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Re: [TowerTalk] Coax

Subject: Re: [TowerTalk] Coax
From: Jim Lux <>
Date: Mon, 09 Apr 2012 08:01:44 -0700
List-post: <">>
On 4/9/12 7:28 AM, Jim Thomson wrote:
> From:
> Sent: Monday, April 09, 2012 6:22 AM
> To: ;
> 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 
        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

> 73,
> 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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