Topband: Topband Digest, Vol 116, Issue 13

[wa3mej at comcast.net]

Tom, 

  The only problem with measuring field strengh is that mos people dont have the tools to do it correctly or accurately. It really takes a total system of antenna, Feed line and Analyzer all of which are of known and traceable uncertianty. Otherwise you can only get relative measurements which allow us to compare one system/configuration to another but will not give you an accurate field strength measurement. You should also be in the antennas farfield to get good reliable measurements. 

Having done these types of measurements years ago when I worked at ECAC in Annapolis MD. I can tell you it is not easy and takes a lot of care to get good, consistent results. One can however make the case that for this endevor the NBS traceable equipment is not necessary all you really want to do is tell when one configuration is better or worse than another. One of the other problems I ran into doing this  type of measurement from home is none of my equipment is battery powered .. 



Jim   


------------------------------ 

Message: 3 
Date: Sat, 4 Aug 2012 13:47:34 -0400 
From: "Tom W8JI" <w8ji at w8ji.com> 
Subject: Re: Topband: "return" current - what is it? 
To: "Bob Kupps" <n6bk at yahoo.com>,        "topband" <topband at contesting.com> 
Message-ID: <25C910C29361408E9563B20E98595197 at tom0c1d32a93f0> 
Content-Type: text/plain; format=flowed; charset="iso-8859-1"; 
        reply-type=original 

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