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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sat, 09 Nov 2024 12:21:41 -0800
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On Sat, 09 Nov 2024 20:02:05 +0000, 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 was never interested in rag chewing, but signals is still signals.