RE: Question

[dan.Strassberg@att.net]

Mark: So does direct measurement use an RF wattmeter, 
which would automatically compensate for variations in 
the common-point resistance, or does it involve taking 
the square of the common-point current and multiplying 
by the (presumed) commom-point resistance. The wattmeter 
method sounds more fool-proof, especially if the common-
point impedance has a significant reactive component,
which could make determining the resistance a bit 
tricky. This might be particularly true if the reactance 
were unstable.

And if direct measurement involves measuring the 
current, squaring it, and mutiplying by a previously 
measured resistance, how does the old method (the method 
that direct measurement replaces) work? And what was the 
old method called--indirect measurement? Seems as though 
deriving the power from I^2*R could qualify as an 
indirect-measurement technique.
> I believe the question refers to the method that am stations use to
> determine power.  It is done by multiplying the antenna input resistance
> by the square of the antenna current in amps to yield watts.  This is
> basic Ohms Law, where power equals resistance times the current squared.
> 
> For directionals the input resistance is measured at the input to the
> phasor and is known as the common point.  It is typically 50 ohms.  WBZ's
> main site has a common resistance of 50 ohms and runs 32.43 amps of
> current.  Which is about 52,600watts.
> 
> For non directional systems, the resistance of the tower is used and is
> know as the base.  The value varies greatly from installation to
> installation.  Shorter towers are typically lower resistance - sometimes
> as low as a few ohms.  Half wave towers are usually pretty high and can
> be more than several hundred ohms.