[TowerTalk] Lightweight Antenna Wire For Portable Work?

[jimlux]

On 11/2/18 9:35 AM, Richard (Rick) Karlquist wrote:
> 
> 
> On 11/2/2018 8:29 AM, jimlux wrote:
> 
>> Tungsten is 3 times the resistivity of copper, and results in about a 5% 
> 
> Whenever skin effect is involved, the RF resisitivity
> is proportional to the SQUARE ROOT of the DC resistivity.
> Therefore, the RF resistance of tungsten is only SQRT(3) or 1.8
> times as much as copper.

yes.. that's true.. the skin depth gets deeper with higher resistivity, 
(as sqrt(rho)) so you wind up with a rho/sqrt(rho)

This only applies if skin depth is << wire diameter (say, wire diameter 
is >8-10x skin depth.

For 10 MHz, skin depth is about a mil, so unless you're using AWG30 or 
smaller, probably not an issue.



> 
> Another example, aluminum has 58% more DC resistance than copper
> but only 26% more RF resistance.  These numbers happen to neatly
> translate into 4 wire gauges and 2 wire gauges respectively.
> For example, 4 gauge aluminum wire has the same resistance at RF
> as 6 gauge copper wire.


And, it's handy to remember that 10 gauges is 1/10th the area, 20 gauges 
is 1/10th the diameter. and AWG10 is 0.1" in diameter.
For DC resistance
3 gauges is twice/half the resistance (just like dB!)

> 
> This calculation doesn't apply to magnetic alloys like nickel, iron, or 
> stainless, because the permeability affects only the RF resistance,
> so there is no simple relationship.

Well, the skin depth goes as sqrt(rho)/sqrt(mu) If you have a good 
number for mu (see below) you can calculate it. Non-magnetic stainless 
steel is, unfortunately, not mu=1, it's just "mu is small compared to iron"

The real challenge is finding a good permeability number for some random 
wire alloy - not only does the resitivity of steel vary a lot with 
carbon content, so does the mu, and both vary with frequency too.

If it's a critical thing, the approach is "measure it"

We spent some time at work attempting to characterize the RF properties 
of steel tape measures - they make handy deployable antennas - 
ultimately, we wound up just bounding the problem and showing that in 
the worst case, it wasn't "too bad".

just like antenna traps, *measuring* the RF properties of some arbitrary 
"thing" that's not a discrete component can be challenging.







> 
> 73
> Rick N6RK