I routinely measure at the back of the amp and as close to the antenna as is
easily possible. I don't usually climb the towers but measure at the base of
the tower knowing that there is additional loss going up the tower. Formulas
are interesting but actually measuring is more enlightening and accounts for
all the connectors, switches, etc. I usually use the highest antenna
frequency for the measurement. It can be depressing.
73, Larry W6NWS
-----Original Message-----
From: Jim Thomson
Sent: Monday, November 24, 2014 10:56 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Coax loss. Has anybody measured it ?
Has anybody actually measured the loss on their coax from back of amp to top
of tower, for various freqs if a multiband yagi used..... or what ever band
your monobander is? I mean using accurate watt meters at both ends?
That above data could then be compared to the online loss calculators..like
the times microwave one in the below url...... or your favourite method of
measuring coax losses.
http://www.timesmicrowave.com/calculator/?productId=139&frequency=10&runLength=90&mode=calculate#form
The only time I have seen this done was back in the 80s. A buddy ran 1kw
into the long run of 213U coax, 300 feet.... and measured a lousy 350 watts
at the top of his 100ft tower.... on 28.6 mhz. Flat swr on his 10m
yagi.
K9YC sez the losses on HF are all copper dc resistance losses..and nothing
to do with the dielectric etc. I see you can get Andrew .5 inch heliax
that consist of a Cu clad solid AL wire for the center conductor. You can
also buy .5 inch heliax (think it was cablewave)... that consist of a
solid cu center conductor. Andrew .875 heliax consist of a hollow .375 OD
cu tube for the center conductor.
What baffles me is that 213 U per times calculator is 1.1 db per 100
feet...at 29 mhz, yet RG-393 is 1.1 db per hundred ft. 213 has a single
copper braid. 393 has TWO silver plated braids. 213 has a 12 ga CU
stranded center conductor. 393 has a silver stranded center conductor.
VF on 213 is .66 VF on 393 is .69
I highly suspect if folks actually measured the power at the top of the
tower, vs the back of amp..... they would be in for a rude
awakening......esp on long runs...and on upper bands like 20-10m. I might
be wrong here, but it would prove to be an interesting exercise. I mean the
difference between a 5-6 el yagi and a 3 el yagi is typ 2 to 2.5 db. If
you are losing 1-3 db in the coax, that’s a bunch, compared to the gain of
the yagi itself.
Jim VE7RF
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