On 12/13/2013 3:32 PM, kr2q@optimum.net wrote:
Contributor B says, "What if the impedance at the antenna is only 33 ohms....
and you have a quarter wavelength of 75 coax." Then...Oh horrors.
Well, don't look now, but if the feedpoint is 33 and the coax is 50, I
calculate that to be the
same VSWR as if the feedpoint were 50 and the coax were 75. No? Nobody
worries about
the former, do they?
Let me clarify this. If you have a 50 ohm transmitter and use 50 ohm
coax to drive a 75 ohm load, the SWR is indeed 1.5:1. OTOH, if you
have a 50 ohm transmitter and use 75 ohm coax to drive a 50 ohm load,
the transmission line mismatch is 1.5:1, but the SWR as far as the
50 ohm transmitter is concerned is 2.25:1 in the worst case of an odd
multiple of 1/4 wl. Thus a load mismatch is not as bad as a coax
impedance mismatch.
How likely is it to get your coax to be in odd multiples of quarter lambda? I am
guessing, "Not too likely."
And if it is, well, just add some more coax (if you care about SWR). Would
that not work?
If you are driving a multiband antenna, it is actually
quite likely to have near a 1/4 wl on some frequency especially
if you have a few hundred feet of coax.
Remember, the original poster asked "does it matter"?
In general it might matter, but you might be able to work
around it. But you shouldn't just assume it doesn't
matter at all and ignore it. The one case where it
doesn't matter is when the coax is close to a multiple
of a half wave. Everyone knows that principle.
I highly recommend that, if you are able to implement it.
Also to clarify some questions about directional couplers:
the concept would appear to be associated with
traveling waves in transmission lines, but
it is possible to make audio directional couplers
out of ordinary transformers, such as in legacy
phone patches. Everything is essentially of
zero size relative to a wavelength. You can
also make a directional coupler entirely from resistors.
The bandwidth of such a coupler is essentially
"DC to daylight" if you use very small resistors.
"directivity" is just a mathematical concept.
Rick N6RK
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