Topband: "return" current - what is it?
k3bu at optimum.net
Sun Aug 5 06:51:21 PDT 2012
Here we go again, speaking of misinformation, peer review, spreading
false "guru" stuff.
You can not apply Kirchoff law from DC circuits to the current behavior
along the STANDING WAVE RF radiator.
Current magnitude changes along the standing wave radiator, on the
resonant dipole it is maximum at the center and minimum at the ends,
When loading coil, stub or resistor (load) is inserted along the circuit
(wire) current into the "load" and out of it can vary according to the
ratios of load values and position of insertion along the circuit
Please see http://www.k3bu.us/loadingcoils.htm
73 Yuri, K3BU.us
www.MVmanor.com home of future DXcontestvention
On Sat, Aug 4, 2012 at 1:47 PM, Tom W8JI wrote:
> 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
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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