Path: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: Grounded grid VHF front-end Date: Sat, 09 Nov 2024 12:21:41 -0800 Organization: A noiseless patient Spider Lines: 84 Message-ID: References: <1r2rj8l.msi28f14weovyN%liz@poppyrecords.invalid.invalid> <725vijtq4c4jj21uavvjevu3a9npum08jp@4ax.com> <1r2rp4o.1w2tcwvw8pjuoN%liz@poppyrecords.invalid.invalid> <47fvijhj8g018ps9unh419o8enmslja5m9@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Sat, 09 Nov 2024 21:21:42 +0100 (CET) Injection-Info: dont-email.me; posting-host="0b121c3243c9592d62c68c8069785ae2"; logging-data="4158103"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/CSlepJmyPAoPxQrvdgPCg" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:Nz7h4TJsM7lhizWgOY7hNxY0Pa0= Bytes: 5619 On Sat, 09 Nov 2024 20:02:05 +0000, Cursitor Doom wrote: >On Sat, 9 Nov 2024 19:27:13 +0000, liz@poppyrecords.invalid.invalid >(Liz Tuddenham) wrote: > >>john larkin 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.