Tom Horne wrote in
:

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.


If you do not change anything about your antenna system (and that
includes the feedline if it carries significant common mode current),
more current means more power radiated, radiated power will be
proportional to current squared.

If you knew the radiation resistance and current, you could calculate the
radiated power, couldn't you. But, measurement of radiation resistance is
a challenge. For some antennas that are known to have good efficiency,
you could assume that the feedpoint resistance is almost entirely
radiation resistance.

If by changing the VSWR, you mean that you change the antenna is some way
that changes the radiation resistance, then the above does not apply
because now two factors, current and resistance are changing).

If you adjust say an ATU on the transmitter side of the current
measurement, maximum antenna current would usually occur near minimum
VSWR, but not necessarily at exactly minimum VSWR.

But, you could do things that reduce VSWR and reduce radiated power. For
example, if you insert a 10dB attenuator in the output of your ATU, you
will improve the VSWR, but in most cases, reduce the power radiated.

For most radiated power, you usually need to strive for efficiency rather
than slavish pursuit of low VSWR. It is not as easy to evaluate
efficiency, probably the main reason people focus on VSWR as it is real
easy to measure! Viewing the world through a VSWR meter is a very limited
view.

I offer an example of efficiency over VSWR: my 40m antenna is a dipole
that is cut a little shorter than a half wave (yes, non-resonant), so as
to cause a 1.5:1 VSWR on the RG6 feedline (yes, the target VSWR is
greater than 1), and the feedline is cut at a length that results in a 50
+j0 (VSWR(50)=1) load that I can plug straight on to my transceiver. (You
can read about it at http://www.vk1od.net/antenna/7MDipole/7MDipole.htm
..)

The bottom line is that transmission lines are a very interesting topic,
and a sound understanding is better than learning Rules Of Thumb (ROT)
from others, they are often wrong. With knowledge you can make informed
design choices rather than excluding a whole lot of worthwhile solutions
because they breach dumbed down or false ROT.

Owen