Hi,
This also applies to quads and yagis, like Dave talked about. Quads have
very strong
electric fields in the middle of the elements opposite the current points,
concentrating the fields right where a mast or tower might be.
Loaded, trapped or interlaced yagis could have this problem also (depending
on design and construction).
Even without resonance, a large conductive object (even an ungrounded
object) in a strong electric field can mess up antenna operation. Resonance
can make a problem worse for this and other forms of coupling, but is isn't
a requirement for problems to occur.
The problem is almost certainly not exclusively with Rohn 55G. Rohn 55 is
absolutely not self-resonant in cross members or internal structure at HF,
the lowest resonant frequency in the tower structure (not related to tower
length and guy lines) is well up in the UHF range! I've had problems with
UHF antennas near many different cross braced towers, but the problem
relates to face width and cross brace length where the brace forms a "loop
antenna" with the tower rails. Rohn 55G has no electrical differences with
45 and many other towers at HF, and even at UHF differences are small.
Antennas with poor balance (like a balanced antenna fed with coax without a
balun, a quad or non-gamma matched yagi would fit this description) could
aggrivate or cause problems also.
> Myself and WO0Y came to the same conclusion 2 years ago, That's why mine
> is in a pile in a corner of the basement. It's not for sale. when my
> body no longer functions, someone will probably stake up tomato plants
> with the aluminum. I could only get mine up 60'. So I gave up on
> building it. The stations I want to talk to are Asians in the early
I don't have a 402CD, but I can't imagine the problem relates to antenna
height. It is much more likely one of antenna interaction, and we'd have to
be exact in the model construction when modeling it. Let me explain why.
When an antenna is loaded, voltage and current is very much out-of-phase
beyond the
loading coils. The antenna has the same current at each end of the coil,
but voltage on the outside end of the coil becomes very high because of
this phase shift.
The high voltage produces a very strong electric field across the coil (or
you could view it the other way around, that the strong electric field
produces a high voltage, hi hi) and between the element and anything
around the elements. Increased energy storage in the electric field is one
thing that reduces bandwidth.
Sticking a 20 meter antenna in the middle of or near that very strong
electric field will cause a lot more problems than sticking the same
antenna in the less
concentrated field of a full size monoband antenna (where the electric
field is spread out over a much larger area). You might do fine with a 40
meter full size antenna near something that is non-resonant, but not a
loaded antenna.
The problem even extends to guy lines. Even if the guy lines are
non-resonant ( just like the twenty meter beam is not resonant on 40) the
guy lines can mess up a 40 meter loaded antenna more than a full size
antenna. Keep the area beyond
the loading coil in loaded antennas as far as possible from other
conductors
or dielectrics other than air.
73 Tom
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