4 May 2013

The £20 DECENT HF transceiver challenge

Ideas for the £20 (or less) new HF transceiver so far, based on past experiences:
  • VXO controlled, single band- GQRP crystals are inexpensive, but will allow around 10-15kHz easy pulling on 14MHz, more on 21 or 28MHz.
  • Single balanced diode RX mixer - homebrew with low cost diode and toroid matching
  • 2N3904s everywhere - these low cost (5p or less), ubiquitous transistors work well as oscillators, PAs (if several paralleled up) and are OK in audio stages. Maybe an IRF510 PA if a lower HF band chosen,
  • Deadbug build - on  a small piece of copper laminate and low cost using a few MeSquares to help ease construction
  • Low cost metal box - e.g. Maplin Aluminium box or a diecast enclosure. A mint box would be less expensive but too small.
  • Few controls - a 1k pot for RF gain is all that's needed on RX
  • Capable of running from a PP3 9V supply or external 12v - rechargeable 9V packs are low cost from 7dayshop.  69p each non rechargeable and £2.60 for a rechargeable one.
  • Low-Z stereo headphones will be less expensive than a crystal earpiece these days - may need an extra audio stage but at around 10p for this, this is a price worth paying.
  • Probably CW, although DSB not out of the question.
  • No ICs at all - these are too expensive (maybe an NE602 might be cost effective and I'm not sure about a low cost IC for the RX audio).
  • TX-RX offset essential - so that listening on the actual TX frequency is possible
  • Does not have to be pretty - the aim is a transceiver that can be thrown in a rucksack and survive reasonable abuse levels.
  • Wire antenna - a simple dipole or end fed half wave.
  • At least 2W RF out, ideally 5W - so that the rig has a reasonably good chance of being heard on a busy band
Is anyone else up for this challenge? The target is a really credible HF transceiver with decent, not compromised, performance that can be made with all new parts for less than £20.

GQRP club sales offer some very good prices for components if you are a GQRP Club member. If you are not a member then you are missing out on the very best magazine (SPRAT) available. I cannot recommend SPRAT and the GQRP club highly enough.


threeme3 said...

Very inspiring article, I am up for this challenge. Maybe it is interesting to allow a 0.80 GNP attiny85 microcontroller that acts as pll, CW keyed, ssb modulator, AF band filter and class-d AF PWM driver? BS170 will do good as few watt driver. Cannot wait to see first drafts. 73 guido pe1nnz

Pedro Colla said...

Any such circuit with the AT Tiny85 available? TU Pedro/LU7HZ

threeme3 said...

Hi Pedro / Roger,
The best I have now is an C code that runs on my RaspberryPi that can directly generate SSB signals on GPIO4 just by microphone input. The algorithm determines the phase and amplitude of the microphone signal and then applies accordingly frequency changes to the RaspberryPi PLL that replicates the same phase, the amplitude is replicated by changing the GPIO drive-strength but could also be reject and then you have a SSB signal with continuous carrier and still sounds ok.
Maybe the same idea can be put in a tiny micro controller, together with some FIR filter, AGC, speech compression and other DSP code that makes it a decent 20 GBP transceiver.
73, Guido

Pedro Colla said...

Any way to have a hand into your source code? We're working on an extremely small SDR based SSB (sort of a SSB Pixie) here but driven with a PC. It won't be out of the question to device something far cheaper like a AT Tiny or even a Raspberry PI (still cheaper and less energy hungry than a PC). Pedro/LU7HZ

Anonymous said...

The BITX20 looks like what you describe. 73 Ron PA2RF

threeme3 said...
This comment has been removed by the author.
threeme3 said...

Hi Pedro,

For your interest, here is more information about the SSB experiment: http://pe1nnz.nl.eu.org/2013/05/direct-ssb-generation-on-pll.html

73, Guido