[TowerTalk] Reference plane for FCC power limit

[Jim Lux]

----- Original Message -----
From: "Bill Turner" <wrt at dslextreme.com>
To: "Chuck Counselman" <ccc at space.mit.edu>
Cc: <towertalk at contesting.com>
Sent: Wednesday, September 03, 2003 3:58 PM
Subject: Re: [TowerTalk] Reference plane for FCC power limit


> On Wed, 3 Sep 2003 15:12:32 -0400, Chuck Counselman
> <ccc at space.mit.edu> wrote:
>
> >The value of Zo does _not_ completely
> >determine the ratio of E/I, whether you  take E and I in this formula
> >to be real or complex.
>
>
> I think you're using the word "real" where I would use "RMS" and
> "complex" where I would use "reactive", but aside from that, ok.

Not at all... RMS volts times RMS current will give you apparent power which
is the vector sum of real and reactive power (that is, apparent power = sqrt
(real power^2+reactive power^2)...  Chuck is using the correct terminology
here.

>
> >You can easily see that Zo does not determine
> >the ratio of E/I by considering a lossless line terminated in a
> >short-circuit.  At this termination and every integer half-wavelength
> >back from it, the ratio of E/I equals zero, completely independent of
> >the value of Zo.  The ratio of E/I equals Zo only when there is no
> >reflected wave.
>
> Correct, in that specific hypothetical example, which the FCC couldn't
> care less about.  I was referring to the RMS E and I at the output of
> the transmitter, which is what the FCC will be interested in.  I'm
> sure if you try to slip your hypothetical stuff by them, they will
> slap on a 50 ohm load, retune to whatever your anode E and I meters
> said beforehand, and quickly fill out your citation.  When you go to
> court, I'm sure the judge won't care any more than the FCC did.

There's no "slipping by" here.... it's purely a question of what's the
reference plane for the measurement, and does the regulation apply to net or
forward power, something that people selling RF hardware and their
sophisticated customers have to deal with every day.  I have spent many days
carefully writing specifications for just such things, and having to develop
test plans to verify compliance with the spec.  In fact, as a result of this
discussion, I intend to ask the FCC precisely what their interpretation is,
in view of the increasing use of automatic antenna tuners, multiply driven
arrays, and the like.

And, as far as the citation goes,  the judge and the FCC will care.... The
FCC enforcement people are fully aware of the difference between reactive
and active power.  And, the anode E and I are just the RMS values (if that)
so their product is voltamps, not actual watts.  This isn't something
obscure or esoteric, it's something that the FCC deals with every day for
just about every transmitter other than amateur radio.  Anybody who is in
the business of designing, building and installing RF hardware is mightily
aware of the differences between forward, net, apparent, real, reactive,
etc. power, otherwise nobody would bother specifying VSWRs on RF components,
invest in $100K+ network analyzers, test with sliding shorts, etc.

Mind you, you're going to have to have some reasonably high quality forward
and reflected power measurements, etc.  If you're sitting there with your
Henry 5K running at full throttle, and claiming that you're merely
accounting for the estimated transmission line loss, and you don't own or
have rental receipts for the appropriate test gear, then you'd better have
some pretty good analysis to back up your case.

At every opportunity I've had to deal with the FCC on regulatory matters,
their folks have been fully skilled and conversant with all the esoterica of
RF measurements.

Jim, w6rmk
>
> --
> Bill, W7TI
>
>