BITX 40 "semi-kit"

Although most of us build from scratch, kits have some advantages; all of the components are available, the layout is done, others have had it working, you just need to assemble it. Heathkit refined this to an art and many of us learned the basics of assembly and testing from their products. Now Ashhar Farhan, VU2ESE, has made a version of his BITX (BI-directional Transceiver) available to the world as a “semi-kit”, with control and transceiver boards pre-assembled and shipped with most of the external parts needed to build your own single band SSB transceiver.

This saves the builder the tedium of loading and soldering the board. He can install it in any form that he wishes, as a nice portable in a weather resistant case, or perhaps in a big solid cabinet for the shack. He can spend his time dressing it up and adding features.

For a mere $59 USD (about ₤46) the BITX40 is shipped to your door with the 4.5” x 5” printed circuit board, the “Raduino” VFO module, two 3-conductor 3.5mm audio jacks, a volume control with power switch, a 10K linear taper potentiometer (for tuning), a BNC antenna jack, a set of power connectors, an electret microphone element, a push button switch, and all of the connecting wires needed to wire it up (with plugs attached!). Not only that, but 8 brass stand-offs, mounting nuts and bolts included. The boards are all assembled and tested, just waiting for the drive to be set for the Power Amplifier.

The transceiver is the simple single-conversion superhet design originally conceived by Farhan more than a decade ago. During the ensuing years it has been constantly refined and is now a logically arranged, cleanly designed module with large-format surface-mount components on an uncrowded board. This is ideal for modification and servicing. There is nothing better for demonstrating theory to beginning hams and engineers. The board “reads” like a schematic. Signals flow in easily traceable directions through clearly identified components. Test probes have easy access to all parts of the circuitry, unlike most “through-hole” equipment. A real joy to explore.

The BITX was originally designed to use an analog VFO. In fact, the current model is delivered with everything needed to use it analog fashion. A single-turn 10K pot controls the varactor-tuned VFO so you can enjoy the clean sound and simplicity. However, like the Epiphyte and other rigs that use analog VFO, the operator needs to frequently re-touch the tuning to compensate for the drift. We are used to that, right? The problem is made larger by the huge available tuning range. That single turn for over 100 KHz makes for a fine touch, indeed!

Fear not! The BITX40 now includes a spectacular answer, the Raduino. The name is a combination of “Radio” and “Arduino”. If you haven't met the Arduino line of microcontroller boards then you are in for a treat. They are widely available at give-away prices, plug into a usb port on your computer, and are easily programmed using free software. They are like a Swiss army knife, an electronic multi-tool, limited only by your imagination. The Raduino employs the thumb-sized Arduino Nano to control a tiny (yet powerful) Silicon Labs Si5351 three-output synthesizer and a two line 16 character LCD display in a single VFO module. Rock stable, clean output, and reliable. Having a tiny computer in the module makes it possible to configure it do work the way that you want it to. It is fast, uses little power, and has extra pins to add a bounty of features.

To use the Raduino as the VFO, one re-uses the tuning pot for control, disconnecting one end of the original VFO inductor, and plugging the output of the Raduino into a jack labelled “DDS”. The Raduino comes already programmed for a unique tuning solution using the pot to tune 50 KHz portions but reserving spaces at each end where the same knob can step up or down in 10 KHz increments. A plethora of features is provided by the original software (written by Farhan and is, in itself, a course in programming) that includes two VFOs, RIT, CW, calibration, and memory operation.

Want to try different software? Other tuning modes? Want to add features? There are scores of choices available free from the Internet. Join the BITX20 group at https://groups.io/g/BITX20 for a very active (and talented) group just waiting to help and answer questions. Read the postings on http://bitxhacks.blogspot.com/ for ideas. Doesn't work like you think that it should? Then program your own “sketch” (the Arduino name for software). It's easier than you think. Remember that you can reload the old program at any time.

The radio module uses mostly discrete components. Even the mixers use hand-wound toroids. The transmit output transistor is an IRF510 and has a separate DC power input jack so that if you want to run more than the 5 to 7 watts PEP that 12 volts provides, you can connect up to 25 volts and run 20 watts. If you do run higher power then be advised to use a larger heat sink than what is provided.

Knobs, speaker, or headphones are not included. You would want to use styles and sizes appropriate for your enclosure anyway. This is an advantage over most kits in this respect. You buy the pre-tested works and give it form.

The 4 crystal Cohn IF filter yields good sideband suppression while maintaining clear audio performance and selectivity. The crystals are hand-picked and matched before installation. For a single conversion receiver, there are few spurious signals. It is very quiet without the usual NE602 mixers, yet quite sensitive.

Overall, an incredible value. A true single sideband transceiver, complete with configurable digital control (or not!) for less than many most CW only sets. All of the drudgery of basic assembly is done for you. All that remains is to individualize it and have fun.

Check it out at: http://www.hfsignals.com

de ND6T


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