Stuart<
Thank you for the insight. I calculated the % the wire lengths were
off by % the frequency was off, and I also "moved" the model up in
frequency (by shortening wire lengths in the model) by the amount it
differed from the measurement. In other words, the actual was about
200 KHz below the model, so I moved the model up 200 KHz. The model
was at the same height as the measurements. Both techniques produced
results that were close. One thing I found was to not change the
wire spacing at the ends, but just to change the length of wires
between the main supports. I will wait until we hoist it to the
final height which I estimate to be near 30' and take some more
measurements then trim based on that. I think we can operate with
the existing lengths as the top band edge is within the 2:1 SWR
bandwidth at 21'; it may be a little off at 30' as the resonant
frequency goes down as the height is increased, but we don't have to
operate near the top end of the band. We have a LDG tuner, so it
should be able to tune ok. A comment on the vertical pattern. For
field day and special events the high angle of elevation of radiation
on 40 meters you get at a height of 30' is not a disadvantage as we
are looking for local contacts instead of DX. About 90% of the US is
in the horizontal beamwidth from RI. Just northern New England and
southern Florida is out side the 3 dB beamwidth. It used to bother
me that with a dipole 1/2 of the radiated power was going toward
Europe, which really doesn't participate in field day.
Chas, W1CG
At 05:06 PM 9/26/2005, Stuart Rohre wrote:
Charles,
With dual band close spaced beams, everything interacts.
You can try adjusting the wire lengths by the percentage you were off your
target frequency, or just operate them as they fell. They will vary with
height, and there is no compelling need to have them exactly resonant in
cases like this, as long as your are getting about the beam action you want.
Your SWR actual at 21 feet seem to be OK for operations. Or just use a
tuner to touch up the match.
Stuart
K5KVH
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