Yes, I have measured actual coax loss using both dummy loads and antennas at
the end of the cables. IIRC in almost all (maybe all ?) cases (baring obvious
issues with connectors and adapters) the measured results were some what better
than data sheets suggested. I also repeated some of the measurements using
various watt meters and a spectrum analyzer to convince my self the
measurements were reasonably accurate.
IIRC the cables involved were various flavors of Belden 9913, RG 58, RG 213 and
1/4" thru 1/2" heliax.
Connectors and adapters seemed to be the only source of issues for me and in
most cases I believe I would have spotted the problem ones without actually
needing to measure the loss. This was done back in the 1990's.
One outcome of these tests were that I bought my self a nice collection of
genuine amphenol adapters that I still use. I also replace any PL259
connectors that don't look right and avoid cheap adapters. At one point I
also invested in some expensive silver teflon PL259 barrel connectors for an
installation where the use of barrel connectors was unavoidable (they were made
by Kings IIRC.)
I've just recently gotten into high power VHF operations (ie. 300 watts plus..)
and am taking another run at this as I'm pushing the rated power limits on some
of the jumper cables I'm using. (I managed to damage one jumper cable by
modestly exceeding the power limit for a brief period of time.)
Your results may vary.
Regards
Mark VE7AFZ
Sent from my iPad
On 2014-11-24, at 9:00 AM, towertalk-request@contesting.
>
>
>
> Message: 4
> Date: Mon, 24 Nov 2014 07:56:53 -0800
> From: "Jim Thomson" <jim.thom@telus.net>
> To: <towertalk@contesting.com>
> Subject: [TowerTalk] Coax loss. Has anybody measured it ?
> Message-ID: <AE52BE79BDDD44528BECA805001C7EDC@JimPC>
> Content-Type: text/plain; charset="utf-8"
>
> 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
>
> ------------------------------
>
> Message: 5
> Date: Mon, 24 Nov 2014 08:37:32 -0800
> From: "Jim Thomson" <jim.thom@telus.net>
> To: <towertalk@contesting.com>
> Subject: [TowerTalk] Yagi gain vs rotary dipole.
> Message-ID: <79332DBCB81347D19B596280B508F4D3@JimPC>
> Content-Type: text/plain; charset="utf-8"
>
> Ok, forget FB and FS for this exercise. Has anybody actually measured the
> gain of a yagi vs something like a rotary dipole.... at the same height ?
> A lot of yagi vs dipole comparisons are not done right.... since the dipole
> is usually not at right angles to the desired signal. ideally, it would be
> nice to have 2 identical height towers..... or at least mount a yagi on 1
> tower..and say a full size dipole... made from AL tubing..on another
> tower..at the same height.
> And with towers side by side..not behind each other etc. And equal lengths
> of coax.
>
> has anybody done anything remotely like this ? Is your 3 el 20m yagi 5 db
> louder than the dipole ? Or another case is a small yagi and a bigger
> one...like a 3 el vs 5-6 el. What do u see between
> them... 2 to 2.5 db ?? Or do u see a lot bigger differences ?? I
> mean....if a 2 el yagi is only 4db gain..that?s only 2/3 of an S unit. Im
> looking at this from a sig strength perspective only, NOT F/B or F/S. 2db
> with the bigger yagi hardly seems worth it on paper...if it actually is 2db.
> Switch in a 2db pad on RX and u will see what I mean.
>
> A buddy back in the 70s had 2 x 100 ft towers up....side by side and 250 ft
> apart. equal length feedlines. tribander on tower1 and a hb 6 el 20m yagi
> on the other. We could not tell the difference between em, while
> switching...listening to EU short path.... night after night. he was not
> impressed. F/B and F/S on the monobander was better..and pattern was
> narrower, but that was about it. We tried it on strong and weak
> signals..and also with rf gain backed way off etc.
>
> Jim VE7RF
>
> ------------------------------
>
> Message: 6
> Date: Mon, 24 Nov 2014 08:38:31 -0800
> From: Jim Brown <jim@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Coax loss. Has anybody measured it ?
> Message-ID: <54735F07.2090002@audiosystemsgroup.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On Mon,11/24/2014 7:56 AM, Jim Thomson wrote:
>> 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?
>
> I've measured loss in various lengths of various coax types on the
> ground by several methods. The first time around, I used an HP generator
> as a source and an HP Spectrum Analyzer as a load and voltmeter. In
> later years, I've used the DG8SAQ VNWA Vector Network Analyzer.
>
> As to your data comparisons between coax types -- remember that skin
> effect dominates resistance above mid audio frequencies, so resistance
> of both shield and center conductor are largely due to diameters of
> both. Also, remember that RG-numbers don't mean much with respect to
> cable specs. Rather, we must look at the PART NUMBERS for the coax.
> Belden makes several dozen RG6s, many of which are quite different from
> each other. Likewise a dozen or more RG11s, and dozens of RG8s and
> RG213s. Differences include center conductor composition (copper, copper
> clad steel), center diameter, dielectric, shield composition (copper,
> aluminum, braid, foil), and diameters of both conductors.
>
> The equations and graphs on the Times data sheets are based on those
> fundamental properties of the coax and the wave equation. Dan Maguire,
> AC6LA has developed Excel spreadsheets that will plot graphs of Zo, loss
> and velocity factor as a function of frequency when you import sweeps
> from a vector antenna analyzer. http://www.ac6la.com/
>
> 73, Jim K9YC
>
>
>
>
>
>
>
> ------------------------------
>
> Message: 7
> Date: Mon, 24 Nov 2014 08:46:49 -0800
> From: Jim Brown <jim@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Yagi gain vs rotary dipole.
> Message-ID: <547360F9.4060008@audiosystemsgroup.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On Mon,11/24/2014 8:37 AM, Jim Thomson wrote:
>> k, forget FB and FS for this exercise. Has anybody actually measured the
>> gain of a yagi vs something like a rotary dipole.... at the same height ?
>
> As I recall, that was part of the testing of a dozen of so Yagis that
> N0AX and K7LXC did about 15 years ago. I've loaned my copy of the report
> to my neighbor, W6GJB. You can buy it for about $25 from Champion Radio
> Products. Well worth the cost.
>
> 73, Jim K9YC
>
>
> ------------------------------
>
> Subject: Digest Footer
>
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>
>
> ------------------------------
>
> End of TowerTalk Digest, Vol 143, Issue 68
> ******************************************
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