main +12V line of the transverter. This chip is capable of produc-ing much more than the 4 or 5amps required by the PA and so is well under-run in this application. Nevertheless it requires good heatsinking as can be seen in the photo

above. The supply has a built in failsafe circuit to protect the PA in the event of the -12V bias supply failing. The main +12V input is switched to the LT1083 regulator by a relay. When the bias supply comes on, a transistor relay driver is operated by the -12V from the inverter. The small relay (contacts rated at 10A DC)can be seen in the lower right of the photo and this then switches the +12V input to the 10V regulator. Similarly the +5V supply, which enables the PA, is only ac-tivated when the same relay contacts are closed. The NMS1212 bias supply IC was chosen because it supplies -12V even when the portable battery supply drops from a fully charged state. Normal regulators need some 2 volts headroom and thus an in-put of almost 14 volts would be required, not easily come by under portable condi-tions. The circuit of the PA power supply is a modified version of one supplied by John, G3XDY, and can be recommended for other PAs where voltages other than 12V are required at several amps. The Sequencer is a DB6NT SEQ3. It can han-dle a PA supply of 12 volts at 18 amps, well in excess of my system! I believe in rating components very conservatively. That way you avoid system failure later. The se-quencer comes ready made for around 25 Euro (about $25US) and is hardly worth the bother of making it yourself. I mounted this one in a diecast aluminium box with all supplies in/out via 1000pF feedthroughs. The sequencer provides proper TX/RX/TX changeover protocol so that the sensitive receiver GaAsFETS are protected from the RF damage that would almost cer-tainly occur if no sequencer were used. A good friend of mine tried out his 15 watt PA 3.4GHz transverter in a contest and blew his receiver "front end" almost immedi-ately ... he was not using a sequencer! The sequencer also ensures that the relay contacts are fully closed before transmis-sion takes place. If you put RF through the relay before the contacts have fully changed over, serious damage could be caused to the contacts at this level of RF. All supplies and the sequencer are protected by suitably rated in-line fuses(see the circuitry). Do not cut corners and miss out this protection ... it could be a costly mistake if you did!

3.4GHz Transverter 3.4GHz Transverter 3.4GHz Transverter 3.4GHz Transverter ---- The Antenna System The Antenna System The Antenna System The Antenna System

The 3.4GHz transverter would not be complete without a suitable antenna system. I already had a 1.2m (4 foot) diameter dish, which I use on 5.7GHz and 10GHz. Unfor-tunately I could not make a suitable feedhorn to slot into the feed mount already fitted to this dish. My 5.7 and 10GHz feedhorns do fit the mount and thus I can change bands very quickly from 3cm to 6cm ... but not 9cm! So, for 3.4GHz, I decided to make a whole new set of mounting struts and a horn mount for the dish. These could be fitted for the times when I would go out port-able with the 3.4GHz system and be changed for the 5.7/10GHz struts for the times when I would activate those bands. I tried a simple log periodic feed for the 23, 13 and 9cm bands on pc board as an alternative. This did fit into the existing 10GHz mount but unfortunately it could not handle the 15 watts at 3.4GHz.

Photo above: The completed dish feed A commercial "chaparral" or scalar feedhorn was on hand but the rear part of it had a large waveguide flange. I wanted to use a circular horn feed of the VE4MA type since this was eminently suitable for the dish's 0.4 f/D. So I made a "soup can feed" and arranged a simple clamp of copper strip to fix it to the waveguide flange. The waveguide port on the diecast chaparral horn was filed to make it into more of an elliptical cross section, in an attempt to provide an "easier" transi-tion to the circular soup can section. The soup can measures 65mm inside diameter and is 110mm long. A probe, 24mm long, extends into the can and is some 38mm from the backplate of the horn (can). The probe is fitted to an sma socket which was soldered to the outer surface of the can. A suitable length of 3mm o.d. brass tubing was soldered over the centre con-ductor of the socket to make the probe. Directly opposite the end of the probe, I fitted a similar length brass screw, set in a brass nut soldered to the outer sur-face of the can, the idea being to give a small amount of adjustment to the cav-ity. In practice, however, the screw adjustment was found to be unnecessary, as it made no difference to the performance of the feed. The soup can arrangement, to-

gether with the existing circular feed section of the chaparral horn, forms a suf-ficient length to make a reasonably efficient dish feed. No definitive gain meas-urements have yet been made but the performance in actual use is very encouraging. The horn is fed with a 1.8m length of Andrew FSJ1-50 heliax that was already pro-fessionally fitted with an sma connector at one end and an N connector at the other. The sma connector fastens straight onto the transverter while the N type connects to a short length of semi-rigid line which in turn connects to the feed horn via an sma plug. This arrangement, with the heliax included, results in ap-proximately 0.65dB loss ... quite acceptable for terrestrial use!

Photos left and below: The soup can feedhorn showing the position of the input sma/probe and the adjustment screw

This photo shows the commercial chaparral or scalar feed. Though designed for a slightly higher frequency than 3.4GHz, it does appear to work well.

This photo shows the complete 3.4GHz antenna system on its heavy duty tripod, with the 3.4GHz transverter fitted, ready with the G4NNS Noise Amplifier for some sun noise and sky/ground noise measure-ments

USEFUL REFERENCES: 1. DB6NT, Kuhne Electronic: www.db6nt.com 2. IC202 Interface circuit and relay power supply: Dave Robinson, G4FRE. Microwave Update proceedings 1992 page 95 3. 3.4GHz feedhorns: Horns for the Holidays, by W5ZN. Microwave Update

Proceedings 1997, page 147-157. 4. Surplus Ionica 15W solid state amplifiers: South Birmingham Radio Society,

England. Email Ian G8IFT at g8ift@sbrs.org.uk 5. The author of this article, Peter Day, G3PHO. Email to:

microwaves@blueyonder.co.uk 6. “The World Above 1000MHz” webpages: www.g3pho.org.uk