[TowerTalk] Legal Power measurement Discussion

[jimlux]

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.
>
>