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Old August 16th 04, 09:16 AM
Roy Lewallen
 
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Jerry Martes wrote:

It might be that I'm wrong with my conclusion that when receiving, the
loss of received signal wont be appreciably worsened by VSWRs as high as,
say, 3:1, when the line atenuation is under a couple DB. But I dont think
I've been confused about the Line to Load Mismatch and VSWR.


As long as we make sure the readers understand we're talking about the
actual SWR on the transmission line, that's true for both transmitting
and receiving.

I recognize that the lowest VSWR and lowest line loss and the lowest noise
figure are all important.


If the VSWR and (matched) line loss are roughly within the bounds you
mentioned, the transmission line VSWR isn't really important, since the
extra loss it causes is generally negligible. However, as I somewhat
indirectly mentioned earlier, the match between the antenna and its load
determine the amount of signal that gets to the receiver. And as Ian has
mentioned, the receiver noise figure might be best when the receiver is
mismatched to the source impedance it sees. Further qualifying the
matter is the fact that at HF, losing signal when receiving due to
mismatch, line loss or SWR doesn't impact the signal/noise ratio unless
the attenuation is very great, since the dominant atmospheric noise is
attenuated by the same factor. I do realize, though, that the original
posting was directed toward VHF or above, where those things do matter.

Since I'm always working with systems that arent perfect, I dont get
concerned with low VSWR when working with receivers except when there is
need for phasing arrays. It is my understanding that the transmission line
loss isnt increased excessively when the line loss is under about 2 DB and
the VSWR is as high as 3:1, and that S-meter readings arent measureably
degraded when the receiver sees these signals thats not coming in with a 50
ohm internal impedance.

It occurred to me that the original poster of this "Phasing Harness" might
have some sophisticated need for phase or low mismatch. If the two antennas
being harnessed are only to maximize radiation toward the horizon, I'd
consider the question a simple one to answer. ie Feed them in phase with
any convenient equal lengths of line. If bandwidth is a factor, expect
sidelobes to vary with frequency.

But, my knowledge and experience is very limited, so any 'redirection' of
my thinking is appreciated.


Your thinking looks fine to me.

Roy Lewallen, W7EL