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Re: LF: LF-antenna with top load

To: "LF-Group" <[email protected]>
Subject: Re: LF: LF-antenna with top load
From: "Dick Rollema" <[email protected]>
Date: Fri, 17 Sep 1999 13:55:46 +0200
Reply-to: [email protected]
Sender: <[email protected]>
Peter wrote:

Question to PA0SE: Dick, did you choose 1.5mm of diameter or
radius? Some programs are using "radius" other "diameter"...
I used diameter.  AO requires the material to be put in. I choose copper,
for which the program uses a conductivity of 1.7241E-08.

Here are the results from simulations with MININEC
I'm including the maximum gain as the program reports at
"x-direction" (g in dBi)
AO also provides gain figures of course but I refrained from including them
in my report because they are not applicable to the surface wave by which
the majority of contacts on LF are made.

If you want to know field strength figures for the surface wave (both E- and
H-field) you can use
the "near  field" option of AO or EZNEZ. As was explained by W7EL on an
earlier
occasion this in reality computes the sum of the near field and the surface
wave. At a distance of 2 wavelength or more from the antenna the near field
becomes so weak
that for all practical purposes the result of the "near field" computation
gives the surface wave. You can check the validity of the results by
calculating E/H (E in V/m, H in A/M). The result should be near
120 * pi = 377.
The method only gives E and H at selected points, not as a radiation
pattern.

I am happy that Peter's calculations of Rs agrees very well with mine using
AO. However this should not be taken as a proof that the results must be
true as both
programs use the same algorithm.


Refering to the last part of my second report (16.9.99 14.17) it is clear
that top loads are hardly a substitute for height. Even raising the antenna
from 10 to 15 m increases radiation resistance more than even the largest
top load can do. This is easily understood as radiation resistance is
proportional to height squared whereas the most a top load can do is
doubling the radiation resistance.

This also explains the fine signal from Steve, GW4ALG. His helium balloon
raises the antenna to 20 m and that gives him a radiation resistance of
almost 32 milli-ohm, even without any top load at all!

73, Dick, PA0SE



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