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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sun, 10 Nov 2024 18:14:33 +0000
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On Sun, 10 Nov 2024 10:09:22 -0800, john larkin <JL@gct.com> wrote:

>On Sun, 10 Nov 2024 16:03:25 +0000, liz@poppyrecords.invalid.invalid
>(Liz Tuddenham) wrote:
>
>>john larkin <JL@gct.com> wrote:
>>
>>> On Sun, 10 Nov 2024 08:00:17 +0000, liz@poppyrecords.invalid.invalid
>>> (Liz Tuddenham) wrote:
>>> 
>>> >john larkin <JL@gct.com> wrote:
>>> >
>>> >> On Sat, 9 Nov 2024 20:26:18 +0000, liz@poppyrecords.invalid.invalid
>>> >> (Liz Tuddenham) wrote:
>>> >> 
>>> >> >Cursitor Doom <cd@notformail.com> wrote:
>>> >> >
>>> >> >> On Sat, 9 Nov 2024 19:27:13 +0000, liz@poppyrecords.invalid.invalid
>>> >> >> (Liz Tuddenham) wrote:
>>> >> >> 
>>> >> >> >john larkin <JL@gct.com> wrote:
>>> >> >> >
>>> >> >> >> On Sat, 9 Nov 2024 16:35:45 +0000, liz@poppyrecords.invalid.invalid
>>> >> >> >> (Liz Tuddenham) wrote:
>>> >> >> >> 
>>> >> >> >> >My current receiving aerial system is very inefficient at 2 metres
>>> >> >> >> >(144 Mc/s) and I have thought about making a sleeve dipole for that
>>> >> >> >> >band.  My VHF receiver is an Eddystone 770R, which covers the band
>>> >> >> >> >but only in a small portion of the whole scale.  While I am
>>> >> >> >> >improvomg the aerial system, I could also make a crystal-controlled
>>> >> >> >> >down-converter, that would allow me to use an HF communications
>>> >> >> >> >receiver or the lower ranges of the 770R, so that the  band 2 Mc/s
>>> >> >> >> >wide would cover a much greater scale length.
>>> >> >> >> >
>>> >> >> >> >It's been a few years since I designed anything with valves, so I
>>> >> >> >> >thought I might have a go at making a down-converter using valves -
>>> >> >> >> >but not necessarily the expensive 'cult' ones which everyone seems
>>> >> >> >> >to regard as having magical powers.  The EF91 is plentiful and
>>> >> >> >> >cheap as New Old Stock, so that seems like a good valve to start
>>> >> >> >> >playing about with.
>>> >> >> >> >
>>> >> >> >> >The EF91 was used as an RF amplifier in the input stages of
>>> >> >> >> >television sets working at about 45 Mc/s, so it can't have too bad
>>> >> >> >> >a noise figure (although Mullard don't quote one in their data
>>> >> >> >> >sheet).  If I triode-strapped it and ran it in grounded grid mode,
>>> >> >> >> >that would reduce the noise and increase the maximum frequency it
>>> >> >> >> >could usefully amplify. From the data sheet, with 200v on anode and
>>> >> >> >> >grid 2 and an anode current of 6mA, the gm is about 6mA/V, which
>>> >> >> >> >gives an input impedance at the cathode of  160 ohms.  A 75-ohm
>>> >> >> >> >feeder could be matched to this with a Pi tank or by tapping the L
>>> >> >> >> >or the C of an input tumed circuit.
>>> >> >> >> >
>>> >> >> >> >The voltage gain may not be as high in this configuration as in
>>> >> >> >> >grounded cathode mode, but it allows the valve to be triode
>>> >> >> >> >strapped for low noise without instability problems or the
>>> >> >> >> >dependence on neutralising that a cascode stage would have
>>> >> >> >> >(especially the need for correct neutralising to obtain the best
>>> >> >> >> >noise figure).  If I also use an EF91 as a mixer, I might need one
>>> >> >> >> >more stage of RF gain to get the signal up to a level where the
>>> >> >> >> >mixer noise is negligible - but this isn't such a bad thing because
>>> >> >> >> >it would allow extra tuned circuits to give better image rejection
>>> >> >> >> >and allow a lower output frquency if I wanted one.
>>> >> >> >> >
>>> >> >> >> >Anyone with experience of doing something like this with valves?
>>> >> >> >> 
>>> >> >> >> 
>>> >> >> >> 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 12.6v to run the heaters.
>>> >> >> 
>>> >> >> 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 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.
>>> >> 
>>> >> 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.
>>> >
>>> >Series-tuned input circuit.
>>> 
>>> Or a parallel tank with taps. The Q killers are the radiation
>>> resistance of the antenna and, a little bit, the ohmic component of
>>> the grid impedance from electrons being ornery.
>>
>>The point I was making about grounded-grid operation is that the input
>>impedance of the valve is very nearly the characteristic impedance of
>>the co-ax (voltage ratio 3:2 for a triode-strapped EF91 drawing 6mA from
>>a 200V HT line).   A Pi network or a 3:2 winding on a ferrite core could
>>be used to match them
>
>A tuned circuit into the grid has voltage gain, but the grounded-grid
>with ohmic matched impedance throws away at least half the available
>signal voltage. Impedance matching isn't good when it throws away
>signal.
>
>It's the voltage difference between the grid and cathode that gets
>amplified against the tube's inherent noise.
>
>Of course you can never get a better s/n than what the antenna
>provides, and that will be pretty bad, so working hard to get a very
>low noise fig in a HF receiver is entertaining but not terribly
>useful.
>
>At some wavelengths, in the microwave, looking at things way overhead
>with a very directional antenna, low noise figures are worth the
>hassle. The effective temperature of the universe is low.
>
>A very directional antenna is a big win on s/n. It ignores a lot of
>junk. I don't think it improves the inherent thermal background if
>it's receiving terrestrial transmitters. That would violate COE.
>
>I wonder if one can tell the difference in thermal noise by aiming an
>antenna north or south from the USA or Europe. It's certainly less
>aiming up.
>
>I guess a good antenna feeding a matched resistive load will heat up
>the load; steal power from the BBC. Or aim up and cool it.

Under 5Mhz is where it gets particularly bad. 80m is often unusable.