"Buck" wrote in message
...
(My information for this comes from a number of websites including
commercial antenna sites. Needless to say, the accuracy of any
information, especially that of commercial sites is suspect.)


I have been reading about reduced size antennas using capacity hats
instead of, or in addition to, inductor loading. Most sites claim
that capacity hats reduce size with much less (or no) signal loss
compared to a full size antenna. At least one site claims a
vertical
1/4 dipole using cap-hats has gain over a 1/4 vertical ground plane.
The consensus seems to be that size for size, the antenna shortened
by
capacity hats has less loss than the same size antenna shortened by
inductors as per http://www.sommerantennas.com/gain.html (taken from
another thread in this newsgroup.)

Some claims are that the capacity hat antennas have equal signal
strength to their full-sized counterparts.

My research, thus far, my theory is:

1) any shortened antenna will have some loss compared to its
full-sized counterpart. (i.e. an 80 meter dipole shortened by one
foot using capacity hats will not be as efficient as the full length
version, even though one might be hard pressed to find the
instrument
that could measure it.)

2) antennas shortened with capacity hats have less loss than those
shortened by inductors

3) capacity hat antennas exhibit slightly more bandwidth than
inductor
loaded antennas

4) given equal length, a cap-hat vertical dipole will exhibit equal,
(or according to some sites, greater) signal strength to a vertical
monopole either reduced or full-size.



Size Loss vs efficiency of a cap-hat dipole.

Assuming my first point of theory is correct, there must be a point
in
which the reduced size of a dipole using only capacity hats is
noticeable. Continued reduction finds additional noticeable points
of
loss.

What I would like to know is approximately where those points might
be
so the 'value' of a cap-hat dipole antenna can be determined given
some acceptable size or loss.

An example might be I have 25 feet of antenna pole. I can build an
antenna with what I have. However, it may be that for ten more feet
of pole, I can have a much better signal. Should I use what I have,
or order the additional ten feet of aluminum?

Another example would be to estimate the maximum power I can run on
60
meters using a given length antenna. If a 1/4 wave dipole will
radiate almost as effectively as a 1/2 wave, then I would not worry
about adding the extra 5 or ten watts I would need to max the ERP
out,
but if the loss were significant, I would know I can leave my radio
on
full power (100 watts) without committing a violation.

========================================
Buck,

For the same reduction in height a top hat has greater efficiency than
a loading coil. It can amount to 3 or more decibels.

But a top hat gets in the way and is more unsightly than a coil.

Have you seen program TOPHAT available from website below.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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