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## Re: [TowerTalk] Coax

 To: towertalk@contesting.com Re: [TowerTalk] Coax Jim Lux Mon, 09 Apr 2012 08:01:44 -0700 mailto:towertalk@contesting.com>
 ```On 4/9/12 7:28 AM, Jim Thomson wrote: > > > 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 > > 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..) _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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