Hi Guys,
Some of you may be interested in the following.
Today I completed the change over from a sloper with its high point at 65ft,
to a top driven mast arrangement. I have been using the sloper ever since
being QRV in ZL, and it has served me well. I have always felt, however,
that the horizontal field component radiated by the sloper was probably
wasted under many propagation conditions. I have been planning to change
over to a top drive arrangement for a couple of years, but have only just
completed the implementation.
The top drive arrangement was described by G3LNP in an April 2002 issue of
RadComm. I have referred to it previously on this Reflector. In my case, I
use a 3-element SteppIR as the top loading. I have grounded the director
and reflector to the boom to achieve more capacity, as the standard SteppIR
comes with these floating. For my mast height, EZNEC predicts an impedance
of 37 - 300j, so I used a series inductor with about 1200 pF of shunt
capacitance at the drive end. (L match) The resistive part of the
simulated impedance was very close, but the reactance appeared about 15%
higher than predicted, and I had to add about 4 turns on to the coil.
The SteppIR is insulated from the main mast by a 6 ft length of 2"
fibreglass tube, and its control cable is wound with many turns on an old
plastic cable drum, so that it doesn't shunt out the tuning inductor. (Come
to think of it, the shunt inductance of the control cable is probably what
made the reactance appear higher than expected, as it is effectively in
parallel with my tuning inductor) I switch over from straight through coax
to the L match using two vacuum relays. The whole arrangement is in a
plastic box at the mast top.
I also measured the comparitive field strength of my ground wave at about
1000m distance before and after the changeover. I used a tuned loop,
matching transformer, bandpass filter, and log amp. I applied 10 W to the
antenna under test in both cases. In an easterly direction I found that the
top driven mast was 2.5 dB better than the sloper, and in a northerly
direction it was 3 dB better. I would be interested to hear how people
think that these ground wave measurements will correlate with DX
performance. With a relatively small change like that, the difference will
be very difficult to estimate using on-air measurements, and an A-B test is
out of the question because of interactions.
Let's hope for some improved condx so that I can try it out on the air.
73, Greg, ZL3IX
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