N1AW Homebrew: QRP transceiver for 80 meter c.w.

modified: 05 Jan 2006

[80 mtr QRP xcvr] I built this 80 meter transceiver in 1975, when I was still licensed as W1GSJ. In this view the cover has been removed and several subassemblies have been extracted for troubleshooting.

This rig was built almost entirely from scratch. The one exception is the audio amplifier, which was scavenged from a junked 1960s-vintage a.m. broadcast receiver. It is the largest of the circuit boards extended above the enclosure in the picture.

VFO and transmitter

The heart of the rig is the VFO. The physical construction is important. It is built around a high-quality variable capacitor with a 2:1 gear drive and a planetary reduction drive shaft, so the calibration for 180 degree rotation of the capacitor spreads across 360 degrees on the calibrated dial, and corresponds to 6 full turns of the tuning knob. This is the bandspread capacitor and it covers about 125 khz for a single setting of the bandset capacitor. The bandset capacitor is adjusted with a screwdriver, with access from the bottom of the cabinet. Most of the VFO circuitry is built on a piece of perf board that is rigidly fastened to the frame of the tuning capacitor, so oscillator components cannot move relative to each other even if the U-shaped cabinet is flexed. To further rigidify the oscillator circuitry a square frame of 1/8" thick aluminum surrounds the oscillator. The bandset capacitor is attached to this framework. The oscillator is very stable. A note about the physical construction techniques used was published in QST's Hints and Kinks column.

The VFO is a 3N128 FET, and runs at a low power level, using 9 volts from an integrated circuit regulator. The VFO circuit is a series-tuned Colpitts, its inductor is a toroid would on a tuning slug from a a junked a.m. radio i.f. transformer. It is followed by an FET buffer, the output of which supplies local oscillator injection for the direct conversion receiver. For transmitting a 2N3641 bipolar junction transistor buffer is the keyed stage, and drives the final amplifier, which uses three 2N3641s in parallel. The collectors are tapped down on a toroidal inductor which is tuned by a variable capacitor scrounged from a junked a.m. radio; this is followed by a pi network filter, with its output tuned by another junkbox variable capacitor. These capacitors were modified by using a hacksaw to make a screwdriver slot in the ends of their stubby shafts, so they can be adjusted through holes in the side of the enclosure.


As I try to describe this circuit thirty years later, I cannot remember why I designed it as I did. It was not the first solid state r.f. circuit I had made, but most of my previous homebrew experience had been with vacuum tube circuits and I suspect the output tuning came from this experience. In other cases, "design" is not really an appropriate term -- I used what I had on hand. That is almost certainly the case with my choice of an obscure integrated circuit, an LM703, as a mixer. I've never seen a description of another ham project using this device. Nevertheless, it works as a direct conversion mixer. The rest of the receiver portion is an op-amp active filter and the junkbox audio amplifier mentioned earlier.

Other features

In addition to the basic transmitter and receiver circuitry, there are several other features I added that made this rig quite a usable self contained system. The receiver input is taken from the collector tap on the power amplifier output tuned circuit, and no additional tuning is really necessary, but I added an extra tuned LC circuit with a front panel control so I can peak the input, or tune it off-peak to reduce overload from very strong signals. Another feature is an RIT (receiver incremental tuning) circuit that permits moving the receiving frequency slightly above or below the transmitting frequency. This is very helpful with a direct-conversion receiver. Yet another feature is a simple sidetone generater that injects a tone into the audio amplifier when the transmitter is keyed.

Metering and calibration

A final feature is metering -- the miniature 0-1 ma. meter is switched to perform several functions, two of which are somewhat unique. First I'll mention the ordinary functions: power supply voltage, regulated low voltage, and collector current. The input voltage measurement is convenient when using a battery supply. The low voltage measurement is not really necessary, but is useful when the battery is low, so I can be sure the voltage regulator is still doing its job. The collector current position allows me to tune the transmitter output for a current dip at resonance. This is not really necessary, however, there is another way to tune the power amplifier.

One of the unique functions of the meter is for tuning the transmitter for maximum power output and matching to the antenna. An Operate/Tune switch on the back of the rig switches the output of the power amplifier between a direct connection to the antenna connector and a connection through a classic resistive bridge circuit. The input side of the bridge is made of four 50 ohm half watt resistors in series-parallel, so with no antenna connected it provides a dummy load for the transmitter. The Tune position of the meter switch connects it to the diode detector for the bridge, and with no antenna connected the indication is proportional to the power output into this dummy load. So with no antenna, the Operate/Tune switch in the Tune position and the meter in its Tune position the output circuit of the transmitter can be adjusted for maximum power into the 50 ohm load. With the antenna connected the circuit functions as an SWR bridge; when the antenna and possibly an external antenna tuner present a 50 ohm load the bridge is balanced and the meter will show a null reading. Thus the final tuning can be adjusted off-the-air, and then the antenna can be connected and tuned for a match. The upper arm of the output side of the bridge is 50 ohms and capable of dissipating 1 watt, so when tuning with the bridge in the circuit the power actually reaching the antenna is reduced. And for any antenna impedance that can possibly exist the final transistors cannot see anything worse than a 2:1 mismatch during tuneup. When the antenna has been matched the Operate/Tune switch can be moved to the Operate position for full output to the antenna. This circuitry was also described in QST Hints and Kinks.

The other special meter function is receiver calibration. On the back of the enclosure is a place to plug in a crystal. The crystal connects the VFO output to a diode detector whose relative output is measured when the meter switch is in the Calibrate position. I set the bandspread dial to the crystal frequency and tune the bandset capacitor for a peak on the meter. I keep two crystals in FT243 holders, left over from my Novice and early General days, with the rig. They fit snugly in a plastic 35 mm film canister. A 3.598 Mhz crystal provides a calibration point for setting the bandset capacitor so the main dial covers 3.5 to 3.615 Mhz. A 3.735 Mhz crystal calibrates the 3.61 to 3.76 Mhz range. I also used this rig as the VFO for some experiments with a homebrew phasing-type SSB generator, and the 3.735 Mhz crystal also calibrates a the rig for 3.735 to about 3.92 Mhz.


Using it!

In the summer of 1975, when my son was one year old, I brought this rig along on a trip to Seattle, Washington. One purpose of the trip was to conclude the sale of the house my wife and I had bought when we were graduate students at the University of Washington. Because of a depressed real estate market we had rented out the house, rather than try to sell it, when we moved from Seattle to Massachusetts in 1972. I had operated from this location as W7FHK when we lived there, but I had left no antennas or other ham equipment there, and my operation during the 1975 trip was much like a camping trip, with an improvised antenna. For that matter, the entire trip was like camping, as there was little furniture in the house, and we slept in sleeping bags on the living room floor. I did not, however, try to bring the radio along on the backpacking trips we made to Mt. Rainier and the Cascades. With a one year-old child and associated supplies on my back extra weight was definitely not an option.

This rig was also carried to Florida for use during a vacation trip by one of the students in the Hampshire College Amateur Radio Club, of which I was a trustee, and I believe other members used it on other occasions.


Two notes based on this project were published in the QST "Hints and Kinks" column.

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