Crystal Radio Receivers.

[D Cassidy]

Hello.
This is my first "technical" post so please be gentle with me if it's a bit wobbly. I've always thought about improving the crystal set's performance and wondered if a full wave detector or bridge type would work.
Has anyone tried this?
Regards.
D Cassidy.

[Radio Wrangler]

No, not tried, but diodes have initial turn-on voltages, below which you won't get any output. Having a bridge doubles this, so that's not an improvement.

Doing a full-wave rectifier using the tuned circuit with an overwind to get the reverse phase would work, but for a reasonable Q tuned circuit, the half-wave rectifier is no worse. Think of the tank as a flywheel.

David

[G8HQP Dave]

You have to define what you mean by 'improve'. More audio from a strong signal? Ability to hear weaker signals? Better selectivity?

More audio from strong signals probably means heavier loading of the tuned circuit, so you extract more power.

Ability to hear weaker signals probably means lighter loading of the tuned circuit, so you reach the diode threshold voltage more easily. Alternatively, add a little DC bias.

Better selectivity probably means lighter loading of the tuned circuit, both by the detector and the antenna.

[wireful3]

I have seen several articles about using full wave and bridge rectifiers in crystal sets. Though I have not tried it myself the general impression is that there is no improvement.

I did experiment with crystal sets years ago and found "The Xtal Set Society" a US web group, had some very helpful content. The essence seems to be very high Q coils and correct matching between all the stages. In fact my biggest improvement was gained by using sensitive phones and a transformer arrangement to match them to the crystal.

[D Cassidy]

Thank you both for your most helpful comments.
I remember making Short Wave Crystal sets when I was a lad, I suppose I was lucky then to have been raised on a farm, my long wire aerial was long. I must admit I'd like the best of both worlds, no doubt not easy especially when the hearing isn't so good!

[terrybull]

I recall seeing a crystal set design many years ago that used the rectified signal to power an amplifier employing SB transistors to amplify the audable signal. Needed to be close to a big transmitter. It may have used a separate coil tuned to the close transmitter but I don't recall the details.
I also have a small pamphlet which has about 12 designs for crystal sets claiming high sensitivity, selectivity and a sw design. I'll dig it out if you're interested.

[David G4EBT]

The main limitation of crystal sets in lack of selectivity rather than sensitivity. It didn't matter so much in their heyday as there were so few stations, but nowadays, you'll receive several stations, though all on top of one another. Improving the sensitivity only makes matters worse.

Some time ago, out of curiosity I built the 1959 BBC 'Focus' radio - basically a crystal set with a transistor amplifier stage. Hopelessly lacking in selectivity - half a dozen stations on MW, but just a cacophony of sound, all on top of one another. Not sure if a directional MW 'DX' frame aerial would improve matters, but a long wire certainly wouldn't. (Is there such a thing as a 'Q Multiplier' circuit for crystal sets to improve their selectivity I wonder?).

I still have the 'Boys' Book of Crystal Sets' by Babani Press and made several of the designs in the mid 1950s in my early teens.

(A bygone era, when there was such a thing as 'boyhood').

Happy days!

[G8HQP Dave]

You could add a Q-multiplier to a crystal set, but it seems like cheating unless the Q-multiplier is powered from the antenna alone. I suppose you could make a broadband arrangement with an efficient rectifier to harvest as much power as possible from the antenna to feed the Q-multiplier supply rail, then notch out the one signal you want to listen to and feed that to the audio detector.

[Herald1360]

Have a look at the stateof the art when nothing else was available:

http://www.sparkmuseum.com/MT.HTM

[Kala_12]

Originally Posted by David G4EBT

I still have the 'Boys' Book of Crystal Sets' by Babani Press and made several of the designs in the mid 1950s in my early teens.

I still have a book called "The Boy's Book of Crystal Sets" with practical circuits. Mine however is by W.J.May published by Bernards Radio Manuals first published in March 1954, (this is the Twelfth impression in October 1960). There is a section on soldering, aerials, earth, headphones and crystals.
There are twelve designs mostly using home wound coils.
Like most of us over a certain age, I spent many hours listening into the snall hours.
Stuart.

[Herald1360]

http://www.vintageradio.me.uk/radcon...rc/circuit.jpg

This works quite well.

[D Cassidy]

Hello.
While my roof was undergoing repairs I have now completed a new long wire aerial project. It runs from the apex of the roof to a pole at the extreme end of the garden, just under 100m. Later this week I will run a feeder to the workshop that is built into the house. I hope that doing the improvement will boost the efficiency of my crystal sets. Not sure though the best way of feeding the long wire to the workshop. Coaxial cable or a balun? What is the feeling here.

[G8HQP Dave]

A better antenna will feed a stronger signal to the crystal set. I suppose you could call this an efficiency boost, although it is the signal which is better not the receiver.

A 100m antenna could present a high or low impedance, depending on what frequency you are receiving. If low, then a coax cable would do. If high, then the capacitance of the cable will lose much of the signal. It could be argued that 100m is too long for MW reception as its impedance will vary so much over the band. For best results you will need an ATU. You don't need a balun, as nothing is balanced.

You also need to think about earth, which forms the other half of the antenna.

[Peter.N.]

I tried everything in the '40s/'50s and the arrival of germanium diodes was the best leap forward. A diode with the lowest possible switch on voltage is what you need.

Peter

Best to feed both aerial and earth (big stick in the ground) to your workshop, then you can play with various matching schemes without going outside. Is the workshop upstairs or down?

[Herald1360]

An ATU, several hi Q tuned circuits in the set and a 1N34 or OA47 diode will give the best you can get. It'll be a bd to tune but will deliver good results when it is.

It'll also look remarkably like professional early '20s kit, circuit wise.

[Ian - G4JQT]

A Schottky diode (aka hot carrier diode) is the modern equivalent of the germanium diode, but with even less forward voltage drop, and faster too.

Although as diode in a crystal set (a not very demanding task!), I haven't noticed a significant difference between one and a good germanium diode.

[G6Tanuki]

Originally Posted by Peter.N.

I tried everything in the '40s/'50s and the arrival of germanium diodes was the best leap forward.

Look at "Gold-Bonded" diodes - OA47 etc.

They have a really worthwhile forward voltage-threshold.

[ms660]

Another take:

http://www.arrl.org/files/file/Techn...pdf/culter.pdf

Lawrence.

[ms660]

I like the vari couple tech in this one, nice litz wire coils too:

https://www.youtube.com/watch?v=c-vtvxskcP0

Lawrence.