On 23 mar, 02:24, "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 hehttp://frrl.files.wordpress.com/2009...f-small-an...-
- 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

Hello Joel,

As with many questions, the answer to the temperature rise method can
be "yes" or "no".

For this discussion I assume an antenna as a device or system to emit
radio waves to certain directions. Mostly designers try to maximize
radiation intensity over electrical input ratio, or total radiated
power over electrical input power.

For situations where obstacles are wavelengths away from the antenna,
temperature rise can be a means of evaluating antenna efficiency. I
once used temperature rise to accurately measure efficiency of a high
efficiency RF amplifier.

In cases where obstacles are very close to the antenna, just
determining temperature rise of the metallic structure being the
antenna does not satisfy me. You will know the dissipated power inside
the antenna, but not inside the obstacle in the reactive field. When
this obstacle dissipates 90% of the electrical input power, overall
efficiency will not be high.

By using the temperature rise of the antenna only, you will notice
higher efficiency when the (loop) antenna is closer to an obstacle
(for example a thick wall). In case of a loop, the Q-factor drops,
resulting in less reactive currents, hence less dissipated power in
the loop and tuning capacitor. Of course more power is dissipated in
the wall.

When the antenna is close to metallic structures with certain
geometry, the real efficiency (so Prad/Pelec) may increase. The large
structure may extract energy from the loop and reradiate it (instead
of converting into heat). The extraction of energy from the loop
results in lower Q-factor, hence less heat loss in the loop and tuning
capacitor. Theoretically spoken, the temperature rise method is a
good one.

How will you relate temperature rise of arbitrary structures to
dissipation? If this question remains unanswered temperature rise
method will also not solve the antenna efficiency question.

Regarding temperature rise methods in general, it is good way to find
where losses are and whether it is worth to do some redesign to lower
losses.

Best regards,


Wim
PA3DJS
www.tetech.nl
PM will reach me, but don't forget to remove abc.