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Re: [TowerTalk] Legal Power measurement Discussion

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Legal Power measurement Discussion
From: jimlux <jimlux@earthlink.net>
Date: Mon, 26 Sep 2016 07:47:41 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 9/26/16 6:05 AM, Ed Sawyer wrote:
Clearly, this is "settled science".

Actually, no it's not.

Sure, if you want to measure the "output power" of an amplifier, the obvious reference plane for the measurement is the connector on the amplifier.

But I think as we get to more and more complex antenna systems, some with multiple amplifiers feeding multiple antennas, the "reference plane" is harder to define. Particularly when you start getting away from the 50 ohm connector with coax model.

Take two cases:

Case 1: If I built an amplifier with a 1200 ohm output impedance that varies with frequency and load, and then some transmission line that's a few meters long to an outboard matching network to ladder line to a non-resonant dipole

Case 2: The amplifier has the matching network inside.

Case 3: what about case 1, with the line between amplifier and outboard matching network is 50 meters long?



Where is the measurement plane? at the output of the amplifier before the matching network? Or at the output of the matching network? Why would the different physical configuration make a difference from a regulatory standpoint?

I can think of other schemes where the distinction between "transmitter" and "antenna system" is less obvious, or, at least doesn't provide a convenient measurement point. What if I have two 3.7 MHz amplifiers in the shack, phased 90 degrees from each other, then a couple 100 foot transmission lines, and a hybrid coupler to combine them before the run up the tower?

Topologically, this is a very common microwave system (90 degree hybrid, 2 PA devices, 90 degree hybrid), and in a microwave system, the transmission lines might be several wavelengths long.

Is the power measurement at the MMIC device (that's what the mfr of the MMIC will ask for), or is it at the output of the hybrid coupler (that's what I, as a system engineer, care about)?


- if you're selling amplifiers, then clearly you want a "at the output of my box" kind of spec - if you want a simple to implement requirement that covers 99.9% of the cases, "stick the power meter in the feedline at the end next to the amp" is easy.
- But I'm exploring the 0.01%


Let's be clear.. nobody is going to be hunting down someone running 1900W into a 1dB loss transmission line, with 1500W showing up at the load end. We're not talking here about "active antenna tuners" that "seem" to radiate a signal equivalent to 5kW.

Nor are we talking about contest rules - which by definition are arbitrary.. if the rules say "all transmitters within 1000 ft are to be considered as one", then that's what the rule is.

This is sort of an academic discussion - but one of some importance in the long run as more sophisticated radio systems start to be built. The idea of amplifier per element hasn't got a lot of traction yet, but it's coming. - SW broadcasters do it, why not hams. Particularly with solid state devices, the losses in a combining network in a SSPA with a single output jack are substantial, particularly with microwaves. Spatial combining (which has essentially no loss) is pretty attractive, when you're ganging up 20 100 watt modules to get 1500W out. Why burn the extra 5 modules just to overcome the combiner loss.


https://en.wikipedia.org/wiki/ALLISS for an interesting take on this.. their power levels are substantially above what hams do, but the architecture is interesting.. They use a variety of basically high power digital sources that are combined to make a low distortion, high power RF signal.

The Harris AM broadcast transmitters are sort of similar: essentially a giant power Digital to Analog converter, but without the spatial combining.

And shouldn't we be looking at architectures that have better "wall plug to Radiated RF" efficiency.. That's part of "advancing the state of the art" that's one of the purposes of ham radio.









 Power is measured at the final of the
radio or amplifier, whichever is last.  1500W max PEP from the transmitter
is clearly in the rules - as its quite clear how the FCC states ERP for
other services.  And they don't mention ERP except for the new VLF.



This dialog reminds me of the splitting hairs a decade ago of whether 1500W
was "per antenna" in the contest rules and rules finally settled the issue
by rewording that its 1500W total power - regardless of number or direction
of antennas.


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