Saturday, October 30, 2010

Swing Set

Swing Set is a double-parametric, regenerative, 80m receiver. Despite the unusual circuity this is a classic regenerative receiver setup (Q-multiplier -> diode mixer -> AF amplifier). As shown above, a single RF signal source (I've been using my bench, RF signal generator) can be used to pump both the Q-multiplier and the AF amplifier. This tactic minimizes the overall parts-count at the cost of having to track a second RF resonator while tuning across the band.

A degenerate parametric oscillator produces the negative resistance required for proper Q-multiplier operation. A pair of varactors are parametrically pumped at twice the operating frequency with sufficient vigor to drive the input L-C tank circuit just over the threshold of instability. The result is much the same whether one employs a traditional three-terminal feedback amplifier, a tunnel diode or a parametric amplifier. In each case the total positive equivalent conductance appearing across a parallel resonant L-C tank circuit is partially canceled by the application of a negative conductance of nearly identical magnitude. The oscillating Q-multiplier produces high-gain at narrow-bandwidth while providing the necessary "beat-note" for proper CW or SSB reception.

The varactor-based audio frequency (AF) amplifier may be thought of as an RF modulator followed by a diode demodulator. Subsequent to the discovery of synthetic ferroelectric materials this circuit gained a brief popularity in research circles. Another burst of interest followed the introduction of semiconductor varactor diodes, such as I'm using in the above circuit. For best results the RF tank circuit in the AF amplifier is tuned to either side of peak resonance. In practice, one simply tunes this tank for maximum AF gain. So long as the varactors aren't driven into conduction the input impedance of this amplifier is extremely high; thus placing a negligible load upon the Q-multiplied resonator.

The power gain of a three-frequency, parametric upper-sideband up-converter (USBUC) is numerically limited to the output frequency divided by the signal frequency. The power gain of a three-frequency, parametric lower-sideband up-converter (LSBUC) is limited to the negative of this same ratio. Thus, the USBUC can produce modest, but stable gain, whereas the LSBUC can theoretically provide extremely high, but potentially unstable gain.

My varactor-based AF amplifier may also be thought of as a four-frequency parametric up-converter. These devices support both the upper and lower sideband outputs in order to combine the virtues of both the three-frequency USBUC and LSBUC, while avoiding their potential shortcomings. However, the output is an amplitude modulated RF signal which must be demodulated in order to reproduce an amplified version of the input waveform. Despite the loss associated with a demodulator the overall power gain can be substantial.

I've been using SMV1212's for all four varactors; mostly because that's what I have on-hand. The zero-bias capacitance per varactor is approximately 75pF. At a bias of 4v the capacitance drops to 9pF (with a Q of at least 150 @ 50MHz). I've also had luck in the past using certain common rectifiers (such as the 1N4002) in my HF parametric regenerative receiver experiments.

The AF amplifier resonator is simply tuned for maximum signal or noise. The Q-multiplier tuning takes a bit of practice. With the antenna connected, I initially adjust the Q-multiplier resonance (with the pump amplitude set to a fairly high value) until it pops into degenerate parametric oscillation. I gradually lower the pump amplitude while continually adjusting the tank resonance in order to maintain parametric oscillation. With the pump amplitude reduced to the lowest value that affords stable parametric oscillation, I further reduce the oscillation amplitude by slightly detuning the resonator to a lower frequency. As with any regenerative receiver, the aim is to set the oscillation to a value just above the onset of oscillation in order to maximize the gain and narrow the bandwidth.

If the AF amplifier squeggs, or is otherwise unstable, I lower the pump amplitude or raise the number of demodulator coupling turns on the RF resonator (or both). Stability is also increased (at the expense of gain) by raising the pump coupling capacitor value (I've used values from 5 to 15pF). It's important to use a low-loss RF resonator in the AF amplifier in order to achieve a high amplifier gain.

It's been fun listening to the 80m CW band with this receiver in the past two weeks. The Europeans are quite loud in the early and late evening hours. I've mentioned elsewhere how I enjoy hearing fellows operating vintage equipment. One evening I heard K8FN running an Eico 720 into an open-wire fed dipole through his Johnson Matchbox. On several nights I copied K4JJW putting out a lovely signal with his Johnson Ranger @ 40w into an inverted-L antenna.

The stronger stations provide armchair copy with the headphones resting on the bench. In fact, I had a perfect copy of W1AW one evening with no antenna connected. Speaking of which, it's necessary to attenuate the very strongest signals; especially as my varactors are unbiased.

1 November 2010
I popped in two pair of 1N4001 rectifiers in place of my SMV1212 varactors this evening. A bit of retuning rewarded me with 80m signals. The Q-multiplier appeared to be working much the same as before. However, the AF amplifier gain was considerably reduced. I suspect the dc/dv slope for the 1N4001 isn't as steep as with my SMV1212's. The Qu might be lower as well. Still, tooling around the 80m CW band with a 0-V-1 regenerative receiver made from four power supply rectifiers and my bench, RF signal generator is pretty cool!

