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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