[Antennas] Re: [Elecraft] Two Band Moxon/wire beam

Tue Sep 27 07:51:48 EDT 2005

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