im working on a simple homemade shortwave reciever from here: http://www.youtube.com/user/mailomail#p/u/5/z3_iwtgnRwc

got a question on the coil. can i substitute a smaller coil on a ferrite rod not only to make the coil physically smaller but to increase the Q? i asked him via email what value he had. i dont have it in front of me at the moment, but i wound a very small coil with the same uH rating. he also said its not critically important. am i on the right track or am i totally wrong? is there a rule of thumb as far as where to make the tap? one other question; the base of the transistor is going to be oscilating on the resonant frequency of the tank circuit, so that means that the output of the transistor is going to be at the same frequency. where or at what point does it get "converted" to a audible frequency?

 

i read that "
if the Q is high, it will make it really hard to zero in on one channel."

 

A smaller coil on a ferrite rod may or may not have better Q. The ferrite materials used for typical antenna bars typically work best for the medium wave and long-wave AM bands, that is from about 150kHz to 1.6MHz. Trying to use one at the higher frequencies used for short waves may not be so successful - you would just have to try it.

I don't know what the best position would be for the tapping on the coil. Why don't you make a coil with a number of tappings, and see which one works best?

The conversion from AM modulated carrier to audio takes place because of the non-linear response of the transistor. The transistor will be biased so that with no signal the collector current is quite small. When there is an RF signal present, the high-frequency signal causes slightly bigger upward swings of collector current than downward swings. The mean collector current will vary with the strength of the RF, so the collector current will thus have some content at the audio frequency.