Dear Gary, LF Group,
Looking at your diagram, I would avoid
having a downlead near the central mast for two reasons;
firstly, unless the mast is well insulated from ground, the capacitance between
the metal part of the mast and the uplead will result in a significant
proportion of the current going “up” the uplead
flowing back “down” the mast, resulting in partial cancellation of
the radiated signal. This implies a larger antenna current at the feed point
required to obtain the same radiated signal, increasing the losses in the
loading coil and other conductors. If you do go for this arrangement, make sure
the mast has a low-resistance direct connection to the ground point of the
antenna feed, in order to minimise the losses caused by these circulating
currents. Secondly, there will be a high voltage stress between the uplead (carrying the full antenna voltage)
and the mast (more or less at
ground potential), increasing the likelihood of problems with corona.
I think the optimum arrangement would be
to put the loading coil on the garage roof (but spaced from it, especially if
made of corrugated iron or other lossy materials),
close to the sloping end of the wires, to which it could then be connected with
a short downlead. This minimises the amount of high
voltage conductors close to ground level, which have minimal contribution to
the radiated signal, but do lead to increased loss. The fibreglass top section
of the mast will maintain good separation and insulation between the main mast
and the antenna wires – when using a fibreglass pole in a similar way to
support my antenna, I found it was necessary to add a
corona ring around the fibreglass tube to prevent burning where the wire joined
the mast. As far as the arrangement of the top loading wires goes, the main objective
is to maximise the effective height of the antenna. If the antenna was a simple
vertical 14m high, the effective height would be 7m. Adding loading wires above
the 7m level will lead to an increased effective height, whilst adding wire
below the 7m level will reduce the effective height compared to the simple
vertical. I’m not sure if there is any one “optimum”
configuration – it would depend on all the geometrical restrictions in
the particular situation – but as a general rule, for an antenna with a
single mast like this it would certainly seem sensible to maximise the amount
of wire above ½ the mast height, and eliminate as much as possible below that. This
would also keep the high voltage wires well clear of the aluminium supporting
poles. However, some compromise may be required here, since trimming the
loading wires too much would reduce the overall capacitance to the point that
it is difficult to make a big enough loading coil to resonate the antenna that
does not have excessively high losses.
Cheers, Jim Moritz
73 de M0BMU
