Grounded grid VHF front-end

How about a tube/valve XO and a diode mixer to start?

A good HF receiver may have a low enough noise figure that atmospheric noise still dominates.

Good thinking but there are several snags with that system:

If the down-converter is at the aerial end of the feeder, the HF receiver is almost certain to suffer from strong HF signals picked up on the downlead. If the down-converter is adjacent to the HF receiver, there will be significant losses at VHF in the downlead, as the aerial needs to be mounted as high as possible.

If there is no amplifier ahead of the mixing diode, the local oscillator signal could be radiated by the aerial - especially if it happens to lie at a frequency where the dipole has another resonance or the dipole and downlead form a resonant system.

I was thinking in terms of the converter being right next to the aerial (the sleeve dipole has a 'cold' bottom end and could be joined directly onto the converter box). The HT and LT could be supplied either by a separate multi-core cable or by superimposing 40v A.C. at 50c/s on the co-ax and feeding it into the 200-220-240v tappings.of a mains transformer primary. The full primary winding would act as an auto-transformer to give 250v H.T. and the secondary could give 6.3v or

This is really ham territory so I don't think JL - with all due respect - will be able to assist you very much in this endeavour. However, there should be tons of info on this in one of the old ARRL handbooks. If you have any from the early 60s lying around it should be well worth a look through.

I was never interested in rag chewing, but signals is still signals.

I have read most of that sort of literature in the past and still have copies of most of it but don't remember this particular approach being used before - that was why I though it might make a good fun project. There are some grounded-grid circuits but they use triodes intended for the purpose. There are cascode circuits with double (and sometimes two single) triodes but, again, the triodes are intended for that purpose. The idea of using a bog-standard descendent of the ubiquitous EF50 for frequencies it wasn't supposed to cover - and making it do that adequately - appealed to me.

The only place I have come across anything like this is in Geoff Woodburn's design for the Eddystone Panoramic Display Unit, where a triode-strapped E180F is used as a grounded-grid untuned wideband front-end amplifier. I did copy that successfully with a ZTX450 as the wideband front end of a noise-measuring set that I designed; it gave very satisfactory results.

Indeed, but this is niche and there are so many fine points and trade-offs and gotchas that need to be factored in that only a dedicated VHF RF designer could assist here. For sure the best people here could come up with a workable design, but in practice it would stink for the above reasons. There's not a single person on this group today who can really add any value here. Ham group, Liz; ham group.

You do them a dis-service, there are some people here who can think with an open mind. Even if the ideas they come up with wouldn't work, trying to find out or explain why they wouldn't work is a good exercise and may lead to a better solution than just asking hams what they have done before.

I know a lot of receiver designers, and the older ones are all Hams.

Joe

HF receivers don’t have to have good noise performance because the atmosphere is so noisy, and AFAICT they usually don’t. Intermod is more of an issue.

The atmosphere is quieter above 100 MHz, though, so you care more about the Rx noise figure.

A mixer front end is going to have a noise figure of 6 dB or so, on account of the conversion loss, and that adds to the NF of the HF back end.

Some gain ahead of the mixer, and some more following the band select filter should help a lot. Don’t overdo it, of course.

Cheers

Phil Hobbs

Seems to me that the lowest noise voltage gain - no noise in fact - comes from a high-Q LC resonator. And that best drives a small capacitance like a grid.

Driving a cathode can be wideband, but a cathode looks like a low value resistor, a Q killer.

Tubes are noisy. There are MMICS with NFs below 1 dB.

All correct.

On 2m, a decent noise figure is worth achieving. It seems to me that at

On Sat, 09 Nov 2024 23:23:59 +0000, Dan Green snipped-for-privacy@hotmail.se wrote: [...]

Unfortunately there are a lot of people working with valves who aren't engineers and simply cobble together circuits based on myths, legends and 'golden ears'. The voices of the real engineers who understand valves are lost in a din of technobabble.

There are quiet valves (Nuvistors) and noisy transistors (many), engineering is a matter of chosing the right ones or using them in a way which minimises the noise or makes it less significant in the overall design.

Series-tuned input circuit.

Thanks, that was the path my thoughts were going down. A grounded grid stage on a 200v HT supply should have reasonable linearity according to the data sheet. I hope to get a voltage gain of x10 from it from it if the anode circuit can be designed to be around 2 kilohms at resonance. That should lift the signal above the noise of the mixer stage with a bit to spare but still give good immunity to overloading.

With one tuned circuit at the front and a slightly-overcoupled bandpass circuit to the mixer, the bandwidth over the band from 144 to 146 Mc/s should be fairly flat. Alternatively a bandpass troughline at the front and a single tuned anode load would give the same bandwidth and better out-of-band rejection ahead of the EF91.

OK, that's entirely your call. But I think you'll find the number of people here with expertise in VHF receiver design is zero and the proportion of those with an adequate familiarity with designing for toobz is a vanishingly small subset of that figure. :)

I'm sure Liz is aware there are far better modern alternatives out there, Jan. However, this is (I believe) a purely fun project - and I've been thinking for many years about doing a similar thing myself with tubes. In fact I've retained a substantial collection of the damn things for precisely this purpose. I always thought, 'one day I'll retire and I'll have time to build something using these valves and it'll be a blast.' Anyway, I retired 25 years ago and have even less time now for fun projects than I had when I was working. Such is life...

Dunno if you have any of the RSGB-published books by Pat Hawker (G3VA IIRC) but he had several designs for very effective antennas at VHF and UHF made from lengths of old coax. The emphasis was always on economical designs with old Pat. He grew up in the shadow of WW2 where you had to make do with what you found lying around in the rubble of bomb craters.

Elektor was one of the better hobby ones. I was also subscribed to Wireless World which was a bit more heavy-duty. No idea if any of these are still being published and can't be bothered to find out. There was also Everyday Electronics and Practical Electronics, too. I'm guessing they've all gone now since the kids just want to code it seems.