From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 5 Apr 2001 09:41:17 +0100
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Subject: Re: LF: LF Antennas
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G4CNN wrote:

> Regarding the use of a high loading coil, Mike (XDV) comments:
> 
> >                     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. 
> 
> Perhaps I have misunderstood your point Mike, but this sounds wrong to me.
> The less top loading, the greater the inductance required for resonance.
> With little top loading above the high coil, a very large inductance is
> required to have any effect, since the capacitive reactance will be
> extremely high. A small coil will have little to no impact on this
> reactance.
> 
> It would appear to me that the reduction of high voltages in those parts of
> the antenna that are near ground, trees, buildings, etc. is probably where
> the most gain is made. Moving the current node up the antenna obviously
> makes a difference, but the reduction in losses because of the lower
> voltages is probably much more significant. The high voltages are still
> there of course, but they are now confined to the top load, which hopefully
> is well in the clear.  

Taking your own philosophy of experience being better that theory, the 
experience of myself and others is that a short antenna will benefit from an 
elevated coil much more than a long one. The argument about close-in 
absorption applied to both types of antenna.

The point is that with, say, an inverted-L, the purpose of the horizontal section 
is to load the vertical section such that the =effective= height is maximised. A 
good size top section - perhaps 50m long - will already make the effective 
height very close to the physical height. Therefore the gain to be made is 
small. In this case the coil loss will probably be more than the gain available, 
unless a very low loss coil is used, and this can be too heavy or bulky to 
elevate.

With a much shorter top section - mine is some 18m - the gains to be made 
from an elevated coil are much more, and greater than the coil loss.

I agree that there is a point where there is a trade-off between coil loss and 
the inductance required, but I think is much less significant than the trade-off 
between effective height and coil loss.

In any case, it works and that is the main thing.

I have a 7mH elevated coil but it is quite lossy. My plan is to use my similar 
size 3mH Litz wire coil and load it with ferrite. This should be a better 
compromise between loss and height.

73


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