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Path: ...!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sat, 09 Nov 2024 21:48:10 +0000 From: Joe Gwinn <joegwinn@comcast.net> Newsgroups: sci.electronics.design Subject: Re: Grounded grid VHF front-end Date: Sat, 09 Nov 2024 16:48:10 -0500 Message-ID: <l1mvij9unnvfkmers68g81jk5oi90nm951@4ax.com> References: <1r2rj8l.msi28f14weovyN%liz@poppyrecords.invalid.invalid> <725vijtq4c4jj21uavvjevu3a9npum08jp@4ax.com> <1r2rp4o.1w2tcwvw8pjuoN%liz@poppyrecords.invalid.invalid> <47fvijhj8g018ps9unh419o8enmslja5m9@4ax.com> <l0hvijtdrqo2997g0lf1bkncpmmlj0rv8n@4ax.com> <8ajvij1nnu2h3arj7719ftja07vbiq50on@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 99 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-wAuwzCUfugHb5tdLTygcrg4reJ+wE7+niOBsOEc45X9IkqUBHwrgm9BaH395tt4nXulyZVSDljwpAq/!iq12UROzqcAVS4lcBxDW2xp/vmbUbKvwdfs2ipMbAqWQ4zXO6J0CMgi21RrspZI4AwMuyts= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 6591 On Sat, 09 Nov 2024 21:03:12 +0000, 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. I know a lot of receiver designers, and the older ones are all Hams. Joe