Joel Koltner wrote:
I know that many people think G3LHZ is a little bit off his rocker, but
out of curiosity... what he suggests on slide 15 he
http://frrl.files.wordpress.com/2009...heuristics.pdf
- - is that a valid approach to measuring antenna efficiency? -- Use a
thermal camera to note how much an antenna heats up with a given input
power, find out how much DC power it required to heat it to the same
temperature (the antenna's loss), and -- poof! -- antenna efficiency =
(input power-loss)/input power?

What are the significant loss mechanisms that he's not accounting for?
(He claims his matching network isn't getting at all hot.)

Thanks,
---Joel

A thermal camera is NOT a good way to do calorimetry. It's a fine way to
look for hot spots. Here are some of the potential problems: 1) the
thermal camera converts long wave IR brightness to temperature using
some assumptions about the emissivity of the surface; 2) convective and
radiative losses to the surroundings will change the surface
temperature; 3) surface temperature may or may not correlate well to
dissipated heat.

It's an RF device, so the physical distribution of the power dissipation
will be different than with DC. In a classic substitution RF power
measurement, a lot of effort is made to try and make sure that the
thermal properties are identical for the DC and RF dissipation cases
(well defined broadband load that is physically small, etc.).

In the subject case here, think of this: say you had a 2cm diameter
copper bar and you run 100 Amps of DC through it. The current is
distributed evenly, as is the power dissipation. Now run 1 MHz RF
through that same bar. The skin depth is about .065 mm, so virtually
ALL the RF current is contained within a layer less than 1/3 mm thick.
That's a very different heat and thermal distribution (sort of like the
difference between putting that thick steak in the 200F oven and
throwing it on the blazing hot grill).

One can calibrate for all this, but, still, it's tough.

A better way to do this measurement is to put the antenna in a suitable
far field test site, accurately measure the power flowing into it,
accurately measure the power flowing out of it (e.g. E & H field
strengths in the far field)

Now, finding a suitable site is difficult, particularly at lower
frequencies: you want to be "many" wavelengths away from the ground, for
instance.

How about hanging it from a balloon with a battery powered transmitter
(or receiver: I assume nobody is claiming that reciprocity doesn't work)
and have the field strength detector also hanging from a balloon.