[TowerTalk] ANTENNA RESONANCE VS HEIGHT ABOVE GROUND - DATA POINTS?
Larry - K7SV
k7sv at comcast.net
Mon Aug 24 15:51:24 PDT 2009
Thanks to the several folks that responded.
After reading my first posting and the responses, it's apparent that I need
to better explain the situation.
The antenna is 6 elements on a 56 foot boom. The design is patterned after
some 5 el 15M 36ft boom yagis that we've successfully used from NR4M for
maybe 15 years. Just a different band and an additional element.
The antennas use a T-match. We tune the antennas by adjusting the driven
element length and the tap between the shorting bars between the T-match and
the driven element. We depend on the natural capacitance developed in the
T-match rather than using external capacitors. As a result, the length of
the physical length of the driven element gets pretty close to the length of
the reflector. There's really nothing in the modeling that helps with the
For the 15s we simply raised the antenna up and down on a tram line over and
over to get the match right at about 70 feet. The 20 is obviously much
larger so doing all that tramming would be a pain in the butt. We currently
have 10 feet of AB105 sitting in concrete. It has the top section of the 190
ft of 105 bolted to it. The mast and rotator are mounted in the tower
section. The yagi was built on saw horses and the plan is to raise it to the
top of the mast which will put it at maybe 20 feet. The intent is to tune it
somewhat low in frequency there to get it in the ball park and then do the
final tuning as we run it up and down a tram to about a hundred foot level.
As I said our guess based on what we have experienced in the past (based on
what we can remember!) is we'll start 200khz low and see what happens. We're
really wondering if anyone has done anything like a study that would give us
a better starting point than the 200khz guess?
73 de Lar K7SV
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