Topband: Adding radials, a question

[Sinisa Hristov]

W5UN wrote:
 
> I have a question for any of you who have experience with adding radials
> and making measurements of the changing impedance on a 1/4 wl ground plane.

...


Please describe your measurement setup and procedure.

Assuming that nothing went wrong with measurements,
there are two possible explanations of your findings.

The first (and more probable, I'd guess) explanation
concerns the use of 1/2 wl surface radials.
It is rarely mentioned that such wires are resonant
and have standing waves on them (YU1CF and myself
verified this by actual measurements last December).
And standing waves imply impedance transformation.

In the case of 1/2 wl surface radials,
ground loss resistance gets transformed
to a substantially higher value as the
feedpoint moves away from the current maximum.

This may be viewed the other way around.
The amount of current entering a 1/2 wl surface radial wire
transforms to a substantially higher current
in the middle of the wire. And higher current
means more power lost in soil, resulting 
in higher feedpoint resistance.

This means that it is possible to INCREASE ground
losses by increasing the length of surface radials
from 1/4 wl to 1/2 wl. The effect may be illustrated
by EZNEC modelling of radials slightly above the surface.

Therefore, increasing surface radial length
above 1/4 wl has twofold effect:

  * losses are reduced by improved soil shielding effect
    (preventing the field to enter soil);

  * losses are increased by the impedance transformation described above.

One or the other may prevail, depending on circumstances.

The second explanation is that the actual current
distribution changed in such a way to move the current
maximum away from the feedpoint. I'd guess that 
this probably cannot account for the entire increase
you've found.


73,

Sinisa  YT1NT, VA3TTN