Can the transmit and receive functions of a boat anchor transceiver share the tank circuit?

Question

I'm starting design work on a two-tube (two-valve, for those who prefer that term) reflex-regen receiver that I hope will cover AM broadcast through 160m-10m -- both amateur bands and those "in between" spectrum segments including SW broadcast bands.

Longer term, I'd like to build this into a transceiver, with at least CW transmit and possibly a later upgrade to AM or even SSB.

Part of the definition of a transceiver is that parts count is kept under control by sharing parts of the circuits -- the audio amplifier in a receiver might be reconnected as the voice modulator for AM transmission, for instance, almost always the antenna is shared, and in older handy-talkies (especially the "toy" variety) the speaker becomes the microphone.

What I'm wondering is whether it's practical, without sacrificing desirable qualities like stability, to make the receiver's tuning tank circuit also act as the tuning section of the VFO for transmit. It seems so, at first glance -- we've got a tuned network already set to resonance at the selected frequency; at most we might want (for CW, especially) to offset the transmitter by a small amount, easily done by switching in a low-value capacitance.

Most of the transceiver schematics I've looked at, however, were either very simple circuits that really only share the power supply and antenna, are crystal controlled without the ability to genuinely tune either receive or transmit, or are so complex I can't be sure I'm following the functions correctly (ex. the 22 tube Heathkit SB-102).

So, is it practical or advisable to expect to use the same tuned circuit to control both receive and transmit frequency in a VFO-controlled transceiver?

Answer 1

You can use the tuning circut of the regen and, to be exact, the regen stage as a VFO for your transciever! Couple it to the separator amp with a 100pF cap between separator grid and the to of VFO tuning circut. Keying can be achieved by the key turning a relay on, the relay's contacts being connected to separator's cathode resistor and ground with a cap in the range of 100nF to 10nF for smooth keying in parralel. Another contact of relay switches some component to turn regen on to max - we could really use the schematic here. The third relay contact switches the antenna between the reciever input and power amp output. I suggest using two relays, fir the first we contacts you can use DPDT relay (two switches that switch between two contacts) or DPST (two switches that have only one contact), for the antenna switch you can use a SPDT. You can use the heater supply for powering them, the antenna relay should be faster than the first relay so the power output tube doesn't transmit into a open circut, if you can, use small relays here since smaller relay = faster relay. The separator and power output tube can be one of those triode-power pentode combos. Use a П network to tune tube to antenna and high quality mica capacitor between anode and П network. Remember to test VFO stability and screen each stage to separate them, especially the power supply and power amplifier.

Answer 2

That idea may be okay for a toy, but it is probably bad for a practical transceiver.

Some reasons are more regulatory. Regen receivers radiate power into the antenna unless there is a separate RF amp stage. Most tube-era regen receivers omitted that. Also, the transmitted spectrum must meet -43dBc harmonic level for amateur stations in the US, and that standard is usually much tighter in other countries. Most tube-era QRP transmitters fail on that unless you add a filter.

Regen receivers are made to oscillate very very weakly, and when there is a strong signal (like the signal from your own transmission) the oscillation frequency moves. So, the bias and the loadline need to change when you switch transmit/receive. However, when you do that, the oscillation frequency also changes. There is no good way to synchronize them unless you calibrate them at each operating frequency. That's a UI nightmare.

There is also a greater risk of inadvertently transmitting off-band. When those simple tube VFOs were commonly used and those articles were published, everything was laid back, but that is not the case anymore.

Designing and building a stable LC oscillator requires a lot of skill and it's not easy. The know-how is not fully described in any book or article and takes a lot of experience. I would recommend that beginners use a crystal-controlled transmitter.