Someone (I think that it was Brian Beezley, K6STI) suggested a few years
ago that HF beams may be tested before installation by pointing them
straight up in the air with the reflector just a few feet off the ground.
I have heard from many hams who have tried this, and it seems to work well.
This simulates an antenna installation height of more than 70 feet on the
bands 40-10 meters. The only thing that may cause problems is wires,
fences, etc. parallel to the beam elements near ground-level.
As far as fine-tuning the DE with the antenna on the tower, I don't have
any problem with this. A change in length of a single DE has very little
impact on gain, F/S or F/B as long as the efficiency and match remain the
same or improve. A change of a few inches of a DE at HF is reasonable.
However, I would not recommend fine-tuning a DE in a log-periodic dipole
array, dual-driven array or an open-sleeve without some guidance.
I also do not recommend adjusting parasitics without knowing what effect it
will have on the beam. Lengthening a reflector or shortening a director
will usually produce a broader VSWR curve, but it also affects the F/B and
gain too. Computer programs such as YO, NEC, AO, etc. can give you some
guidance here if you have an accurate model.
73 Roger WB0DGF
Telex Communications, Inc.
http://www.hy-gain.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|