Peter, DL3PB, suggested that I try LEDs for my varactors. He notes their higher forward-barrier voltage would be a virtue in non-biased receiver circuit. Secondly, they come equipped with a visual forward-bias indicator.

I briefly tried single, red, green and yellow LEDs last night, as well as paralleled pairs, as replacement varactors. While they didn't work as direct replacements for my SMV1212's (I couldn't pump the Q-multiplier over the threshold of instability), this was anything but a conclusive test. I'll need to make some measurements on the LEDs. Thanks again, Peter, for the excellent idea!

Thursday, October 28, 2010

A One-Tetrode CW Station

Hi Myrton,
The rig that you worked last night was the result of a chance find at the Deerfield, New Hampshire hamfest a few weeks ago. Dunno why, but I paid a buck for a neat-looking old vacuum tube. It was a 24A, made by RCA. Turns out, this is the direct successor of the UY-224; the first American-made, indirectly heated, screen-grid tube. My example bears the inscription, "Wards Airline" on the base. I suppose it was an OEM item for one of those radios. This tube was state-of-the-art in 1929 but pretty much obsolete by 1935.

The circuits used here are entirely run-of-the-mill. For example, I'm sure you could sketch the grid-leak/ticker-feedback regenerative receiver circuit in your sleep. The transmitter is a crystal-controlled, "Miller" oscillator. I initially tried 58Vdc on the transmitter plate (the maximum voltage obtainable with my bench power supply), but I could only tease out 4mW of power at 3.5MHz. I then placed a "wall wart" transformer back-to-back with a filament transformer to produce 150Vdc. A bridge rectifier made from 1N4007's is followed by a 47uF ripple filter. The transmitter output power increased to 50-60mW with the anode supply potential at 150Vdc.

This tube is clearly "played-out." In fact, my attempt to "rejuvenate" the thoriated-tungsten emitter only brought about a temporary increase in cathode emission. Still, the old girl had enough kick left in her for eight CW QSOs (please see list below).

I used three DIP-type, DPDT relays to switch the tube electrodes (grid, screen and anode) between the transmitter and the receiver circuitry. A hand-thrown toggle switch controls two of these relays. The third relay "keys" the RF output power between a 50 Ohm resistor and the antenna. I first tried keying the oscillator cathode return but it chirped like a Chickadee. Allowing the oscillator to free-run helped quite a lot on that account.

Some of my QRP pals have built roughly the same circuit, only using a junction field-effect-transistor (JFET) in place of my old vacuum tube. One complaint that I've heard about the JFET version is due to the temperature rise of the transistor while transmitting. Apparently, this results in an annoying frequency-drift once the set is switched back to "receive." However, my fire-bottle runs hot enough in receive to swamp any differential anode dissipation. Upon throwing the transmit/receive switch my receiver plops right back on frequency.

I had one more pleasant surprise on the initial shake-down of this circuit. Hams have had trouble spotting their transmit frequency on regenerative receivers since the days of Hiram Percy Maxim. A strong signal simply chokes the sensitive regenerative detector. I've read how some of the fellows would place a wash tub over their receiver. Others used a low-power auxiliary VFO as an aid for spotting their transmit frequency.

I've twice heard mention of a (1950's?) spotting technique for use with crystal controlled transmitters. The "trick" involved building the set such that the quartz crystal is very weakly coupled across the receiver antenna terminals. Tuning the receiver through the crystal's series-resonant frequency, a sharp "blip" was supposedly heard in the headphones. Well, that's exactly what I heard as I first tuned my 24A genny across the CW band! I quickly learned to tune to the low-frequency side of this marker. It works perfectly.

The receiver board is located in the foreground of the above photograph. The transmitter/relay board is in the middle. I couldn't find a five-pin tube socket at the hamfest, so I drilled some press-fit holes in the plywood and hard-wired the pins.

I borrowed a power supply from the office to energize the heater, as I didn't have anything readily available to generate 2.5V @ 1.75A. One could practically toast miniature marshmallows on this receiving tube. It uses nearly 4.4w of heater power in order to produce 60mW of RF output; horribly inefficient, but it certainly makes a warm and pleasant glow!

I'll be pulling this rig to pieces as the power supply has to go back to work today. I guess I've had my dollar's worth of fun! A couple of my contacts last night ran over a half-hour. It's amazing what you can do with half-a-hundred milliwatts when the band cooperates.


  1. W4AG, Hillsborough, NC, 1039km, 599/449
  2. N1UIY, Athol, MA, 172km, 599/549
  3. KD1JV, Randolph, NH, 130km, 569/549, "...insane set up you got there Mike, but it works!"
  4. N1GKE, Hope, RI, 278km, 589/579, 30 minute QSO; I peaked at S-9
  5. W4AG, Hillsborough, NC, 1039km, 599/559, Thanks for my best DX, Stan
  6. WA1JHV, Chepachet, RI, 257km, 579/559, 43 minute QSO; I peaked at S-7
  7. NY3A, Glen Rock, PA, 586km, 599/459
  8. KA2REY, Whitehall, NY, 78km, 599/559
Mike, AA1TJ

Tuesday, October 19, 2010

The Kingdom of Auschwitz

Tete de Jeune Fille, William Bouguereau, 1898

This passage has long, haunted me

"The Auschwitz hospital illustrates the central paradox. In theory, there was no reason why a death camp should have a hospital at all, and yet the one at Auschwitz grew to considerable size, with about sixty doctors and more than three hundred nurses. It had a surgical department and an operating theater, and special sections for infectious diseases, internal injuries and dentistry. The lord of this domain was Dr. Mengele, the chief physician at Birkenau, who labored long hours on testing and then killing captive twins in a futile effort to find new ways of increasing the German birth rate.

