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

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 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.

For the same amount of delivered power, the highest
current will occur at the highest SWR assuming a
current maximum point exists on the transmission line.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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

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

Owen Duffy wrote in
:

....
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.

I should have written that clearer... you will improve the VSWR seen by the
ATU, ie upstream of the attenuator. Of course, the attenuator does nothing
to the VSWR on the load side of itself.

Owen

On Apr 5, 12:13*am, 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- Hide quoted text -

- Show quoted text -

Monitoring the current would be of about as much use as monitoring any
other single parameter lets say VSWR. Once optimal values are
established it would provide a useful reference to monitor the
condition or changes in your system. Should a significant change occur
it is important to find the correct cause of the change. In the case
of current or VSWr one could make adjustments to make either read the
nominal value but this may not correct the actual problem.

Jimmie

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

There's an article in the Feb 2009 "QST" titled
"Keeping Current with Antenna Performance", by
Eric, KL7AJ, that will answer a lot of your
questions about the benefits from measuring the
RF current.

Here's a tidbit for you. Put a Bird wattmeter
at the antenna feedpoint and set it to indicate
reflected power (power rejected by the antenna).

The maximum radiated power from the antenna will
occur when the power rejected by the antenna is
at a maximum - assuming the feedline/antenna
configuration doesn't change. Present that fact
of physics to your license preparation classes
and see what happens. :-)
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Sun, 05 Apr 2009 13:13:24 -0500, Cecil Moore
wrote:

The maximum radiated power from the antenna will
occur when the power rejected by the antenna is
at a maximum

This must be one of those Sparsely Hypothetical Informative Tasers
where you have to do a linguistic rubik cube to parse just what does
"rejected" mean? We have a statement book-ended with "maximums,"
hence what falls between them (another duopoly of antennas) must be
minimally significant. When we apply the random babble dispersal we
obtain the zen:
maximum from antenna antenna at maximum
Zoots alor! But of course.

- assuming the feedline/antenna
configuration doesn't change.

Assuming? This is the hypnotic cusp of self-consciousness. Why would
they change? New billing address? New area code? An existential
crisis during a phase reversal from manic to depressive? Would it
occur on the basis of rejection - like divorce (Please don't leave
honey - I can change. Honestly!)?

Present that fact
of physics to your license preparation classes
and see what happens.

Rejection would seem to be the key word here simply because facts seem
to be so sparse and preparation a ghost of the last election debate.

73's
Richard Clark, KB7QHC

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