Dear LF Group,
I had quite a lot of problems with insulators on my LF antenna (basically a
40m long x 10m high inverted L with a single top wire). When I increased TX
power from about 350W to 1200W, the antenna voltage increased from very
roughly 10kV to 20kV. This led to corona occuring at the ends and corners of
the antenna wire, which tended to melt or set fire to the plastic
insulators. This was solved by attaching corona rings about 100mm dia. made
from stiff wire to each of the affected points to reduce the field gradient.
At this point, I started to get corona problems with the straight parts of
the antenna wire itself, which were cured by using thicker wire.
For the downlead part of the antenna, I switched to ceramic insulators,
since these are not flammable, allthough in themselves they would not be a
cure for the corona, since the discharge would eventually erode or crack the
insulator as well as wasting TX power. I retained the plastic insulators for
the top section of the antenna, since they are lighter and cause less sag in
the top wire, and seem to be OK with the corona rings; also, the position
they are in means they probably won't set fire to anything! I kept the
distance between the supporting metal poles and the ends of the wire about
4 - 5 m; allthough this reduces the length of the wire, having the wire
closer to the masts causes two problems - first, there is a very high
potential gradient between the end of the wire and the grounded mast if the
two are close together (this could be what caused PA0SE's rope to melt), and
second, the capacitance between the wire and the mast is increased, which
increases the RF current flowing "downwards" in the masts, reducing the
effective height of the antenna.
As far as insulator materials go, the plastic ones I used were the ribbed
"dogbone" type about 100mm long, which came from WH Westlake for 75p. I
think they are made of polypropylene. I think most plastic materials are
pretty good insulators at 136kHz, provided they are not exposed to corona.
People seem to have had problems using plastic tubes; perhaps this is
because moisture can accumulate inside. I have used thick nylon monofilament
with good results, provided precautions against corona were taken. I have
not tried phenolic laminate - I know this does absorb moisture to some
extent, allthough I don't know if this has an adverse effect on its
insulating properties. I believe silicone rubber based insulators are
becoming popular for HV power distribution, because this material is
resistant to corona, and is water-repellant too, but I don't know where you
would get it from.
As far as the relative merits of L vs. T antennas go, It is true that a T
configuration will have a shorter path for the ground currents to flow
along, and therefore reduce resistance due to this cause. Ground resistance
is accepted to be the major source of loss in large commercial LF antennas,
so a top load arranged about a central feed point should reduce the loss for
this type of antenna. But previous discussions on this reflector have shown
that the actual resistance of the ground path is only a minor contribution
to the total loss resistance of much smaller amateur antennas - most of the
loss in small antennas seems to be due to dielectric losses in the ground
due to the high voltage gradient under the antenna. So it should be possible
to reduce the losses by extending the top load, as G3AQC found out
experimentally some time ago. The length of the ground path will be less
significant than the area over which the electric flux of the antenna is
distributed, so the size of the top load will be more important than the
position of the feed point for amateur antennas (Except perhaps if you have
a G3KEV-sized antenna!)
A happy new year to all those on the reflector,
Cheers, Jim Moritz
73 de M0BMU
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