On Apr 4, 9:13*pm, 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

"This alternating current stuff is just a fad. *It is much too dangerous
for general use." *Thomas Alva Edison

At the lowest SWR, 1:1, the steady-state RMS current along a lossless
transmission line is everywhere the same. *At high SWR, the steady-
state RMS current along a lossless line varies between some high value
and some low value; the SWR is the ratio of the high value over the
low value, assuming the line is long enough that you won't find higher
or lower by extending the length of the line.

If the antenna feedpoint impedance is, say, 10 ohms and you feed it
100 watts, you should measure sqrt(10) amps at the feedpoint. *You'd
have a 5:1 SWR on a 50 ohm line with that load. *With a 250 ohm load,
you'd also have a 5:1 SWR on the same line, but the current at the
antenna end for 100 watts delivered to the antenna would be sqrt(0.4)
amps, or 1/5 as much current. *If the antenna represents a 50 ohm load
on the same 50 ohm line (swr = 1:1), the current is sqrt(2) amps for
100 watts, an intermediate value.

Hope that helps (and that I haven't screwed up my mental arithmetic).

Cheers,
Tom

Tom

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

--
Tom Horne

"This alternating current stuff is just a fad. *It is much too dangerous
for general use." *Thomas Alva Edison

Hi Tom,

Well, addressing one of your last points first...it's really not so
difficult to monitor the strength of your signal, at least in a
relative way, with a simple field-strength meter. Especially if you
are using a fixed (non-rotating) antenna, and assuming the antenna's
response doesn't vary too drastically with frequency, a field strength
meter fed from a fixed antenna (always in the same position) can be a
help: you can tune the transmitter for maximum field strength, and
you can check that the field strength stays reasonably constant from
one day to the next.

As Owen pointed out, measuring the current at the antenna feedpoint is
a reasonable way to monitor the power fed to the antenna. So long as
the resistive part of the feedpoint impedance is constant, higher
current means more power fed to the antenna. For most "full-size"
antennas, most of the power fed to the antenna is radiated, and
anyway, the ratio of power radiated to power dissipated as heat should
remain essentially constant, so the more power fed to the antenna, the
more power radiated. It's common for AM broadcast stations to monitor
their output power using an RF ammeter at the base of the antenna
tower (at the feedpoint), with the feedpoint impedance having been
measured accurately so that the power can be calculated (I^2 * R).

Many (most?) modern transmitters operate at maximum power output into
a very limited range of impedances, typically 50 ohms, so there's
often an incentive to make the SWR on a 50 ohm line connected to the
transmitter output be low. However, you can put an "antenna tuner"
between the transmitter output and the antenna feedline to transform
the impedance seen at the input to the antenna feedline into something
close to 50 ohms, if it isn't already. Perhaps your transmitter or
transceiver already has an antenna tuner built in.

But also, please note that you can't use antenna feedpoint current to
compare between two different antennas, unless you know the feedpoint
impedance of each antenna, since the power depends on not only the
current, but also on the resistive part of the feedpoint impedance.
Power = resistance times (current squared). Even then, there's the
possibility that one antenna is significantly less efficient than the
other, and you have to take that into account, too, if you are
interested in power actually radiated. That would be true, for
example, for antennas that are very small compared with a wavelength:
for example, for a mobile antenna for 3.9MHz. There are other factors
to consider, too, like directionality and polarization and just what
it is that you're trying to accomplish (where the station on the other
end is, and what the propagation is like between that station and
yours).

You can go into a lot more depth about all this if you want. You
should be able to find some good books about antennas and how to feed
them. I'm not very current on what's good in this area; maybe some
others will post some practical references.

Cheers,
Tom