Well,
0.1 mf will roll some audio, depending on the impedance of the circuit. The
rule of thumb is to have the reactance 0.1 of the circuit operating impedance
at the frequency you want to pass to ground. You can use less, and not affect
the audio as much, but bypass the RF, since the reactance varies with
frequency.
AT 1000 Hz, roughly audio band center, the 0.1 will still have 1592 ohms
reactance. No exactly a short circuit to audio, but virtually one to RF. A
0.01 will have 15,920 ohms to audio, and the circuit impedance will dictate
the best choice. At least 0.001 was used in tube days. At the lower
impedance of solid state circuits, you do have to consider not loading a lower
Z situation. Some of the older Ten TEc equipment did have circuits suited for
tube type high impedance dynamic mikes, while the later Scout type audio
circuit works with the 500 ohm electret elements of mikes of today. 200 pf
could be tried, and you then see if it sufficiently couples the RF to ground.
73, Stuart K5KVH
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|