[TowerTalk] Coax loss. Has anybody measured it ?
Joe Giacobello, K2XX
k2xx at swva.net
Mon Nov 24 12:43:29 EST 2014
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 at 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 at 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 at 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 at contesting.com
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