The homebrew 20M QRP CW transceiver was designed and built in 1984. It has recently been refurbished and upgraded as described in this article.
Background and Design Activities In 1984
Motorcycle touring, camping, and ham radio are activities I’ve immensely enjoyed. It only seemed natural to want to combine them. Unfortunately, most commercial ham radio equipment available in the early 1980’s was rather large, particularly for packing on a motorcycle where space is at a premium. The equipment was also power-hungry, with typical receive current consumption of an amp or more. Battery operation for extended periods was not practical with such equipment. (Elecraft would not come into existence for another fourteen years.)
A review of my ham radio logbook revealed the majority of my operating activity over the previous few years was on the 20 meter band, and most of it was CW (Morse code.) The concept of a small, rugged, low-power (QRP) CW transceiver exclusively for the 20 meter band began to take shape. The following design goals were identified:
- Continuous tuning over the lower 100 kHz of the band using a VFO to set the desired operating frequency (not rock-bound with crystals.) Operating frequency displayed directly from a vernier dial.
- Transceive operation, not independent receiver and transmitter.
- Low power consumption intended for 12 volt battery operation.
- Rugged packaging and small size.
- Nominal five watt transmitter output power.
- Full break-in keying (QSK), such that received signals can be heard between transmitted dots and dashes.
- Solid state transmit-receive switching without relays.
- Built-in electronic keyer to automatically form dots and dashes.
- Sensitive superheterodyne receiver (not direct conversion) with six poles of IF crystal filtering, nominal 400 Hz bandwidth.
Full scale development of this transceiver commenced in the summer of 1984 in my spare time. By the year’s end it was assembled and tested. All design goals were met.
Use from 1985 to early 1990’s
The little transceiver was an absolute joy to operate. It accompanied me on numerous motorcycle camping trips, including the 1990 trip to Alaska.
It was backpacked to the summit of Standing Indian Mountain in NC for one Field Day. It was backpacked to near the start of the Appalachian Trail in GA for multiple Field Days. It was also used extensively at home and performed very well in a number of stateside and DX contests. This was one fun radio!
But my work and other commitments reached a point where I had very little time for ham radio and I became inactive. I offered the rig to my friend Dennis, N4NR, for a long-term loan, which he accepted.
Long-Term Loan from early 1990’s to 2014
Dennis subsequently put the radio through its paces, even more than I had done.
He used it on numerous camping and business trips both in the continental US and in Hawaii, including from condos on Oahu and Kauai.
He operated Field Days with it. He used it mobile. He used it from his home as well.
Dennis is shown operating the transceiver from a black sand beach on Hawaii on the cover of this edition of the ARRL’s Low Power Communication: The Art and Science of Qrp publication.
Dennis still operates quite a bit, but only CW and mostly QRP with his Elecraft K1.
My ham radio activity had resumed in the previous decade.
Given the current popularity of QRP equipment and operation, it occurred to me that others may be interested in the radio and how it was designed back in the day. So I decided to post this writeup.
Dennis graciously returned the radio and expressed thanks for its loan. He had taken very good care of it.
The transceiver was powered up and thoroughly tested with no degradation of performance found. This was pretty remarkable for a thirty-year-old rig.
A photo shoot was then performed.
Finally, these refurbishment/upgrade actions were taken:
- All electrolytic capacitors were replaced. Tantalum replacements were used except for two 220 uF capacitors (due to cost reasons.)
- The BFO was recalibrated to precisely match the IF crystal filter response and to provide a 600 Hz pitch on receive.
- The vernier dial was replaced as the original was nearly worn out. The VFO was recalibrated.
- The monaural headphone jack was replaced with a stereo jack to allow the use of stereo headphones.
- The Cinch power connector was replaced with a PowerPole connector.
- The manual key jack was replaced with a stereo jack. The receive audio was coupled to the ring terminal of this jack. With this arrangement a computer interface could be connected to the radio via this connector.
- A filter was added to further reduce hiss generated in the LM386 audio output stage. Hiss was not a problem prior to the modification but this made the receiver even quieter.
A number of contacts have been made post-refurbishment. The radio accompanied me to the 2014 Amish Rally as seen in this photo. For this particular trip the rig was keyed from a netbook computer using this USB Radio Interface.
I think the rig is now ready for another thirty years.
Schematic diagrams of the transceiver can be downloaded from the links below for both before and after refurbishment changes.
Schematic May 1987 before refurbishment.
Schematic August 2014 after refurbishment.
Homebrew QRP Transceiver Photographs
The following photographs were taken in 2014, but prior to refurbishment.
The following photographs were taken after refurbishment.
Circuit Design Notes
As the design came together and shortly thereafter I corresponded by mail with several members of the amateur radio community. Selected excerpts from my own writings during this information exchange are quoted below.
