[TowerTalk] Reference plane for FCC power limit

[Chuck Counselman <ccc@space.mit.edu>]

At 7:28 AM -0700 9/3/03, Jim Lux wrote:
> ...I think the FCC is a bit vague on where the reference plane for 
> the power limit is....


The FCC is (also) silent regarding _what_ power is limited.

The "forward" power at a reference plane/point along a simple TEM 
transmission-line such as coaxial cable or parallel-wire line depends 
not only on the voltage across and the current through the line at 
said plane, but also on an assumed value, e.g., 50 ohms, of the 
characteristic impedance Zo of said line.

OTOH, the _net_ (forward minus reverse) power at said plane is 
determined by the voltage and the current at said plane and is 
independent of an assumption regarding Zo.  [Here I refer to only 
difference-mode transmission; I ignore common-mode transmission, 
which is substantial for many of you.  :-) ]  _Net_ power, also 
called "real" power (as opposed to "imaginary" or "reactive" power), 
is what the electric utility company's kilowatt-hour meter on the 
back of your house responds to.

Since the FCC does not specify a characteristic impedance for forward 
or reverse power measurement, it must be _net_ power that the FCC 
limits.

-Chuck, W1HIS, and not a lawyer.


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