The theory behind this is quite involved, but is well described in Terman's
"Radio Engineer's Handbook" (pp 28 - 37 in my
1943 edition). Basically you can find the RF resistance per unit length at a
given diameter and frequency, and since
copper is a very good heat conductor, it is permissible to equate the power
dissipation for a temperature rise due to RF heating with the rise due to DC
heating.
So if a certain current capacity due to I**2*R(dc) losses for DC for a
conductor is given, you can find the corresponding value for R(ac) with skin
effect taken into account by calculating the RF to DC resistance ratio, and
solve for the maximum RF current allowed.
This will not be the exact solution, but close enough for "engineering
purposes".
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Dan Levin" <djl@andlev.com>
To: <amps@contesting.com>
Sent: Friday, April 15, 2005 7:22 AM
Subject: [Amps] Skin Effect and Wire Current Capacity at HF
> I'm trying to design a coil that will be used in a 1500 watt filter at HF.
> I know that I need to allow for frequency related skin effect that will
> reduce the area of the wire or tubing carrying the current.
>
> First, I check the ARRL Handbook and the Antenna Book. No formula and no
> table showing suggested wire sizes for a given frequency and power level.
>
> Next, I determine that the skin depth at 28 mHz is .0005", and I calculate
> the copper volume that the current is traveling through at one skin depth in
> wires and tubes of various sizes. I get results that seem unreasonable -
> using a current density of 2500 amps / square inch I get a current capacity
> for #14 wire at 28 mHz of under .5 amps and for 1/2" tubing of just a couple
> of amps. Yes, I realize that the skin depth is one standard deviation, but
> even allowing for that the capacities seem very low.
>
> Can someone point me at either a formula or a table that will tell me or
> allow me to calculate what the current capacity of a wire or tube is at a
> given HF frequency? Or, I suppose to put it differently since the capacity
> is related to the acceptable temperature rise - something that will tell me
> the copper loss in a piece of wire or tubing at a given frequency?
>
> Thanks!
>
> ***dan, K6IF
>
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