Tom Horne wrote:
K7ITM wrote:
On Apr 3, 9:38 pm, Tom Horne wrote:
I realize this may be a terribly basic question but at any given
transmitter power will the highest current measured in the antenna feed
line occur at the lowest SWR or not.
--
Tom Horne


If you have the patience please bare with me as I'm hoping to learn
something here. I was trying to figure out what use if any could be
made of a current measuring device located at the antenna feed point.


The current measuring device has to measure the complex current
(relative to the voltage). Active power going into the antenna and
being radiated or turned into heat is the product of the voltage and in
phase current. Reactive power which flows back and forth between
antenna and rest of system is the quadrature phase current.

What relationship would there be between maximum current at the feed
point and affective radiated power.

None, really, unless the system is perfectly matched. Consider a case
where you have a big capacitor as the antenna, and a big inductor on the
other side of the current meter. You could have very high currents
flowing in the LC resonant circuit, but very little actually radiated.


I've been told in both my license
preparation classes that making sure that the transmitter sees a low SWR
does not insure a good signal out of the antenna.

This is true, but usually refers to the idea that a lossy
antenna/feedline can improve the SWR seen by the transmitter.


I'm looking for some
way to actually measure the amount of energy that is getting to the
antenna since it seams impractical to measure the signal strength in the
near or far fields during operation of the transmitter.


No more impractical to measure field strength then accurately measure
the energy going to the antenna. Both can (and have) been done.

Field measurements only measure at some point(s) in the field, so if you
have any directivity, then what you measure in direction A may not
relate well to what's being radiated in direction B.

OTOH, measuring the power flow in the feedline doesn't tell you whether
the power is radiated, or just heating the antenna.