Olga Lengyel, the wife of a Romanian psychiatrist, served as a prisoner-nurse in Auschwitz and recalled after the war that Mengele once insisted on personally delivering the baby of a pregnant prisoner. 'I saw him take every precaution during the accouchement,' she wrote, 'watching to see that all aseptic principles were rigorously observed and that the umbilical cord was cut with care. Half an hour later he sent the mother and child to the crematory oven.'"

Otto Friedrich, The Kingdom of Auschwitz

Tuesday, October 12, 2010

Ameco AC-1

On Sunday evening I answered N1KK's 80m CQ with my one-transistor, Reggie transceiver. Ken was transmitting with an Ameco AC-1 and a folded-dipole antenna from Chepachet, Rhode Island. Wow, I haven't worked an AC-1 in more than 35 years; what a thrill! We exchanged 569 reports; great signals, considering his 5w output power provided easy copy in my gain-less receiver. He was hearing my 80mW transmitter nearly as well. We had a very pleasant QSO lasting some 40 minutes.

The AC-1 was my first amateur radio transmitter. I recall earning the money to buy the kit by cutting my neighbor's lawns with an old cast iron, reel-type, push-mower. Having sent off my order, I kept up a daily vigil for the UPS truck for the next several weeks. It felt like Christmas Day when it finally arrived. Soldering iron in-hand, I followed the instructions carefully. When I thought I had finished, I plugged in my 7187KHz crystal and attempted to "load" a light bulb borrowed from my mother's oven. Only, it didn't work until I had retraced nearly every wire and found my mistake. I remember pressing the key and happily watching the light bulb suddenly illuminate. I had my first radio transmitter.

I must have been about 14 years-old in the above photo. My 40m station consisted of: the AC-1 transmitter, a Realistic DX-120 shortwave receiver, a brass-plated, Radio Shack straight key and a dipole, majestically flying at 15 feet (well, at the end supports). Nevertheless, I made upwards of a thousand contacts with this setup. Mexico was my best DX.

After my contact with N1KK on Sunday I listened a bit with my 80m tunnel-diode regenerative receiver; the one described in my September 30 blog. At 0330 I heard ZS1JX with a very sturdy "S-5" signal working stations from his Worcester, South Africa QTH. Well done, Peter!

Some recent QSOs of note while using my Reggie on 80m:

I worked KB2FEL (559/339) in October's Spartan Sprint. Bob wrote later,

Hi Mike, You had a good signal here in WV. I gave you a call about 0219 but you did not hear me. I was using my FT817 at 1w into a g5rv up about 55ft. I had full attenuation on to try to reduce the noise. I was happy you hear you call at 0137. When I heard you send 80 mw I knew that band was in good shape. The QSB was a bit fast and then at 0223 the band died. Thanks for the Contact es 73 Bob KB2FEL

West Virginia to Vermont (750km) with his 1W and no receiver gain on this end is pretty cool!

And speaking of "cool," last week I worked WA1IIE, in Gardiner, Maine with the Reggie. Chuck was doing very well with 5w to a dipole. However, when he returned for his next-to-last transmission, his signal had dropped somewhat; still a solid copy, just a bit weaker. Chuck surprised me with the news that he had dropped his output power to 100mW and wanted to know if I might still be copying him. Of course, my response was "QRP PSE"! Unfortunately, he had no way of further reducing his transmitter power. Rats! I've no doubt that I could have copied Chuck at 50mW; maybe less. There was no QRM, the QRN was low and there were no Ravens or Bard Owls calling above the Hobbit Hole hatchway :o)

A few night later I worked K1IEE on his Heathkit, HW-9 at 4w. Conditions were not so good on this night. The first time I worked Dick's HW-9 was with my 2N110 transceiver. On that evening my log shows his signal peaked at S-7!

A few weeks ago I worked WU2D in Manchester, New Hampshire. Mike was running his WW2-era, ARC-5 transmitter at 25watts. He had a good copy on my 80mW signal with his Collins R390A, but switched over to his homebrew, two-tube regenerative set. Mike gave me a 579 report on his little Genny.

The next evening I heard Mike's ARC-5 in action again; this time I was listening with heterodyned crystal radio made from a chip of iron-pyrite beneath a cat's whisker made from a bent safety-pin. Again, I had a perfect copy of Mike's signal. The heterodyne oscillator was provided by a 1N3712 tunnel diode. The iron-pyrite was a gift from Seab, AA1MY. It originally came from the Carleton Talc Mine located in Chester, Vermont.