[TowerTalk] RX input impedance

Ford Peterson ford@cmgate.com
Fri, 19 Jan 2001 11:00:30 -0600

More information and comments:

Brian wrote:

>I thought Autek cautioned against directly connecting the meter to the
>RX input terminals.  The signal level is quite high for this direct
>You got away with it this time....  Others may not be as lucky.

>73 de Brian/K3KO

I plugged the Autek into my spectrum analyser.  It's only -10dbm.  Hardly
enough to "burn out" the receiver...  If anything, a guys gotta be very
careful not to "bump" the key when making this measurement--or

Mike wrote:

>Unless your system noise is limited by your receivers internal thermal
>noise floor (usually not the case on 160 meters), the mismatch loss caused
>by the 220 ohm termination will probably be inconsequential. You can check
>for this by listening to the noise floor with and without your antenna
>If on the quietest night of the year, your noise floor still drops when you
>disconnect your most inefficient receive antenna, you have nothing to worry
>about (this indicates that atmospheric noise dominates in your receive
>73 de Mike,  W4EF

I have a shielded loop that is pretty dead.  When I disconnect the antenna
the noise drops to a very low level.  I have to turn the volume way up just
to hear the hiss when no antenna is connected.

Tom wrote:

>Not quite everything.  The test must also be done with the
>narrowest filters to be used. The receiver should be terminated in a
>dummy load equal to the the antenna impedance presented at the
>antenna terminal of the receiver in the test.
>73, Tom W8JI

I don't claim to be a rocket scientist.  I don't understand how any filter
in an IF will change the input impedance at the antenna port.  Just to
satisfy my thinking, I checked it.  There is no difference in the RX input
impedance no matter how I have the filters set or the mode set.  Perhaps I
misunderstood what you were talking about...

I've got the schematic (all 6 "c" sized prints--oh boy) on the IC746 so I'll
study the input circuit to see what's going on.  My suspicion is that the
auto tuner is performing some wild transforms and no matter what I do to the
signal presented, it's going to match to the transmitter output anyway.  It
sounds like about 2 cups of coffee and a major "bonding" experience with the
schematics is in order...

Thanks for all the input (no pun intended) on RX input...


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