Hi Mike and Ford,
> 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.
Receiver input impedance can NOT be accurately measured with a
device that "overloads" the input of the receiver. Measurements
must be made with a signal level well below that point or the
results are meaningless for normal signal conditions.
> You can check for this by listening to the noise floor with and
> without your antenna connected. 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 system).
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.
> Do you have a schematic of the front end circuit? That may give you
> some clues as to whether 220 ohms is expected by design, or if perhaps
> something is broken or mistuned.
or being overdriven.....because that will change the impedance.
I've found receivers misdesigned, and I've corrected it, because with
directional low-efficiency receiving antennas and narrow-selectivity
my noise floor (listening on a quiet winter night) is in the -130 dBm
or lower range. I also use splitters to connect multiple receivers to
antennas, and the systems must be terminated correctly or one
receiver will "hog" signal.
Normally, as m Mike points out, this isn't a problem but there are
cases (like mine) where it is. Remember the noise power is for the
most part evenly distributed with frequency, and is directly
proportional to bandwidth in the receiver. Half the bandwidth will
provide half the noise power, and so if you make a test at 3 kHz
BW and one at 300 Hz the narrow receiver will see 10 dB less
external noise.
73, Tom W8JI
w8ji@contesting.com
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