[TowerTalk] Coax

Jim Lux jimlux at earthlink.net
Mon Apr 9 06:42:34 PDT 2012

On 4/9/12 6:30 AM, Steve Maki wrote:
> On 4/9/2012 9:04 AM, Jim Thomson wrote:
>> ## If you are implying that  dc resistance is the predominant factor
>> in coax cable loss, then  all these various  formulae  + online loss
>> calculators  must be out to lunch, and I find that hard to believe.
> At low frequencies, dielectric loss is very low, gradually rising with
> increasing frequency. At some frequency dielectric loss takes over, but
> that frequency is way above HF. At HF it's all about DC resistance. HF
> is practically DC in the overall scheme of things...

The dominant reason loss goes up with frequency at HF is that the skin 
depth is getting thinner (hence the sqrt(f) term in the loss equation). 
  Most of those equations and formulas have two terms a k1*sqrt(f) term 
and a k2*f term.  At low frequencies, the first dominates.

To a certain extent, the DC resistance probably maps to AC resistance 
fairly well, except where the center conductor is plated steel.  (big 
issue for 75 ohm Cable TV style stuff.. since it's intended for >100 
MHz, where skin depth is tiny, a lot of it uses plated steel for strength)

One might be misled by DC resistance if, for instance, it had a heavy 
braid (low DC resistance, not so low AC resistance). There's also the 
issue of braid over foil kinds of outer conductors.

BTW, this is why you have to be wary of extrapolating a mfr data sheet 
on the bottom end.  If the data sheet starts at 100 MHz and goes up to 
several GHz, I wouldn't just draw a straight line back to 10 MHz to 
estimate loss.

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