Originally Posted by rabbit73
Thanks for the link to the DHC thread, and in particular to your links in post #716. I have looked at those references and they seem to focus on the SWR between the transmitter PA and antenna which can reduce the SNR/MER below the 27 dB required by the FCC for a transmitted signal.
I think it is a mistake to use the antenna reciprocity theorm to assume that what goes on the in receiver's feedline is equivalent to that in the transmitter's feedline. The signal that comes from the transmitting antenna is sinusoidal (analog.....like NTSC).
An increase in the SWR in the receiving feedline causes an additional lineloss that is of the same nature as the inherent coax loss at that frequency and is added to that loss. Both increase the system NF and cause a reduction in SNR which requires a stronger signal to maintain the minimum required SNR.
A multipath problem that shows on the spectrum analyzer as a ragged signal also requires a stronger signal to maintain the minimum SNR between the dips and the noise floor in order for the equalizer to compensate, if it can.OK, you've got me hooked, holl_ands.
What kind of a test can I devise to prove Dr. Bendov's theory?
Would it be sufficient to insert a section of coax in the feedline with an impedance much higher or lower than 75 ohms to give a SWR mismatch of 4 to 1, or is there a better way to do it?
Dr Bendov and I believe
in Reciprocity...but I understand your
reluctance to blindly accept anything without PROOF....
If you pursue the "wrong" impedance coax, it probably should be the SAME LENGTH
as the "correct" impedance coax to replicate the multipath bouncing back and forth.
I'm not so sure you're going to find coax with the requisite impedance....
You might compare to an antenna with a KNOWN BAD SWR, such as the way too
small U-TUBE 4-Bay Bowtie and see if there is "Excessive Noise Figure", which
would be fairly difficult to PROVE in an Apples vs Pommes de Terre (Potatoes)
antenna comparison: http://imageevent.com/holl_ands/multibay/4bay/utube
Alternatively, you could use a 1:1 Coax Balun instead of the usual 4:1 Balun
[could be Coax] with a known GOOD SWR antenna and see if the Noise Figure
degrades when a 300-ohm antenna is terminated in 75-ohms (4.0 SWR).
This would be a better Apples vs Candy Apples comparison....
But you'll need to use very weak stations to see NF degradation,
and you should conduct the test for multiple channels and perhaps also
perhaps multiple antennas to verify repeatability....