Topband: "return" current - what is it?
Tom W8JI
w8ji at w8ji.com
Sat Aug 4 10:47:34 PDT 2012
So I modeled a half wave dipole in free space and sure enough the wire
segments on each side of the feed point carried equal current. I then placed
a resistive load at the center of one half-element (to simulate? a lossy
"return") and now see that those segments no longer carry equal currents,
with less current on the side with the load. Can someone please explain
this?>>>
If you use enough segments so the program calculates small steps along
length, and a ground independent current source, you'll find current on each
side of the feedpoint exactly equal no matter what resistance you insert.
As a matter of fact, very little changes except loss unless the resistance
is high compared to common mode impedance at the resistor's insertion point.
Current immediately on each side of the resistance will be equal also,
unless we have enough antenna area in the segment length to allow a large
portion of displacement current compared to segment current.
The only thing adding the resistor does, if you use enough segments and
enough resistance (compared to common mode impedance at the insertion
point), is change the current distribution shape. Kirchoff's laws, or what
we derrive from them, still applies. A two terminal load or source MUST have
equal currents at each terminal.
Now if that "two terminal thing" is physically large enough to allow
significant displacement currents, that can provide a third (or more)
terminal. Nothing violates Kirchoff's law.
<<<
The Feb 1983 QST article by Doty has 2 tables where they list RF "antenna"
and "return" current measurements as they added radials to an elevated
counterpoise and ground screen. With 6 radials they measured 350 mA of
"antenna" current and 220 mA of "return" current. With 20 radials they
measured 495/445 and with 48 radials 495/495. What were they measuring?>>>
That's a good question.
1.) He used Sevik's method of determining ground conductivity, but that
method is pretty much useless. A measurement of localized current at 60 Hz
doesn't mean a thing at radio frequencies, because skin depths and
dielectric effects are so vastly different.
Anyone using a measurement at 60 Hz to determine characteristics at 2 MHz is
just kidding himself. One can have a skin depth of hundreds of feet, and the
other just a few feet. They are not closely related at all. It was a
terrible method to start with.
2.) I'm not sure what he was measuring, or trying to measure, what any of it
might mean, or how he concluded anything about efficiency or currents.
Nowhere in the article does it mention any isolation of the feedline shield.
I'm not positive what he measured, because he does not describe the
feedpoint system in detail, but it sounds like what he really measured was
the difference between current on the antenna base and current into the
radials. If he EVER read a difference there (which it appears he did), then
he had a third current path he did not account for. That could have been the
outside of the coax shield, or a ground rod that was connected to the coax
shield.
Here is the rule that absolutely cannot be broken. This rule is cast in
concrete:
The sum of currents flowing up into the antenna must ALWAYS equal the sum of
currents flowing out into a ground or grounds of some type.
If I wanted to know efficiency change, I would measure field strength change
as I changed radials using the same applied power levels for every system.
The last thing I would do is measure current distribution and extrapolate
that, through some theory, to reach a conclusion about efficiency.
The reason I say this is because I know for an absolute fact.....base
impedance can vary all over the place with unrelated or unexpected changes
in efficiency. A radial system here that made base impedance of a 1/4 wave
vertical 50 or 60 ohms delivered the SAME field strength as another system
that made base impedance 35 ohms or so. This was for the exact same height
antenna above the radial field height.
Since base impedance changes don't tell us efficiency, and since we all
should know the sum of currents at each terminal of the two terminal
feedpoint has to be exactly the same (unless Kirchhoff law is a joke), and
since field strength was never measured, the entire article is a puzzlement
to me.
73 Tom
More information about the Topband
mailing list