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Re: [TowerTalk] Coax

Subject: Re: [TowerTalk] Coax
From: Jim Lux <>
Date: Mon, 09 Apr 2012 06:28:24 -0700
List-post: <">>
On 4/8/12 9:11 PM, Jim Brown wrote:
> On 4/8/2012 9:01 PM, Jim Thomson wrote:
>> LMR-400db          .666 db
>> LMR-400UF          .799 db
>> RG-213U             1.024 db
> RG213 is not a spec, it's a very broad generic description. There are
> RG213s built with thin copper braid and others with heavy copper braid.
> The loss in cable is a direct function of how much copper they use to
> build it.  Based on resistance data, Davis's 213 is directly equivalent
> to LMR400 for use on the HF bands.
> ##  say what.   Run 200 feet  of 213-U   into a dummy load at the far end, 
> with a wattmeter 1 foot before the dummy load.
> You'll need far more precise instrumentation than a Bird to measure a
> difference of 0.1 dB   Rather, you'll need a scope, RF voltmeter, or
> spectrum analyzer that can resolve 0.01dB, you'll need to measure by
> substitution, and you'd better make a bunch of measurements and compare
> them.
And at that level, small differences in match will affect not only the 
power actually delivered, but also (probably to a greater effect) the 
readings on the meter.

I'd also be interested how they got loss numbers to 0.001 dB precision 

I think what a lot of places do is calculate the loss based on 
construction, and it's a "worst case" or maybe a "1 sigma bad" number. 
That is, most of the stuff is better than the stated number, but 
occasionally, you might get some that is at the limit.

Sort of like saying connector VSWR<1.03.  It's not that the connector is 
that bad, it's that 1.03 is the measurement limit, and is as close to 
"zero" as you can reliably measure.

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