Monday, 10 June 2013
Saturday, 20 April 2013
Tuesday, 17 May 2011
For our 10 minute radio, we will need these parts:
- A ferrite loop antenna coil
In our other crystal radios we wound the coil by hand. In this project we use a much smaller coil with a ferrite rod inside, from our catalog. The ferrite rod allows the coil to be smaller, and it can be moved in and out of the coil for coarse tuning.
- A variable capacitor (30 to 160 picofarads)
We carry this in our catalog. You can also find them in old broken or discarded radios.
- A Germanium diode (1N34A)
We carry this in our catalog.
- A piezoelectric earphone
Also in our catalog.
- Two alligator jumper wires
We use alligator jumper wires here for convenience. They are used to connect the ground and antenna wires to a good ground and a long wire antenna. We carry these in our catalog.
- About 50 to 100 feet of stranded insulated wire for an antenna.
This is actually optional, since you can use a TV antenna or FM radio antenna by connecting our radio to one of the lead-in wires. But it's fun to throw your own wire up over a tree or on top of a house, and it makes the radio a little more portable.
- A block of wood or something similar for a base
Saturday, 14 May 2011
Here is a schematic sent to me by W1FB many years ago. It is very similar to a 6M two-stage preamp that he published in QST in the mid eighties. Doug really favored the grounded gate FET for narrow band preamps. His published work is replete with examples of them on just about every band. I built that amp and remember getting about 10 dB gain, which is all that I wanted for the 6M direct conversion receiver using a diode ring detector that I was building. The great feature of the amp is that it combines a band pass filter and preamp in one. I lost the original schematic that Doug sent me but was delighted to see that I made a bitmapped drawing of it on a floppy disk that was recently re-discovered when we were moving an old desk. The shield shown in the schematic was a small piece of grounded ,double sided PC board in which, I made a small chamfered hole in to pass the lead going to the T2 tap. The shield, along with very short component leads will help minimize parasitic oscillations. The T2 tap is 3 turns down from the end of the T2 main winding that connects to the variable capacitor. Doug specified T37-10 cores for the inductors, but I substituted T37-6 cores and used the same number of windings as specified for the T37-10 core inductors. It worked fine.
Foxhole radio receiver or Crystal receiver is a form of radio that does not operate on local oscillator, which makes it hard to be detected by other electronic device. One of the most interesting thing of Foxhole radio is that it could be operated without the use of batteries, as it is powered solely by the radio waves through its long wire antenna.
Foxhole radio was (supposedly) popular during World War II because it enabled the GI to receive radio broadcast in the middle of the war, particularly in France as the Germans has outlawed the use of radio by civilians, thus the American GI need to build their own receiver to receive broadcasts. Typical component of foxhole radio during those days are : a period razor blade (not the newer galvanized one), carbon (obtained from pencil) and some copper wire with woodblock or cardboard as its base.