n3rr@cais.cais.com (Bill Hider) wrote:
>There is a tried and true rule of thumb: Orient the antenna straight up
>(vertical) with the reflector element nearest the ground and at least 4 feet
>off the ground. (not an easy feat for a large antenna, but doable with rope
>attached to the tower and a tether line onto the antenna.)
>The SWR measured with the antenna in this position will be the same as the
>antenna oriented horizontally at the operating height on the tower at a
>height a quarter wavelength or higher.
>This works FB for Yagis I have tried and was written up in QST within the
>last two years. Someone on this reflector will give you the exact QST
>reference, I am sure.
What Bill Says is true under certain conditions, and not true under
others. More specifaically the antenna in question must have a pretty
good front to back at the frequency being measured. An improperly
tuned beam with a poor front/back ratio will give significantly
different results when hoisted in the air. A beam with a very good
front/back will perform very nearly the same when pointd vertically
or horizontally as noted above.
Because the front to back ratio can change dramatically on some beams
as you tune accross the band trying to generate SWR curves on beam
mounted low and pointnig up vs horizontally at height can give different
results. Thus it is important to know what the front to back characteristics
of the beam in question are before you attempt this short cut approach
to checking the tunning.
Another thing I have found in using this technique is that some beams
can be detuned slightly by the tower as well. As a result I usually
mount the beam in question on a 5' section of mast and "push it out"
away from the tower.
I suspect (but have not tried it) that going up higher, say with the
refelctor at about 15 ro 20 feet would reduce the front to back effect
noted above.
More food for thought ~8^)
Dave
N0DH
dit dit
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