[TowerTalk] Smart antennas

Wed Sep 3 23:10:08 EDT 2003

Power measurements are notoriously difficult to make. I don't
think the FCC is going to raise a stink with anyone who is
running less than 2 KW PEP. There was a guy about ten years
ago who moved from West Virginia to Texas. He wanted to
stay loud with his buddies on 3895 KHz back in WV so he built
a 50 KW amplifier which he used to drive a 75 meter inverted-vee
in a residential neighborhood. That is the kind of thing that gets
the attention of the FCC's enforcement bureau. Pretty hard to
argue about full scale error, real vs. imaginary, mismatch error,
or forward vs. net when you are hard pegging a 10KW bird
slug :):).

Your arguments about feedline loss are good. I agree that
real power reaching the antenna terminals is what counts. Of
course, try convincing an FCC inspector that your system will
pass if he would just climb your 200 foot tower and splice in
his wattmeter at the antenna feedpoint instead of measuring
at the amplifier output :):).

73 de Mike, W4EF.........................................................

----- Original Message -----
From: "Jim Lux" <jimlux at earthlink.net>
To: <wrt at dslextreme.com>
Cc: "Towertalk" <towertalk at contesting.com>; "Michael Tope"
<W4EF at dellroy.com>
Sent: Wednesday, September 03, 2003 10:03 AM
Subject: Re: [TowerTalk] Smart antennas

> At 07:40 AM 9/3/2003 -0700, Bill Turner wrote:
>
> >On Wed, 3 Sep 2003 07:28:27 -0700, "Jim Lux" <jimlux at 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.
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>