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Re: [TowerTalk] Coax

Subject: Re: [TowerTalk] Coax
From: Jim Brown <>
Date: Mon, 09 Apr 2012 09:37:52 -0700
List-post: <">>
On 4/9/2012 8:01 AM, Jim Lux wrote:
> 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...

Agreed on all counts.

Another of those complicating factors is the fact that the Zo of a 
transmission line is NOT a pure resistance, nor is it constant with 
frequency.  Zo starts out highly reactive at audio frequencies, 
gradually transitions to a resistive plus reactance with an absolute 
value larger than nominal Zo, finally converging to it's nominal value 
above the HF spectrum.

Driving and terminating a test sample with 50 ohms causes enough of a 
mismatch at 1 or 2 MHz that there will be "ripple" in loss measurements 
carefully taken over a frequency range. Been there, done that, have the 
Tee-shirt.  To get anything APPROACHING realistic loss data in the 1-2 
MHz range you've got to be measuring at least 1,000 ft of coax, and even 
then, I don't consider my measurements, carefully done at multiple 
frequencies with an HP generator and HP spectrum analyzer as voltmeter, 
any better than about +/- 20%.  My best estimate of the loss of 
Commscope 3227, #10 solid copper center, foil plus braid essentially 
equivalent to LMR400, is about 0.22 dB/100 ft at 1.8 MHz. That's the 
only coax I've had 1,000 ft of to measure.

73, Jim K9YC

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