Hi again Richard.
I think the answer is dont waste your time. LF/MF aerials are so short and
depend so much on ground and none of the systems model them very well at
all. Roy Llewellin admitted as much at an Internet lecture a few years ago
at the HF Convention. Laurie G3AQC and Peter G3LDO have done some work with
EZINEC. It would be a disaster for instance, to fix tuning component values
on the basis of a model.
In particular the programs dont seem to model the reduction in ground loss
with increased top capacity, which is the effect you may have with a short
low aerial. I can imagine that modelling ground strata of a rocky hill top
will be almost impossible. At our level LF and MF aerial work has proved
that empirical techniques are the best. Hence my cry of "measure it with an
RX bridge", I have found that is the only way to really find out what is
happening, and quickly assess changes.
Graham mentions the increase in gain from top loading but it is very small
about 1dB. the main advantage of top inductive loading is to reduce the
voltage on the vertical section so that the loss which is coupled into to
surrounding environment, trees and buildings, is reduced. You never see it
in commercial aerial systems because they just dont have that effect.
The thought of you abseiling down a cliff gives a whole new meaning to
aerial rigging. Yes you could increse the capacity of the lower end to
ground with insulated (from ground) radials. By the time the current gets
down there it has traversed the Radiation resistance and an increased
capacity with reduce the ground loss. It could work quite well!! The top
feed with be just the same as a normal vertical feed.
Dave G0MRF made the 1st trans-atlantic 136kHz crossing using an "urban
cliff".... throwing two wires out of the top-floor flat of a tower block in
West London, and feeding it again the water-distribution system (I believe)
!!
LF stands for "Loadsa Fun" and MF for "More Fun" :-))
Best Wishes
Alan G3NYK
----- Original Message -----
From: "Richard Newstead" <[email protected]>
To: <[email protected]>
Sent: Sunday, May 24, 2009 8:16 PM
Subject: LF: Modelling aerials
> Dear MFers
>
> I am trying to model various aerial configurations for 500kHz using NEC.
> I have set the antenna 0.5m above ground as NEC2 does not like radials
> on the ground. I have used "real ground". Has anyone compared the
> calculated Source impedances to real-world measurements at 500kHz? I am
> trying to understand the likely range of impedances that I need to match
> over and hence the ATU types I need to employ and the components needed.
>
> Any modelling tips will be welcome.
>
> 73
>
> Richard
> G3CWI
>
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