[Dept. of Computer Science]

18. Jampot Ontvanger

The `Marmelade Jar' Receiver

Our Department and University.
See my other radios too; here is the next one.

The Jampot ontvanger (Marmelade Jar receiver) is an easy-to-build crystal set design from the forties. A crystal set contains no tubes or transistors, and no signal amplification is used in any form. This means that it requires a strong antenna signal (20m longwire) to receive just the strongest local stations.

This Jampot Ontvanger uses scrap material and a home brew coil, was built in an evening (April 5th, 1995) and it works! At first I thought the performance (it receives three to four stations) was meager but it isn't. I hear from other people that their crystal sets never work in the first place, and I later tried a `high quality' crystal receiver using better parts, but it didn't work either. Gollum reports his design receives ten stations, but he uses an additional battery.

To build this crystal receiver yourself you need some parts easily saved from a scrap radio: a tuning condensator, a diode, a condensator (1-10 nF), connectors. The coil is made by winding isolated wire on a toilet roll or similar object; the handwork is easy, but to determine the number of winds some formulas must be used: I found 105 winds (length 8.6 cm) to be appropriate (with antenna tap halfway). This gives you the desired inductance to tune the Medium Wave band (550-1500 kHz roughly). Enclosing it in a glass jar is easy (though I didn't find any way to fix the coil, it just dangles) and cheap, and you can always see what is inside.

Here is the schematics I used originally; later I connected the diode to the tap rather than the top of the coil and this gives a better result.

These are the necessary parts:

L  - Tuning coil.  Use 105 turns of thin wire
     on a toilet roll.  The antenna tap is
     approximately half way (at circa 50 turns).
C1 - Tuning condenser.  Value is circa 400pF max.
C2 - Condenser, value is 1000 - 10,000 pF.
D  - Detector diode.  A Germanium type should
     give the best results.
The headset: I could hear stations using a cheap modern device, a walkman earphone set; its performance can be improved slightly by rewiring it to have the two parts in series. However, this still doesn't compare to these old-fashioned headsets with high impedance. I get the best results with a 5000 Ohm headset I once bought in Prague.

The areal: use 20-30 meters of random wire and ground the set. With a 20m aerial I hear about 3-4 stations.

Gerard Tel, gerard@cs.uu.nl