Receiver RF Amplifier and Mixer
“I found during breaboard testing that an RF amp on 20 meters was needed at times when the band is marginal… I was often able to work stations that were weak enough that copy would have been very difficult or not possible without the RF amp.”
“Following the RF amp is a trap tuned to the 6.551 MHz IF. It was designed-in as a preventive measure and possibly could have been eliminated. IF rejection has never been a problem, especially given the oddball IF chosen – little or no shortwave broadcast station activity is on that freq. Such was not the case with the 6.00 MHz IF originally selected (and subsequently abandoned!).”
“The mixer stage is a fairly conventional design. The 510 ohm drain resistor serves two purposes – it properly terminates the crystal ladder filter, and it limits the maximum mixer load impedance as a function of frequency … the xtal filter can appear as a high impedance load at some freqs…”
“A total of six poles of IF filtering was chosen to achieve good selectivity without letting things get too complicated. W7ZOI has written on several occasions about the desirability of having some filtering after the high gain IF stages to reduce the broadband IF noise bandwidth prior to detection. Accordingly, I placed two poles after the IF amp… the receiver is extremely quiet. Enough so that with no antenna connected, several users (myself included) have wondered if it were broken!”
“The crystal ladder filter sections were designed in accordance with W7ZOI’s classic May 1982 QST article. I ordered ten 6.5536 MHz microprocessor clock crystals from Digikey and selected the six most closely matched ones from the lot. Note that although the crystal oscillation frequency was specified as 6.5536 MHz, the series resonant frequency of the crystals measured somewhat lower.”
The overall IF frequency response was measured in 2014 and is shown below. The graph scales are 500 Hz per horizontal division and 10 dB per vertical division. The response peak is located at 6.551175 MHz.
AGC and Audio Stages
“I used audio-derived AGC as described in K1BQT’s April 1983 QST article and ‘Solid State Design for the Radio Amateur.’ If I were doing it all over again, I would still use audio-derived AGC but I would pick it off the top of the volume control instead of the AF amp output. Then the volume control setting would have little effect upon overall AGC action. On the downside, taking AGC from the volume control would probably require addition of an AGC amplifier stage. The diode limiter across the speaker output is a last minute compromise to protect my ears when using headphones and tuning across extremely strong stations…”
“The speaker is a small 1.5 inch square type from Mouser.”
“The VFO is built in a shielded enclosure fabricated from double-sided PC board stock. Tuning is very linear with no drift, chirp, jumping or other instabilities. An inexpensive 0-100 vernier dial drives the VFO tuning capacitor, and as the rig tunes the lower 100 KHz of the band the frequency can be read directly off the vernier dial.”
“… the oscillator was built using point-to-point wiring above the ground plane and the buffer was built on a small etched circuit board within the enclosure… The VFO coil was wound on a ceramic form with an air core.”
“The BFO frequency is shifted slightly when switching between receive and transmit. One minor side effect with the freq shifting scheme shown is a popping sound made in the receiver during keying… the amplitude of the BFO output changes… a step change… is coupled into the product detector, amplified, and makes its way to the AF output… The popping sound was brought down to a tolerable level by adding the diode on the FET gate and removing a pull-up resistor from the BFO shifter transistor collector… An audio muting circuit was considered but rejected due to insufficient board space.”
Transmit Mixer and Pre-Driver Amplifier
“The transmit mixer is disabled during receive by the mixer switch transistor. On transmit, the output from the mixer is amplified, filtered, and amplified further in the cascode pre-driver amplifier stage.”
Transmitter Driver and Power Amplifier
“The transmit driver and PA stages are adapted from W1FB’s November 1982 QST article. One exception is the TX keying circuit, a 2N3906 and 2N2905 Darlington pair. This pair is connected as an emitter follower stage. As the rig is keyed, the 0.047 capacitor on the 2N3906 base is charged and discharged, which causes the 2N3906 base voltage to rise and fall over a period of a few milliseconds. Since the 2N2905 emitter voltage tracks the 2N3906 base voltage, the driver stage builds to and decays from full output amplitude over a period of a few milliseconds. Hence, the output signal is free from key clicks…”
Keyer and Timing Control
“The 470K pull-up resistors were added to the Auto Key jack after Field Day. We were operating on a mountain top on the Appalachian Trail and a rainstorm started. Whenever a water droplet landed on the keyer paddle contacts the rig would go berserk and send code at random!”
- Hayward and DeMaw, Solid State Design for the Radio Amateur, 1977.
- Hayward, “A Unified Approach to the Design of Crystal Ladder Filters,” QST, May 1982, pp 21-27.
- Littlefield, “Construct an Audio Amplifier with AGC for Your Simple Receiver,” QST, April 1983, pp 28-31.
- Demaw, “The 8P6 Special – ‘Hamcation’ Backup Rig,” QST, November 1982, pp 17-21.