[AMRadio] Coax Loss versus SWR


Jim candela jcandela at prodigy.net
Mon Mar 12 18:09:01 EST 2007




-----Original Message-----
From: D. Chester, K4KYV
The tolerable level of SWR depends on  the feedline used.  With ordinary
coax, I would try to avoid anything over 3:1.  The problem caused by high
SWR is dielectric loss, plus the possibility of exceeding the rf voltage
reading at high voltage points, and overheating marginally sized cable at
high current points.


Reply by Jim, WD5JKO

   Donn, everything you say is true. I will take a differing point of view
for a specific exception. What if the coax to be used is very short?
By short, I mean like 6-12 feet long like you might have in a mobile, or at
home where an outside balun resides.

Consider the case where you have a 100 watt class rig feeding a ATU, and
then 10' of coax to a legal limit rated balun. The balun can handle
considerable reactance, and miss match since it is being used at 1/15th the
power rating. The feedline SWR might go upwards to 10:1, and yet the
transmitter is happy, and the losses are low, so therefore the radiated
signal will be fine. The loss from the coax as calculated from this web page
is:

http://www.ocarc.ca/coax.htm

.12 db at 3.880 for Belden 9913 / RG-8 @ 10:1 SWR

The remaining question is the efficiency of the balun. If the load is
primarily resistive (= or > balun ratings), then I bet the balun will behave
pretty well. Now take this scenerio and add a linear amplifier, and all hell
will break out!!!

Regards,
Jim
JKO
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