Crystal sets

It is some time since I built a crystal set, but these are great fun and very easy to build.  Remember you are using the TX station for all power. There are no DC supplies in a crystal set.

Most designs are still for Medium Wave reception, but I had most fun with short wave designs based around T50-2 (red) or T50-6 (yellow) toroids which allow small hi-Q coils to be wound. Larger T68 or larger diameters are also suitable.

There are PLENTY of resources on the internet (just Google "crystal set" ) but a typical one is:
http://www.mds975.co.uk/Content/crystalsets2.html .  Never assume crystal sets are useless: I have copied stations (not via relays) directly from all over the world including All India Radio, Radio Australia and Cuba.  The short wave design  used just a handful of parts. Signal levels will be in 500uV to 2mV region. A good crystal set design can detect down to about 400uV or less, but a lot will depend on your earpiece(s) and your ears. One has to ride QSB as some distant stations appear to be co-channel with less distant stations.

It would be fun to build a multi-band (LW, MW, SW and  VHF) design some day. There are even designs for Band II FM using slope detection. They should work well on VHF aircraft band which is still AM with local airports and planes. I have never seen an aircraft-band design. You can always add a small audio amp, but this rather defeats the purpose of a crystal set.

See also my own website about crystal sets :https://sites.google.com/site/g3xbmqrp3/hf/crystalset .

Crystal Set DX log from 1991

As I clear out my stuff ready for our move, I've discovered my Crystal Set DX Log from 1991, which was 2 sunspot cycle peaks ago. Looking through the entries in the log (all received with a VERY simple shortwave crystal set using a toroid, a germanium diode, a resistor and a crystal earpiece and a longwire antenna about 15-20m long) it is quite astounding how the HF world has changed.

Shortwave Crystal Set DX log - from 1991

For a start, many of the coast radio stations used to transmit in CW on HF and these could often be read due to the cross-modulation from other stations. This was the technique used to detect those shown in the log above.

Also, the shortwave bands were still filled with English language broadcasts. In the few days covered by this log I managed to receive broadcast stations from Cuba, UAE, India and Australia directly (not via relays) as well as plenty of Europeans. Before this date I had no idea that worldwide reception on a crystal set would be possible.

There is no doubt that conditions were very good back in 1991 and I am not so sure that it would be possible to repeat this in 2013, but I would be delighted to hear of success by others with crystal set DXing on HF in more recent times.

I still enjoy crystal sets: they are simple and fun to make yet there is still something unbelievably good about using the electricity from the TXing station to hear them, not my own, especially when the signal comes from very far away.

Crystal set sensitivity experiments

This afternoon I had a little play with a simple crystal set to see what sort of minimum discernible signal (MDS) was possible. My previous experiments with single germanium detectors had yielded an MDS of around -55 to -57dBm in the 5-30MHz range with a high impedance crystal earpiece.

Today I tried the same test with both germanium diodes and a hot carrier diode (HP2835) with and without some bias applied to the diode(s). Best results were with the HP2835 and with about 0.11V forward bias applied. Only a few uA are needed so battery drain is next to nothing. MDS was around -62dBm for a well modulated AM signal. For the tuned circuit I used a T50-6 toroid with around 20 turns with an antenna link winding adjusted for best sensitivity/selectivity and a 365pF variable capacitor .

Incidentally, forward bias for the diode could be obtained by rectifying other signals available on the antenna such as MW/LW broadcasters using a separate tuned circuit, rectifier and reservoir capacitor. I must give this a try sometime. Several hundreds of mV DC should be possible.

Then I tried switching the diode with a second signal generator at a power of between 1-10mW to see what sort of sensitivity could be obtained on a CW signal. MDS improved by around 25dB to around -86dBm i.e. I could just hear down to around 10uV. This would be a just usable level as a basic HF SSB/CW receiver.

I also tried the same tests with a pair of high sensitivity ST-3 headphones that Michael AA1TJ kindly managed to find for me. These are much lower impedance than the crystal earpiece so I experimented with antenna match and the match of the diode to the tuned circuit, expecting a better performance at the best settings. Disappointingly, the results were several dB worse than with the crystal earpiece.

Now, the results with the crystal earpiece are very close to those (without bias) that I obtained some years ago, so I don't think my hearing (at 63 years old) has changed much for the worse. So, Michael must have incredibly sensitive ears as there is no way I was able to hear down to the levels he could with his passive receivers.

At the moment I am playing with the AM crystal set with -62dBm sensitivity listening to shortwave broadcasters. Fun.

Shortwave broadcast QSLs

Talking about QSL cards makes me think of my very first QSL cards received back in the 1960s. In those days I used the ISWL QSL bureau which handled broadcast band QSLs (I think it still does) and well remember the thrill when I got my very first cards. The first ever QSL was from Radio Nederland and it was an exciting moment. Somewhere I still have that red card with a windmill on!

In the subsequent years I did a fair bit of QSLing when using just my shortwave crystal set and got a few cards back from these reports. I heard stations all over the globe using that simple crystal set with best DX being Radio Havana Cuba, All India Radio and even Radio Australia, all direct and not via any relays. Not bad for just a tuned circuit, a diode a resistor and a crystal earpiece. I must have another go at a shortwave crystal set just for fun, especially as HF conditions are so good now, but these days shortwave broadcasting is not what it was. Using a decent toroid and a bridge detector it should be possible to arrive at a sensitive design with quite good selectivity.

Boy's Book of Crystal Sets

One of my favourite books when I first got interested in radio back in the 1960s was the Babani series book number 126 entitled The Boy's Book of Crystal Sets. The link takes you to a PDF copy of this book. I was fascinated by the shortwave version shown in the book and remember building a variety of it using parts I had. It was such fun copying distant DX stations with something so simple. Years later I managed to copy Radio Havana, All India Radio and even (directly from Australia) Radio Australia on a simple crystal set. All good simple fun showing the magic of radio. There are several websites on the internet today dealing with the design of sophisticated crystal sets capable of quite exceptional preformance.

Portable crystal set DXing (in 1922)

Wonderful silent movie from 1922. See http://www.britishpathe.com/record.php?id=2616st5

A crystal set transceiver?

JF1OZL has some fascinating ideas on his inspiring website. Typical of his wacky ideas is his 20uW TX for 7MHz powered only by the human voice bellowing into a mic. I wonder if a crystal set transceiver is possible using either the rectified human voice (as per JF1OZL) or rectified received RF to power a micropower TX working alongside the crystal set RX? Even a 0.5uW TX could cover miles on VHF line of sight with a beam and generating this sort of power should be possible through rectification of MW and lower HF broadcast stations.

DX Crystal sets

Jim's Crystal Radio Page at http://www.hobbytech.com/crystalradio/crystalradio.htm has some very neat crystal set designs with high performance receivers. These units also look like works of art as they are so elegant.

There are other excellent resources at Gollum's Crystal Receiver World http://www.oldradioworld.de/gollum/ .