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Old 27th Dec 2016, 5:21 pm   #1
clay shooter
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Default Home brew aviation band receiver using valves.

Can any of our experts out there recommended where I can find the schematic for a valve based aviation band receiver, there appears to be a lot of stuff for transistor /ic based sets, but not a lot for thermionic valve based sets.
Thank you in anticipation and I hope you all have a great new year
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Old 27th Dec 2016, 6:55 pm   #2
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Default Re: Home brew aviation band receiver using valves.

I made a valved airband set just to see whether it could be done but I cheated a bit by using the front end from a scrap airband ground station receiver I had. This is crystal controlled one channel only but probably could be converted to multichannel if required. The 10.7MHz IF output from the front end fed a simple one stage first IF amp, which then was converted to the second IF of 455KHz using a conventional mixer, again crystal controlled. The rest of the circuit was just what you would find in any old common or garden AM domestic receiver. Obviously a mute circuit would be handy, maybe consisting of a twin triode, perhaps driving a signal meter from the AGC line. My receiver performed well and was quiet enough not to need a mute circuit. The performance probably falls short of a professional receiver but it's good enough for general listening. It can be done on a budget. Fortunately I had most of the parts required already, which was the main reason for attempting the project. I don't have a complete circuit unfortunately, but I think it would be possible to use parts of circuits from other sets, and cobble them together. I often do this to make project designing easier.
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Old 27th Dec 2016, 8:26 pm   #3
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Default Re: Home brew aviation band receiver using valves.

Unless you go crystal-controlled (which limits you to specific channels) the big problem's going to be frequency-drift of the VHF local-oscillator.

You'll need double-conversion to get the necessary combination of image-rejection coupled with a narrow bandwidth for AM. Look at what the early-1960s-era VHF mobile radiotelephones [Pye Cambridge AM10 or Vanguard AM25] did here. Some of these used valve frontends and semiconductor 460KHz/audio-processing strips.

if you're persistent then you *could* perhaps split the signal at the second-IF and feed some of it to a FM-style discriminator then use the 'off-frequency' DC component from this to provide AFC to a free-running VHF local-oscillator.


Alternatively - buy a boxfull of crystals and do a "switch-tuned mixer synthesizer" for the local osc - that's the way the professionals did it in the 1960s. But it's gonna be horribly expensive.


Also remember that at 100+ MHz all pentode RF amps are noisy even if you use something like a 6AK5 which was the 'hot' VHF-band pentode (better than an EF50) in the early-1950s.

Better to go with a cascode [two halves of a ECC85?] or even a neutralised 6CW4 Nuvistor to keep the noise down.
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Old 27th Dec 2016, 10:08 pm   #4
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Default Re: Home brew aviation band receiver using valves.

Hi Gents, wasn't their an R1132A set available war surplus with a nice big dial and 6MHz IF.
I believe this was used for ground reception at some RAF stations and was variable tuning

Ed
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Old 27th Dec 2016, 10:51 pm   #5
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Default Re: Home brew aviation band receiver using valves.

Other than using the Pye RT sets, it was "standard practice" on the 2m amateur band to build a separate converter and feed the output of that into the ubiquitous HRO, AR88 or similar HF Rx, which may explain why designs for complete VHF receivers are less common.

I vaguely recall designs for simple VHF TRF designs, using Acorn valves, but nothing more elaborate.

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Old 27th Dec 2016, 11:05 pm   #6
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Default Re: Home brew aviation band receiver using valves.

Yes the 32A was 100-124 Megs Ed. Often converted to 2 metres/144 Megs!
Might be relatively rare now?

Dave
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Old 27th Dec 2016, 11:13 pm   #7
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Default Re: Home brew aviation band receiver using valves.

The Eddystone S770 series of valved VHF communications receivers covering 17 to 165 MHz was launched in 1953. Some of the circuit techniques could be relevant to your project. Circuits and manuals can be obtained from the Eddystone User Group website at http://eddystoneusergroup.org.uk/
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Old 28th Dec 2016, 12:10 am   #8
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Default Re: Home brew aviation band receiver using valves.

I have an 1132A on the to do pile. It's been there since about 1984. They are quite large beasts designed for 19" rack mount and need a separate PSU. They do have a nice big variable dial. Not sure how stable they were. If they were designed for fixed ground station working, and permanently powered, they may be quite stable, but to switch it on and off in a domestic environment may reveal local oscillator stability problems. Don't let that put you off though.
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Old 28th Dec 2016, 4:47 pm   #9
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Default Re: Home brew aviation band receiver using valves.

Bear in mind that 100KHz (or possibly even more?) channel spacing was used in the heyday of valve receivers, even the early transistorised ones. You'll need to have pretty exceptional stability to receive modern transmissions without some form of synthesis.
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Old 1st Jan 2017, 2:12 am   #10
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Default Re: Home brew aviation band receiver using valves.

That's why a crystal LO for a broadband downconverter in front of a standard SW RX used as a tunable IF was the usual way to go on 2m back in the days. Works just as well for Airband and a valve converter is straight forward enough, even if noisier than solid state. Modding a band 3 TV front end might be an easy way to go...... or start from an early 2m design.
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Old 1st Jan 2017, 3:57 pm   #11
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Default Re: Home brew aviation band receiver using valves.

The RAF signals museum http://www.rafsignalsmuseum.org.uk/ has a Hallicrafters set to 120MHz (approx) and it is quite happy getting ground to air from Heathrow (I think).

And here is said wireless http://www.rafsignalsmuseum.org.uk/100_1989.jpg not behind a glass wall, you can smell it, and if you ask nicely, are allowed to play with it.

Last edited by Guest; 1st Jan 2017 at 4:06 pm. Reason: Photo of radio added
 
Old 2nd Jan 2017, 1:42 pm   #12
G8HQP Dave
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Default Re: Home brew aviation band receiver using valves.

You probably don't need narrow bandwidth as at any given location there will only be a smallish number of channels in use and these are likely to be spaced apart in frequency.

I would start from an FM receiver design, but use an AM IF at 10.7MHz with AGC. However, something better than the usual ECC85 self-oscillating mixer would be best. TV Band 3 would be a good place to look.
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