Topband: Dumb radial question

[herbs at surfvi.com]

Quoting Vanderydt Theo <ON4ATW at skynet.be>:

> Hello TopBanders,
>
  Not having time to lay down radials right away i tuned the
  vertical without and got it at 1.2/1 SWR X 0 and about 50 Ohms. > Today I
drove a copper rod into the ground at the base of the antenna and a
  number of radials have been layed on the lawn. The ground rod and radials are
attached at the bottom of the vertical. Now I read following figures from the
analyzer. SWR 1.5/1 X 0 and R 80 Ohms. Bandwith stayed the same at about 60
KHz. SWR went up a bit, but what I'm puzzled with is the increase
> in impedance? Is it normal to see a higher feedpoint impedance?  Maybe
someone can answer my question.


Theo visualize a true Marconi vertical over a lossy ground...the impedece you
have without radials is the ground loss resistence plus the antenna.  As you
improve the ground system the loss resistence certainly should decrease as you
approach the "natural" impedence of the resonant antenna over a perfect
ground...or in the case you described a better ground.  Also as the radiation
efficiency improves you should see a drop in impedence closer to the ideal or
modeled values.  In other words without a good ground the reading will be
deceptive as half your radiated power is dissapated in lossy earth in the near
field of the antenna.  My advice is to put in the best ground system you can
and feed the antenna with the proper network matching the feedline instead of
relying alone on the antenna analyzer.

Good Luck in Your Project


73

Herb Schoenbohm, KV4FZ