Alphabetical Index

DXing with Crystal Sets

Just what can be achieved with a handful of parts and NO power supply?


What is your best DX reception of a radio amateur or broadcast station when listening on a crystal set?

With SSB, it is much harder to make out ham callsigns with just an envelope detector, but there are some colossal signals on 21, 28 and 50MHz when conditions are good (e.g. in a strong sporadic-E opening) which might just be identifiable at range on a crystal set. E-mail me any crystal set DX results you may have had, or if you've used any particularly effective circuits. I've had some modest success with the small circuit below.

G3XBM's lower HF bands crystal set

What follows must be the simplest receiver you can build that will hear broadcast, utility and, with patience and some luck, a few radio amateur stations !

This uses a red Amidon T50-2 inductor wound with 34 turns of 0.5mm enamelled wire with a 1 turn antenna coupling. The diode was an AA144 (germanium), which I happened to have in my junk box. Other germanium diodes will probably do as will a hot carrier diode. The earpiece is a typical high impedance crystal "deaf aid" type. C1 is an airspaced variable capacitor salvaged from an old Howes ATU.  The antenna consists of 15m of wire from the guttering to a post at the bottom of the garden and the earth is just the central heating radiator in the bedroom. I have not experimented with the 470k across the crystal earpiece. Other values may be better. The same circuit can be used with different toroids and windings for all bands from LW to 30MHz or more.

With this circuit I can usually hear around 10-20 signals at night between 3 and 20MHz. Best amateur band reception (80m) so far is G2ABR at 45 miles and M0BXT at 2 miles (2 way QSO!). There are plenty of broadcast band signals that can be received although most if not all heard recently have originated from relays in Europe e.g. Radio Taiwan International just above 80m. Also heard was a mysterious raspy sounding CW station around 5.2MHz. It seemed to be sending 5 letter number groups. A numbers station?

This is a graph showing the measured sensitivity (by ear) using a calibrated HP8640B signal generator. It is remarkably sensitive on the LF end of the tuning range, but about 5-6dB less sensitive higher up the HF range. No doubt this could be improved by better antenna and diode tapping into the tuned circuit.

This is a lively circuit and it is hard to believe that it uses no power except that  from the stations being received. Performance can be optimised by choosing a diode with a low forward voltage drop and adjusting the antenna matching winding for the best compromise between selectivity and sensitivity.

Some recent developments have been the use of special zero gate bias MOSFETs as sensitive crystal sets (no power applied). See the QST article.

HEADPHONES

A lot will depend on the sensitivity of the earpiece or headphones used with any crystal set. Best results seem to be achieved with old balanced armature types such as the DLR5 if suitably matched. With some headsets sensitivities can be quite remarkably good.




Transcontinental DX Reception

Yes, it can be done. During the 1990 sunspot maximum I had some fun with a very similar shortwave crystal set using a T50-2 (yellow) toroid, a tuning capacitor, a germanium diode and a crystal earpiece. The results were astounding for something so simple. The design and the results obtained were written up in an article in
Short Wave Magazine, December 1991.

Best broadcast band DX heard included Radio Australia, Radio Havana in Cuba and All India Radio (all received from their originating countries and not via relays) - not bad when considering all the power to drive the earpiece comes from the transmitter!

With crystal sets it is a case of being extra-patient waiting for fades to bring up more distant stations. I can remember waiting for a station in the Middle East to end its transmission just in time to hear the famous "Waltzing Matilda" sound from Radio Australia. Occasionally though one can have armchair reception from stations thousands of miles away. One thing to watch is that many shortwave  broadcasters have transmitters located nearer their intended audience. The signals heard from Radio Australia, Radio Havana and All India Radio came from their respective countries however. It was also possible to hear some of the nearer coast radio stations in Europe which were still using CW then.

The sensitivity of a good crystal set is about 500µV for minimum discernable signal. A big issue is lack of selectivity. Various improvements are possible using full wave bridge detectors and antenna matching circuits together with the use of high Q tuned circuits optimally coupled to the detector and antenna. See also the recent MOSFET crystal set design in QST.

I'm sure there is a lot more that can be done to improve "free power" radios e.g. by using the rectified energy from broadcast stations which is normally rejected by the receiver selectivity. One idea I've seen, but never tried, is rectification of 50 or 60Hz hum "fog" (using widely spaced ground rods) to provide DC power for a simple amplifier. Another is to use a super-cap to store energy from a strong broadcast station to power a micro-power HF or MF active receiver.

Broadcast Band DXing

Roger Lapthorn G3XBM www.g3xbm.co.uk
Heard the following during the 1991 sunspot peak (and some earlier) using a T50-2 toroid inductor,single diode, design and longwire antenna:

    1991 sunspot cycle
    Radio Havana Cuba
    Radio Australia
    All India Radio
    + ....most of Europe

    In earlier times
    Radio National Brazil
    VOA Monrovia

    Remember that many stations have relays nearer their intended target areas. All of the above were received from their original countries.

Roger Love K2HLI, Glen Gardner, NJ
When 10 years old{1942} I started of with crystal sets made with a razor blade detector like the GI fox hole radio.  In short order I made a one tube regenerative receiver which gave me big signals from around the world.  After hearing the results of short waves I wound coils for 3 to 30 MHz for my crystal set.  First station heard within 30 seconds of connecting the ant. was the BBC! After that it was my long distance record of the Belgian Congo, Leopoldville
stood for many years.


Ham Band DXing

A photo of a fine toroid based DX crystal set from Brian N4DKD

Brian N4DKD www.n4dkd.com
I use my crystal set as a receiver on our AM modulation net on 3885 in the southeast U.S. I have copied as many as 15 stations, averaging 70 miles, the best dx was 250 miles N4VMY.

Roger Love  K2HLI, Glen Gardner, NJ

Roger used to hear W8RHZ in Twinsburg ,Ohio on a regular basis on 75 meter phone when he lived in West Orange, NJ at the time.

Roger Lapthorn G3XBM www.g3xbm.co.uk
As mentioned above I have recently worked M0BXT cross band using my 4 component crystal set to receive his 80m AM signals from 2 miles away. Also recently heard by envelope detection was G2ABR who was 45 miles away. He was using SSB but I was able to confirm reception by later listening on my FT817 in the other ear and matching the envelope detected signal with the SSB signal in the other ear. This way I was 100% confident that he was the station heard. He was just barely audible most of the time but for a brief period he was S4-5.  I also used to hear G4PJ at 4 miles away on 160m with my single diode medium wave crystal set back in the 1960s.