[TowerTalk] Coax

[Jim Thomson]

Date: Mon, 09 Apr 2012 08:01:44 -0700
From: Jim Lux <jimlux at earthlink.net>
Subject: Re: [TowerTalk] Coax

>
> From: TexasRF at aol.com
> Sent: Monday, April 09, 2012 6:22 AM
> To: jim.thom at telus.net ; towertalk at 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