Date: Mon, 09 Apr 2012 08:01:44 -0700
From: Jim Lux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] Coax
>
> From: TexasRF@aol.com
> Sent: Monday, April 09, 2012 6:22 AM
> To: jim.thom@telus.net ; towertalk@contesting.com
> Subject: Re: [TowerTalk] Coax
>
> Jim, the example compares 200ft of LMR400 to RG213. The stated difference is
> .358 dB/100ft for a total of .716 dB.
>
> That is a power ratio of 1.179 or 100w vs 117.9w. That is easily measured
> with a bird meter.
More like 115 or 120W vs 100W .. 5% accuracy is probably more realistic.
As an example, the Telepost LP100 manual (which I happen to have here)
says 5% worst case absolute, 3% typical, but doesn't say if that's a 1
or 2 sigma number or a worst case, and doesn't say whether that's into a
perfect match, etc. I usually figure a lab HP/Agilent power meter is
good to a "few percent" unless you're carefully characterizing the whole
measurement process (5% is 0.2dB roughly, so you don't really trust
that 0.01 digit on the meter, but I'm willing to go for the 0.1 digit,
particularly in a relative comparison)
taking 0.75 dB/100ft vs 1 dB/100ft..
at 200 ft, either 1.5dB or 2 dB loss..
100W in, you'd measure either 71 or 63 watts
which would be a measurable difference.
(or more likely, given typical 5% accuracy, you'd measure 70+/- 5
Watts and 65 +/- 5 Watts... but if measured with the same meter at
roughly the same time, the errors aren't uncorrelated, so you'd see some
difference.. )
One could spend quite a while nailing the uncertainty in the measurement
and accounting for all the confounding factors..
1) If there is some mismatch in the system. with 1:1.5 on both ends,
the power uncertainty is about 0.35dB. With 1:1.2, more like 0.07dB.
2) The impedance of coax isn't controlled all that tightly (is it 50
ohms or 52 ohms nominal? What is it really?)
3) What about harmonic content? When you're getting to gnat's eyelash
precision, a -20dB harmonic (which would be pretty bad) is a 1% error in
power.
4) there's more...
So, in any case, the difference between the 400 and the 213 is probably
"detectable" and to a lesser extent "measureable"
Even better, you could replace the antenna with a short and measure the
reflected power, which would double the loss measured.
Or you could have a calibrated receiver some distance away and measure
the difference in radiated power (which would take care of any mismatch
issues, and is really what you're interested in anyway)
Now, would that translate to a hearable difference at the receiving end?
Maybe? Maybe not?
That's one of those "whats 0.2dB worth to you?" kinds of questions
## I think you guys are missing the boat here. Who said anything about
mismatch.
Even if there is mismatch, you just use frw – rvs pwr = net power. The
wattmeters
don’t even have to be dead on accurate. They just both have to be the SAME.
IE: if they are BOTH exactly 5% on the low side, who cares. On those array
solutions
power master meters, you can change the calibration in 1% increments if you
want, over a huge
range. Each coupler has been hand calibrated to an HP. Each coupler is marked
on it as to
what frwd and rvs calibration settings to program into mating display units.
I have bird and CD meters, and slugs from 50w to 10 kw, the power master meters
make
the bird-cd wattmeters look like sick puppies. Its very easy to measure the
difference between
lmr-400 and 213 u at 29 mhz..on a 100-200 ft run of each type.
Jim VE7RF
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