I've done it several times but many years ago at a previous QTH. IIRC,
I loaded my amp to a KW out into a dummy load in the shack through an RF
ammeter and measured the current. Since I had a crankup tower along
side my house and a flat roof with easy access, I took the ammeter and
dummy load out to the roof and connected the other end of the coax and
made a second measurement. Again, IIRC, I made the measurement on 10M
in order to maximize the measured loss and I was probably using the
Wireman's Flexi 4XL coax. I distinctly remember that it met spec
without any issues. (I may still have records of the measurements in my
old shack notes.)
73, Joe
K2XX
Patrick Greenlee <mailto:patrick_g@windstream.net>
Monday, November 24, 2014 12:10 PM
Amen Jim, Amen! Measuring the RF power at the antenna feedpoint can
INDEED be a rude and abrupt bubble buster! What matters then is what
if anything is done about it and aye, there's the rub.
Patrick NJ5G
-----Original Message----- From: Jim Thomson
Sent: Monday, November 24, 2014 9:56 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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Jim Thomson <mailto:jim.thom@telus.net>
Monday, November 24, 2014 10:56 AM
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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_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
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