Topband: Ground mounted 1/2 and 1/4 wave verticals (was GAP)

[Tom W8JI]

> Dave... would it be a fair extrapolation to take your last sentence, and 
> draw the conclusion that if adding radials changes feed impedance, then 
> there was actual ground loss in the near field? Or that if we add more 
> radials and feed impedance change is not seen, then we are at a minimum 
> for ground loss?
>
> The above statements certainly align with my gut feeling, but my gut 
> feeling is different than a mathematical proof :-)
>

I'm not Dave, but maybe this will give you something else to think about.

Once energy leaves the antenna, an external change can have the same effect 
on impedance no matter if it turns applied RF power to heat or more EM 
energy. If we put an antenna in a lossy media, we don't know what portion of 
resistance is from loss and what portion from radiation, unless we measure 
radiation or heat.

Also, since the antenna and radial system has standing waves, a decrease in 
resistance at one point can cause an increase in resistance at another 
point. Take a 1/4 wave transmission line, decrease the termination 
resistance at the far end, and the input end impedance will increase. Each 
radial is a transmission line, and the antenna is a transmission line. How 
do we know what is going on by looking at resistance at one point?

Even without any change in efficiency, we can move impedances all around 
when we change things. A folded monopole with equal size conductors has 4x 
the feed impedance of an identical monopole with a single conductor of the 
same effective area. If we change the ratio of conductor sizes, we can move 
feed impedance all over the place with no efficiency change.

What if we move the feedpoint? If I have a given antenna and move the 
feedpoint around while not altering phase or current distribution, feedpoint 
impedance will change all over the place but pattern and efficiency will 
stay the same.

With all of that in mind, and remembering radials and antennas have standing 
waves, why would we ever assume a resistance change at one point in a 
complex system reliably tells us about behavior at other points in the 
system? That would only be true if we understood all the complex 
interactions caused by transmission line effects, or had a special case 
where field strength and feed resistance were directly related (like by 
adding a resistor at the feedpoint).

When measuring a 40M vertical, 4 elevated radials at 6 feet had about the 
same field strength as a dozen or so radials in soil contact. The feed 
resistance of the elevated radials was in the upper 30 ohm range. The feed 
resistance of the earth-contact system was nearly 60 ohms. Field strength 
was the same.

If you want to know field strength change, measure field strength change.

73 Tom