[TowerTalk] Re: Tower Faraday shield?

[Jim Lux]

> Think about how we specify percent coverage with a piece of coax
> braid. We spend money to get 100% coverage instead of 88%. We call 88%
> cheep coax.
>
> Look at a Rohn tower around wires in the middle. What kind of coverage
> is that? 3% ? 2% ? Is it even 1% coverage? Certainly will not keep the
> shield of coax in the middle from acting as one plate of a capacitor
> to something on the outside.


bear in mind also that the tower is made of steel covered with zinc, hardly
the conductivity of copper.  Lower conductivity means greater skin depth,
and skin depth is very related to the amount of shielding one would get from
a cage.

Also, any conductor penetrating a hole (i.e. the shield on the coax, bolts,
etc.) provide an RF path from the outside of the cage to the inside.

Then, start looking at the attenuation/shielding... Say it were 40 dB (which
would be very, very good for a tower.. 40 dB is doing good for an all metal
solid box )... The power in the lightning (at the outside of the box) is on
the order of 100 kJ/meter dissipated in, say, 10 microseconds. For a 10
meter tower, that's 1 MJ in 1E-5 seconds, or a meagre 100 GW peak power.
Knock that down by 40 dB and you're at only 10 MW on the inside.  True
enough, the power is broadband, spread over 10-100 MHz, and there's probably
some selectivity inherent in the feedline and receiver, as well as some
loss.. so maybe you're only getting a few tens of kW incident into the
receiver front end.  Sure, you've got one of those fancy new receivers with
the +40 dBm 3rd order intercept front end.. but I think that the +70 dBm
transient's going to give it a bit of trouble..


Jim, W6RMK