On 12/28/11 8:36 AM, Rik van Riel wrote:
> On 12/27/2011 04:30 PM, David Gilbert wrote:
>> Fundamentally, when RF hits a conducting element (wire, piece of tubing,
>> tree, side of a building, etc) it induces currents in that element ...
>> currents that are no different than if you had been able to somehow
>> connect your transmit coax to it. Those currents generate an
>> electromagnetic field around that element that is in fact re-radiated
>> RF. If the element is lossy (wood, dirt, wet mattress, etc) the induced
>> currents are dissipated as heat instead of being re-radiated.
> I think I get it now...
> 1) If the element is very low resistance, the RF will
> induce a lot of current, but it will get re-radiated.
> 2) If the element is medium low resistance, the RF
> will induce a fair amount of current, but it will
> get absorbed.
> 3) If the element is very high resistance, the RF
> will induce very little current. You do not care
> that it is absorbed, because it is so little.
And that's really why soil properties are important. Really dry sand
doesn't have much effect on radiation efficiency(it's your #3).. The
proverbial salt marsh also doesn't (it's your #1).
It's also why steel is so bad (compared to other metals): It's in that
#2 category.. good enough to actually be "in the circuit" but lossy.
Ditto for, say, #31 mix ferrite cores.
By the way, turning this problem around is how one makes RF absorber for
anechoic chambers, the stuff you put inside microwave enclosures to kill
resonances/feedback, or on the surface of your airplane/ship/what-have-you
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