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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sat, 9 Nov 2024 22:52:16 -0000 (UTC)
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Cursitor Doom <cd@notformail.com> wrote:
> On Sat, 09 Nov 2024 12:21:41 -0800, john larkin <JL@gct.com> wrote:
> 
>> 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.
> 
> 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.
> 

2 metres is pretty much DC nowadays anyhow. 

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 


-- 
D   r Philip C D Hobbs  Principal Consultant  Electro Optical Innovations
LLC / Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog
Electronics