Dear Andy, Dave, LF Group,
My experience is that improvements to the ground system soon reach a point
of diminishing returns where the ground connection losses are small compared
to other losses - I would expect you have reached that point already. In my
case, literally filling the garden with wire made at best about 0.5dB
improvement by reducing loss resistance, compared to having about 5 ground
rods spaced a few metres around the downlead and loading coil. However
increasing the height of the top loading, even if only in the middle, does
lead to substantial improvements. One benefit of doing this is that the
effective height and radiation resistance is increased by increasing the
average height of the antenna wires. If you are in a fairly built-up
environment, the improvement is probably more than you would expect from
calculating the effective height from the dimensions of the antenna itself,
because increasing the physical height of the antenna puts it further above,
or nearer the top of, screening buildings and trees etc., that are
surrounding it. Also, moving the antenna further above loss-causing objects
leads to a modest reduction in the loss resistance. In my case, with an
inverted L about 10m high at the ends and sagging to 9m in the middle,
propping up the middle of the span with a fibreglass pole to about 14m
increases ERP by 3 - 4dB.
As to possible advantages and disadvantages of a loop compared to a
vertical, in general the loop ought to benefit from lower dielectric losses
due to the generally lower voltages. The directional pattern is often
helpful on receive. But the figure-of-eight directional pattern could also
be a drawback for a transmit antenna where it isn't practical to rotate the
antenna to avoid having nulls in awkward directions. Also, one has to think
about the scale. AA1A's loop is quite big in overall dimensions compared to
G4JNT's vertical. The radiation resistance of a loop is proportional to the
square of the area, which is proportional to the square of the linear
dimensions of the loop conductor - so when scaling down a loop, one would
expect Rrad to reduce much faster than Rloss due to reduction in perimeter,
and so efficiency of relatively small loops to be poor.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Andy Talbot" <[email protected]>
To: <[email protected]>
Sent: Thursday, February 26, 2009 4:16 PM
Subject: LF: LF Antennas - back to basics
Firstly, bearing in mind there is no scope whatsoever to raise one end
of the capacity hat which is defined by the apex of the house roof,
and the other end would also need some major sugery to raise its heigh
substantially, that only leaves the middle, ie the height of the
actual radiator. It wouldn't be too difficult to raise this to 10m
or even more with a fibreglass pole, but will I be throwing away all
the advantage by having the top-hat drooping down to 7m?
Secondly, pictures of other peoples antennas aften show a substantial
grounding plate immediately under and around the antenna base. Just
how far out is is worth going with a really substantial base. I
could cut more conductors into the ground; while the ground is still
is still soggy in March may be a good time to do this.
Any ideas please ...?
Andy G4JNT
www.g4jnt.com
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