On 4/9/12 7:28 AM, Jim Thomson wrote:
> From: TexasRF@aol.com
> Sent: Monday, April 09, 2012 6:22 AM
> To: firstname.lastname@example.org ; email@example.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
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
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
> Gerald K5GW
> ## and its even easier with any digital wattmeter. It gets easier if the
> max power is 100w on the 1st meter,
> and less than 100w on the 2nd meter. Once below 100w, then I’m down to
> reading power in .1 watt increments.
> I own 4 of these power master watt meters. They are superb. The bird and
> also my CD meter are long gone obsolete,
> along with the myriad of slugs.
> Jim VE7RF
The ad for PowerMaster says "Accuracy typically better than +/- 3% over
frequency and temperature ranges"
As is typical for ham gear, there's no indication whether that's a worst
case, some number of sigma, or what, but it's in the same class as the
telepost LP100, and I suspect, typical of almost all inexpensive
broadband power meters these days.
I think most use one of the Analog Devices (or other) power measurement
chips driving a ADC and some calibration algorithm in a micro. The
dominant uncertainty is probably in the coupler and in the mismatch in
the system. Very few of the inexpensive meters (if any) use some sort
of DC replacement bridge scheme with thermistors (why bother..)
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