Jerry-- at your stated 1.5 dB loss coax, with a 3/1 vswr, the extra loss
would be just under .7dB, but remember that the reflected mismatch, also
will affect your recievers front end noise (factot/figure), most likely
in a negative way (no way to calculate the impedence presented to your
rcvr front end ) also, remember that fm sensitivity (because of its
nature) is not near as sensitive as cw/ssb (typical 2 meter rcvr can
detect a signal below .01 microvolt cw - fm rcvr does good around .1
microvolt- 20 dB more sensitive! Something to think about-- jim NN7K

Jerry Martes wrote:

"Jim - NN7K" wrote in message
.. .


Jerry, just because you are wanting to recieve, instead of transmit,
doesn't mean that the V.S.W.R. mismatch isn't there-- nor its effect
on the subsequent coax loss's being multiplied because of the mismatch!
Remember- the antenna , coax, and the mismatch are all bi-directional !
The additional loss in the feedline, because of a high swr, is also
present in the recieve direction! If you have copy of ARRL handbook, or
other sources, look under "transmission lines" - these have a chart of
coax loss, and also the swr effect on that coax loss.
For example, coax with 4 dB loss, at swr of 1.5 /1 would raise the line
loss by an extra .1 dB, while an swr of 3:1 would rais it by an extra
dB, at 5:1, would add about 2.2dB loss! and- keep in mind this is
BI-DIRECTIONAL loss ! Another consideration is that the noise figure is
optimized for 50 ohm, and this (because of a mismatch), will also be
negatively affected! Best to find good impedence match for all
considerations-- Jim NN7K


Jerry Martes wrote:

SNIP

In addition, I have assumed that the impedance mismatch for receiving
antennas can be allowed to be much greater than for transmitting. This
statement is directed to the fact that transmission line loss is

increased

by VSWR, and the reciever isnt restricted to being the same impedance of

the

transmission line.

Jerry



Jim

I sure wouldnt argue with your findings. I guess this identifies my
sloppy engineering. I didnt think 4 Db was a tolerable feed line loss
under any but the most dire circumstances. I wonder what a ~3:1 VSWR would
do to add attenuation to feed line loss os, say 1 1/2 db.
To further identify my sloppyness, I cant distinguish zero db from 0.1 db.
It is even difficult for me to determine with any confidance the difference
between Zero db and 1 db.
What I'm getting at is -- I still contend that a receiver can tolerate
some mismatches that arent tolerable in a sophisticated transmitting
station. Richard Fry has pointed out how critical small mismatches are in
FM transmitters. This original post referanced a FM transmitting antenna.
It would be reasonable (to me) to assume this referanced antenna could
have some rigid constraints if it was to be used for transmitting stereo FM.
But, as a omniazimuth FM receiver antenna, the phasing might be fairly
straightforward.

Jerry