UHF Crystal Radio

[vu2nan-nandu]

A crystal radio was wired up to check whether it could receive VHF/UHF signals in the vicinity of the shack.

No signals were audible, either in the shack or on the shack roof, using a 500 mm whip antenna.

However, a weak digital signal was detected with the sound powered phones, while moving on the shack roof using a 250 mm whip. Position on the roof and direction in which the whip was pointed were quite critical.

It was presumed that the signals were from a nearby cellphone tower. A 3 Element Yagi Beam for 33 cm was made using a broken snare drum stick for the boom and 20 SWG bare copper wire for the elements.

The coil was wound integral with the driven element. The same germanium diode and tubular ceramic capacitor were used.

The signal was much stronger with this setup and the beam pointing North-East.

Regards,

Nandu.

[vu2nan-nandu]

Here's an update.

Later on, another Yagi Beam was rigged up with a second OA85 providing the return path for the audio instead of the coil.

Signal strength was as good on that one too.

Regards,

Nandu.

[vu2nan-nandu]

Another update.

.................A folded dipole version, mounted on a terminal block, followed.

It obviated the need for a coil / additional diode and worked quite well too.

Regards,

Nandu.

[Nickthedentist]

If nothing else, your wire-forming is amazing. How do you do it?

Nick.

[vu2nan-nandu]

Thanks, Nick!

I formed it by hand, using a pair of nose pliers without teeth and a mandrel.

Regards,

Nandu.

[Peter.N.]

Something I never dreamed of when I was making crystal sets, there was no UHF - or if there was I had never heard about it. Interesting experiment though, I didn't realise germanium transistors were good for those frequencies.

Peter

[vu2nan-nandu]

Thanks, OM Peter!

73,

Nandu.

[Hartley118]

By way of encouragement, most navigational radar receivers, working at multi-GHz frequencies, are basically amplified crystal sets, but the crystal detector is fed from a high gain highly directional antenna.

most navigational radar receivers, working at multi-GHz frequencies, are basically amplified crystal sets

I thought they where crystal mixers.

[vu2nan-nandu]

Originally Posted by Hartley118

By way of encouragement, most navigational radar receivers, working at multi-GHz frequencies, are basically amplified crystal sets, but the crystal detector is fed from a high gain highly directional antenna.

Thanks, Martin.

Regards,

Nandu.

[vu2nan-nandu]

Yet another update!

The terminal block arrangement made it convenient to replace the folded dipole with a full wave loop.

Likewise with an AWX antenna.

Results were as good as with the folded dipole.

Regards,

Nandu.

[peter.sables]

Well I do not know about UHF But this is my VHF.

PETE

[Hartley118]

Originally Posted by merlinmaxwell

I thought they where crystal mixers.

Well yes.....modern radar receivers usually have a microwave local oscillator to beat with the microwave input signal so that the main amplification is done at a sensible RF frequency. The amplifier is then fed straight from the crystal - it's usually a logarithmic amplifier to cope with the enormous dynamic range. It can still be regarded as an amplified crystal set because the mixer has no gain.

Martin

[vu2nan-nandu]

Originally Posted by peter.sables

Well I do not know about UHF But this is my VHF.

Good work, Peter.

Wish I could build one like that!

Regards,

Nandu.

[peter.sables]

Hi Nandu.

Not really worth building. It looks good and it works but only if you live within 50mts of the tower. Fun to build though.

Pete G4MRU

[vu2nan-nandu]

Hi OM Pete,

It's really worth it, if only for the fun of having another go another day!

73,

Nandu.

[peter.sables]

Hi Nandu.

The clever thing is though is that this crystal set it FM. That's worth thinking about

pete

[vu2nan-nandu]

Hi OM Pete,

Considering the enormity of the task, very few would have successfully achieved slope detection of an FM signal with a crystal radio.

You are one among them. Hat’s off to you!

‘Having another go’ was intended for a range greater than 50m.

73,

Nandu.

[vu2nan-nandu]

The latest update!

A second Yagi Beam was then rigged up, with another diode providing the return path for the audio, instead of the coil.
The signal strength was not as good. During trials, a chance shorting of the second diode gave a considerable increase in signal strength.
The diode and capacitor were hence discarded. Performance of this single-diode UHF crystal radio is quite good.

Regards,
Nandu.

[vu2nan-nandu]

Diode capacitance

Low capacitance diodes are a must for UHF crystal radios.

It follows that they are to be so wired that the capacitance between their leads is kept to a minimum.

This does not imply that the leads are to be cut to minimum length. In fact longer leads facilitate antenna-diode impedance matching.

Antenna-diode impedance matching

Stub matching is a must for maximizing the signal strength.

Long diode leads may be used to form the stub.

A diode length of λ/4 gives a proper match.

A circular stub is best for minimum capacitance between the leads.

Regards,

Nandu.