[Fwd: LF: Elevated radials]

Andre' Kesteloot akestelo@bellatlantic.net
Tue, 16 Mar 1999 16:12:18 -0500

vernall wrote:

> Comments are made after the earlier material from Steve and Rik:
> Rik Strobbe wrote:
> >
> > At 18:24 7/03/99 +1100, VK2ZTO wrote:
> > >...
> > >The antenna configuration I have decided to attempt first is a
> > >capacitively-loaded vertical mounted on the roof of the house.  This is the
> > >only tree-free area on the block.   The loading coil I want to place as far
> > >up as possible to maximise the current in the vertical part of the antenna.
> > > Elevated radials will cover the roof as far as possible around the base of
> > >the antenna and connect to surrounding metal roofs, sheds, chook runs and
> > >an aviary and also to ground.
> > >
> > >A picture of this can be found at:
> > >
> > >http://www.zeta.org.au/~ollaneg/images/AXSO_ant.gif
> > >...
> >
> > Hello Steve & LF gang,
> >
> > I had a look at the picture on your web-page and it reminded me of an
> > (unsuccesfull) attempt to improve my LF antenna. At my QTH it was not the
> > house but some smaller (5m high) trees under the antenna that I tried to
> > 'screen' with some elevated radials. In the beginning I was very confused
> > by the result, as putting radials over the trees increased the
> > antennacurrent from 0.72A to 0.90A but decreased (!) the signalstrength by
> > about 2dB instead of the increase of 2dB that could be expected based on
> > the higher antennacurrent.
> > Trying to find an explenation for this 4dB 'deficit' I came to the
> > conclusion that elevated radials not only screened the lossy trees (and
> > increased the antennacurrent) but also elevated the hight of the 'virtual
> > ground' (and lowered the effective height of the antenna). So in my case
> > these elevated radials gave a gain of 2dB by increasing antennacurrent but
> > at the same time also a loss of 4dB by lowering the effective height of the
> > antenna, so overal 'gain' was -2dB.
> > Of course it is not sure that you will come to the same result when
> > screening the house but I just want to warn that an increase in
> > antennacurrent not always means an increase in ERP.
> >
> > 73, Rik  ON7YD
> This is very good experimental evidence that elevated radials do reduce
> "ground losses", but simultaneously reduce effective height.  The
> theoretical efficiency depends on EFFECTIVE HEIGHT SQUARED so for an
> amateur situation with realistic limitations in the height of the top
> loading, then elevated radials that go over local trees will inevitably
> end up losing more in effective height (and the penalty is height
> squared) relative to the reduction of losses in the ground (much more
> flux goes on to the elevated radials than via the ground).
> There is also a matter of nuisance value of elevated radials, as they
> can be a tripping or collision hazard.  The general feedback from those
> who I know have tried elevated radials is that they soon "get the
> message" from the XYL and others that "they are in the way, get rid of
> them ...".
> Lots of buried radials take a lot of beating for an amateur LF antenna.
> Regards,
> Bob ZL2CA