On Tue, 25 Jul 2006 15:38:04 +0200, Helmut Wabnig
*_.-_- wrote:

Does anybody know the input impedance of his receivers in use?

Hi Helmut,

Stand-alone receivers may differ, but in transceivers, the receive
path, up to a point, is identical to the transmit path. As such, a 50
Ohm design is universal. Now, if there is any divergence from that
value, you must expect that signal power is being reflected away from
the detector. Certainly, any high Z active amplifier may be used, but
in an untuned front end, this is hardly a virtue.

Stand-alone receivers, like the R-390, can have higher Z inputs with
tuned front ends. The R-390 includes what is called an antenna
trimmer to further optimize the input. It also has two distinct
antenna inputs, balanced and unbalanced.

How would you measure the input impedance?

This takes a special signal generator with a known output Z. You
simply place a series variable resistor in line, and watch the
receiver's signal meter as you vary the resistance until half the
voltage is observed. The input Z is the sum of the signal generator's
output Z and the variable resistor's value. Take warning that a
variable resistor's value is going to be the next difficult thing to
determine. It is not simply a matter of measuring its value (or
rotational angle) with a VOM. However, absolute accuracy is rarely
needed in this regard and a VOM reading will be suitably close.

73's
Richard Clark, KB7QHC