OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

The BITX20A Mono-band 20M Transceiver kit.

The BITX20A (File Photo)

AL GM1SXX (photographed by a workmate)

The BITX20A from Hendricks BITX20A is a 10W PEP 20M monoband transceiver for the DIY enthusiast.  The BITX20A kit is a board and parts only job, so you'll need to find a case and knobs etc. You also need to supply some suitable connectors, a microphone and speaker/headset.   The manual is available online and prospective purchasers can see what's involved... a great idea IMHO.  The kit is an adaptation of the original idea, for a single board cheap 20M transceiver that could be built cheaply by people from poor countries. (So much for amateur radio being a 'Rich Man's pastime'... LA2QAA).  I ordered the kit from Doug Hendricks in California and expected a long wait for it.  In fact it arrived in about a week and the Post Office didn't charge me VAT so in all, this radio kit cost me about £54, an excellent price for a 10W PEP mono-band 20M transceiver. 

Now as you may know, I can't abide the sight of an 'unpopulated' PCB, so after two days of admiring the kit and it's contents, I got 'stuck in' and began construction of this kit. Now being a contrary sort, I decided to go for bust and just populate the board using the parts-placement guide, leaving the active components and toroids until nearer the end.  I can't recommend this method but it did highlight the fact that the values of a very few of the parts were not printed on the overlay. Easy to sort out... I just RTFM'd the unidentified parts I had.  Don't take this as any sort of criticism, it's not intended to be. The manual is excellent.  I just LIKE doing things 'my way'.  The PCB is of very good quality, double sided with plated through holes and solder mask with printed parts-placement overlay. It's not a kit for beginners, but it's quite simple electrically. It's been  a joy  building this kit. No surprises of any sort.

The transceiver is a bi-directional superheterodyne type with a conventional free-running VFO and uses an 11Mhz intermediate frequency. The IF filter is built from four 11Mhz crystals and the mixer and balanced modulator use IN4148 diodes and toroidal cores.  The PA uses the very common IRF510 plastic powerfets in a wideband  push-pull configuration followed by a filter stage.  All of the parts used are cheap and readily available... good if you ever need spare parts. Since the  VFO is a free-running type, care has to be taken to use short and sturdy connections between the PCB and the supplied Polyvaricon Tuning capacitor.  The radio should be housed in a sturdy metal enclosure for screening and to aid mechanical rigidity. I plan not to use the supplied polyvaricon tuning capacitor. Instead I'll use a salvaged Jackson Bros air-spaced variable. The radio doesn't have AGC (what do you expect for that little money... LA2QAA!), so the builder can hear for him/herself the variability of band conditions.  It's educational.

The original BITX design was by Ashar Farhan http://www.phonestack.com/farhan/ and his aim was to produce a cheap and reliable 20M transceiver  In turn, Ashar's original design was 'inspired' by yet another one, The S7C.

Here's what Ashar says of his original design...

BITX is an easily assembled transceiver for the beginner with very clean performance.
Using ordinary electronic components and improvising where specific components like toroids are not available, It has a minimum number of coils to be wound.
All alignment is non-critical and easily achieved even without sophisticated equipment. The entire instructions to assemble the rig are given here along with relevant theory.

The Indian hams have often been handicapped by a lack of low cost equipment to get them on air. A mono-band, bidirectional design using ordinary NPN transistors was developed to cater to this demand. The design can be adapted to any particular ham band by changing the RF section coils and capacitors and the VFO frequency.

BITX evolved over one year from the excellent S7C receiver described in the new ARRL book ?Experimental Methods in RF Design? (an ARRLpublication) into a bi-directional transceiver. Several hams across the globe contributed to its design. A series of emails were exchanged with OM Wes Hayward (W7ZOI) during the evolution of this design. His contributions have been invaluable. He urged me to strive for higher performance from the simple design. The resultant rig has sensitive receiver capable of strong signal handling, a stable and clean transmitter capable of enough power to make contacts across the World.

All the parts used in BITX are ordinary electronic spares components. Instead of expensive and hard-to-get toroids, we have used ordinary tap washers. Broad-band transformers have used TV balun cores. The entire transceiver can be assembled in India for less than Rs.300. I have designed a single side PCB with large tracks that can be easily etched at home or by any PCB shop.

Tap washers indeed!

In short, BITX20 is the culmination of a lot of good work by a lot of people. The original design was built in many cases in 'ded-bug' style over plain PCB's or by some others using Copper-Island/Manhattan style construction.  Now Hendricks and others have taken the radio to a new level with this interesting kit, the BITX20A.

It's interesting that others have also adapted the basic concept for other bands. Not so long ago, I built an MKARS80... also an adaptation of this general design. The MKARS80 is essentially the same design, but for 80M and is equipped with a PIC based DFM and huff 'n puff stabiliser.

Still, back to the BITX20A. It's a very nice kit to build. The toroids, while a nuisance to wind are quite straightforward and for me at least, the least pleasant part of building these kits. There's nothing difficult about toroids... they are just a hassle to wind.

If nothing else, the builder should be able to see the construction of a balanced mixer... all 'up-close-and-personal' rather than just plonking a ready-made mixer into a board and soldering the pins in place.  The kit uses a number of small cores of the type 6 material (Yellow) along with some ferrite cores (grey).

For bifilar and trifilar winding, I like to use different coloured wires twisted together and threaded through a darning needle. I just 'sew' the turns onto the core and I find that this leads to a nice neat winding. This 'trick' comes from LA2QAA, and saves a LOT of hassle.

The VFO For the VFO coil, I can recommend winding the coil then placing it in boiling water  for 5 minutes to anneal it before fishing it out and allowing it to dry completely. Painting a thin coat of epoxy resin or varnish over the windings helps hold them in place and aid stability. This usually results in a nice stable tuned circuit.  It goes without saying that the component leads in the VFO section should be a short as is feasible and in particular the leads to the tuning capacitor should be as short and rigid as possible. The stability (or otherwise) of the VFO can make or break a radio. This one is no exception. Some people like to drip paraffin wax over their finished VFO circuit. This encapsulates the components and helps reduce the effect of temperature changes while providing mechanical support. Epoxy also works, but unlike wax, can't ever be removed without causing major damage, so my advice is use wax! Ordinary cheap paraffin wax candles do a fine job.

Here is a selection of photos of the kit contents. You can click on the thumbnails to see bigger images.

In essence, the BITX20A is very similar indeed to the MKARS80 but on 20M rather than 80M. It lacks the PIC based frequency counter of the MKARS80 and it's huff&puff stabiliser but it does use a pair of push-pull IRF510's rather than the single IRF510 PA used on the MKARS80.

Both the BITX20A and MKARS80 are nice little QRP radios and should give their owners endless hours of fun, building and using them.

Some Links for the BITX20A and BITX20

http://www.qrpkits.com/bitx20a.html

http://www.qrpkits.com/files/BITX20_Assembly_Manual.pdf

http://www.eham.net/reviews/detail/7147

http://www.phonestack.com/farhan/

http://groups.yahoo.com/group/BITX20/

http://pa3cno.qsl.nu/?q=node/3

Allan Copland

GM1SXX