[TenTec] Signal Strength

[Jim Brown K9YC]

On Fri, 11 Jan 2008 17:16:22 CET, shristov at ptt.yu wrote:

>Receiver's input impedance is irrelevant for sensitivity (and noise figure)
>measurements, which are defined in terms of the "available power" from the
>generator. The generator output impedance should be close to 50 Ohms.
>Receiver input impedance can be very different from 50 Ohms.

No, the input impedance is NOT irrelevant -- it loads the generator and 
reduces the voltage at the receiver input. The difference between a 50 ohm 
load and no load is 6 dB. A lower impedance than 50 ohms would cause an error 
in the opposite direction. 

On the other hand, it is not uncommon for the input impedance of a receiver 
to be significantly different from the "assumed" or "nominal" 50 ohms. To 
measure this, you would need to measure the voltage of the generator using a 
bridging (high Z) RF voltmeter or scope with and without the receiver 
connected, and do so in a manner (and at frequencies) where transmission line 
effects were not contributing errors. 

73,

Jim Brown K9YC