From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 4 Apr 2001 10:22:21 +0100
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Subject: Re: LF: LF Antennas
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G3LDO wrote:
> In an attempt to improve the reception on 136kHz by cutting down the Loran
> racket I have, over a period of time, been experimenting with LF receiving
> loops. I now have one that works really well. It was used for the first time
> in a 'reverse' crossband (136kHz/3.5MHz) QSO with VE1ZZ on 21st Feb.
> Details of the construction and pictures are on the site shown below - click
> 'An efficient LF receiving loop'.
> 
> Recently, Mike, G3XDV, elevated the loading coil on his 136kHz Marconi. This
> reduced the antenna current but apparently improved the efficiency. I am
> researching material for a mobile handbook and was looking for a graphic way
> of illustrating why elevating the loading coil improves efficiency. What I
> have written is shown and illustrated on the site (I am sure that it must
> have been all done before!). Click 'Discussion'. Any comments direct or via
> the reflector welcome.

Peter,

Thanks for providing these useful pages. The following may also be helpful:

Although you deal with the matter of increasing the length of the higher 
current part of the antenna, the other way of looking at it is that the effective 
height has been increased. This then ties in with the ERP calculation.

Another potential gain is caused by the considerably reduced voltage present 
on the part below the coil. Losses due to proximity of nearby walls, trees etc 
are consequently reduced.

It is also worth saying this is effective when the part beyond the coil is quite 
small (as in your straight vertical diagrams). For an antenna with substantial 
top loading, the gains and losses (caused by the much large inductance 
needed) can cancel each other out. 

Lastly, it would be helpful if you had a What's New page - or perhaps a box at 
the top of the main page - as it is quite difficult to locate new material with the 
present structure.

73


Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net