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Amplifier for internal antennas

by Bruce Carter

The first figure shows the antenna input circuitry of a typical AM radio. There may be some slight variations on this, but the circuit described here will compensate for them.

 AM radio input circuitry before the modification

The author described assembling this circuit on a 1"*1" perf board, I actually laid out a small PC board with excellent results. In this era of surface mount components, I think a much smaller version can be laid out on a PC board, allowing this circuit to be put inside even the smallest radios, such as Walkmans. If I do such a board, I will put the PC board artwork here.

AM radio input circuitry after the modification
(click here for larger photo)


From the article: After the booster is built, open the radio and locate the antenna. Trace out the secondary winding (it does not go to the tuning capacitor!) and snip the leads. Connect these leads to the input of the booster. Run leads from the booster's output to where the antenna secondary went. You must finally connect up the power and ground and mount the board to be finished. When that is done, turn on the radio and tune in a weak station above 1400. Adjust the set's antenna trimmer and you are done. 

To the above I would add:
Any NPN transistor can be used. The author used a 2N3904, but a 2N2222A should work just as well. A good, low noise transistor would be even better.

Some radios only have three connections to their ferrite bar antenna: the antenna connection to the tuning capacitor, ground (two wires twisted together) and secondary (the smaller number of turns). In this case, treat the twisted wires as ground, and the secondary wire as the base connection to the transistor. The transistor will then receive its base bias from the radio.

R1 changes with the supply voltage. If your radio is 9V, use 56K. If it is 6V, use 47k. If it is 3V, use 33k. The goal is to bias the collector of the transistor to one half the supply voltage.

The schematic shown is for a negative ground radio. For a positive ground radio, subsitute a PNP transistor.

This page is based on the article: "AM Radio Booster" by Gary McClellan, Radio Electronics, April 1972, page 25.

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