Re: [TowerTalk] Smart antennas

[Jim Lux <jimlux@earthlink.net>]

At 07:40 AM 9/3/2003 -0700, Bill Turner wrote:

> On Wed, 3 Sep 2003 07:28:27 -0700, "Jim Lux" <jimlux@earthlink.net>
> wrote:
>
> >Actually, for that matter, I think the FCC is a bit vague on where the
> >reference plane for the power limit is.   Say I have a 5 kW amplifier, and
> >I'm using some sort of horrible lossy transmission line to connect to the
> >antenna.  What's to keep me from saying that the output of the system should
> >measured at the antenna feedpoint (i.e. the transmission line is part of the
> >"transmitter".. it's not supposed to radiate).  Particularly on higher
> >bands, this could be significant.
>
> _________________________________________________________
>
> The FCC is not vague at all.  Here's a quote from Part 97:
>
> §97.313 Transmitter power standards.
> <snip>
> (b) No station may transmit with a transmitter power
> exceeding 1.5 kW PEP.
>
> The operative phrase is "transmitter power".  There is no mention of
> feedline loss or antenna efficiency.

My contention is that one could say that the transmitter ends at the 
antenna feed point(s) (i.e. the point where intentional radiation starts to 
occur).  I agree though that under the current rules one can't roll in 
antenna efficiency (even presuming one could measure it accurately).

Note also that 97.313(f) does actually give a ERP standard for a specific case.

Consider two cases...
1) A transmitter that has an internal matching network (with loss) to feed 
450 ohm ladder line built in
and 2) one where I connect a 50 ohm unbalanced output amplifier to an 
external balun/transformer to transform the 50 ohm unbalanced output of the 
power amplifier to the required 450 ohm balanced signal.

Where do you measure the power?  In the first case, one can ONLY measure it 
practically at the output of the box, so I can make the internal power 
amplifier put out whatever is needed to get 1500W PEP output.  So, by 
extension, I should be able to measure the output power at the output of my 
external balun, setting the PA up to drive however much is needed to 
overcome the losses in my balun.

Now, move that transformer up to the top of the tower, with 200 feet of 
lossy coax in between PA and transformer.  Why should I not still be able 
to measure the transmitter power at the point where intentional radiation 
starts?

After all, the PA is only part of the "transmitter", which also includes 
low level modulating circuitry, etc.  The various spurious emissions limits 
are imposed on the "whole system", including external filters, etc., are 
they not?

For an even stickier problem, consider the effect of circulating reactive 
power in a mismatched system.  Say I have a transmitter which has an output 
tuning network that can create a reactive source impedance (like most tube 
amps).  I use that tuning network to match the reactive component in my 
antenna impedance (since the antenna is non-resonant).  There is now some 
reactive power flowing back and forth between antenna and 
transmitter.  This power won't ever get radiated (although some will be 
lost in the transmission line).  Do I count it against my 1500W PEP 
limit?  Say I have a directional watt meter in the line and it reads 2000W 
forward and 1000W reverse.   The net power to the load (the antenna) is 
only 1000W.  Does this meet the 97.313 limit?

This has real relevance when one is starting to do phased arrays with 
arbitrary phasing networks.  Mutual element interactions can lead to fairly 
significant circulating powers (the classic 4 square can have a negative 
real part of the feedpoint impedance on one element in some circumstances, 
indicating that power is actually flowing out of that element and back 
through the feed network and into the other 3 elements).

If I build a transmitter for that phased array which has 4 separate 
outputs, and all the phasing networks inside the transmitter box, it seems 
that I should be able to measure the sum of the net power flowing in or out 
of each port to assess compliance with 97.313 (that is, the power absorbed 
by the antenna system is the power we're talking about... whether it's 
absorbed by dummy loads or by radiation into the aether)

In fact, the whole issue of power measurement is kind of tricky in 
mismatched systems anyway.  Those of you with "legal limit" amplifiers 
running right at the edge: Do you calibrate your power meter for the actual 
line impedance observed, say, when your SWR is 1.5:1, or do you just use 
the 50 ohm numbers.  If your meter is a "peak voltage" reading meter, 
there's an implied impedance factored in.  If you're using a directional 
coupler into a broadband load with a true power detector, it's a bit 
different, of course, but, the coupling ratio for the usual sorts of bridge 
circuits can vary depending on the impedance of the "thru" line.

I looked through  47 CFR Part 2, where the general regulations and 
definitions are to find a better definition of "transmitter power", 
although I couldn't find much there. Lots of descriptions of how to measure 
it, but no definition of where the "edge of the box" is.

This all goes to show how tricky it can be when we try to use simple rules 
on complex systems, and this sort of discussion, in an informal setting 
(i.e. a mailing list) is particularly valuable, because we can float a 
trial balloon, have folks shoot at it, and through that process come up 
with a consensus that is equally offensive to all. 


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