On 6/11/2013 9:24 AM, Jim GM wrote:
So Jim, what are your final conclusions?
I was convinced by K2AV that the Yagi idea (passive reflector) was a bad
idea because it is too sensitive to ground characteristics. I went to
the Christman phasing method and it plays nicely.
Another mistake I made in NEC was trying to model the radials a fraction
of an inch above High Accuracy Ground (I'm using the NEC2 engine), so I
was making the elements too long. I switched to a much simpler model
with the MiniNEC ground model, the elements grounded with a source at
the bottom, and a small resistance load to simulate ground losses. That
gave me numbers for element lengths that correlated well with
measurement (one element in place at a time).
The physical implementation of this antenna is pretty slick. We're
using the military surplus tubular metal mast sections, supported by the
standard tripod base fixture, with sections of PVC mast section as each
leg of the tripod. There are several vendors who sell these parts mail
order or on the net (several are usually at Dayton). Each mast section
has an overall length of 4 ft, but you lose 4 1/2 inches when you mate
the sections.
One person can raise an element unassisted -- the tripod section is
about 5 ft off the ground, so you simply insert sections from below,
push them up, add the next. We have also used this mast construction to
support small tri-banders and even inverted V dipoles for FD and CQP.
73, Jim K9YC
All good topband ops know how to put up a beverage at night.
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Topband Reflector
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