Hello again Mike, Thanks for the question,
I have had as many as 4 of the directional Hi-Z and a new undisclosed
design experimental Low impedance element arrays running at one time for
comparisons. Some over 1000 feet apart.
From the plots of Eznec there is scant 1.4 dB RDF difference between the 4-8
Pro and the superior all active 8A. Also what I have observed and have nothing
but anecdotal evidence for is that the all active 8A seems to outplay to the
ears what Eznec or 4NEC2 predicts using RDF alone. These NEC comparisons are at
a 20 degree angle of incoming signal elevation. If you look at the 3D plot
comparisons at http://www.hizantennas.com/receiving_antennacomparison_char.htm
you will see that the side lobes put there purposely to get the RDF up in the
8A are very very low in elevation. I have always conjectured that this was the
reason that the 8A would outplay the 4-8Pro Eznec comparisons in my fields.
Assuming that local noise and noise in general is higher angles these low
elevation side lobes would not pick up the high angle noise. This could be
responsible for an observed increase over RDF performance. There are a lot of
variables involved so it remains conjecture at this time.
The slightly narrower forward beam of the 8A may also be responsible. When
you start using antennas with this kind of pattern squeezing to make high RDF
it is absolutely imperative to be at least 1000 or more feet away from power
lines and TX antennas as it takes so little coupling to change the pattern and
resultant RDF. This makes comparisons very difficult.
For instance, it is theoretically possible to take the same 8 element 200
foot diameter circle and get over 15 dB of RDF with a 31 degree beamwidth, a
horrible looking pattern, and 22 dB front to back. The absolute maximum
directivity according to MT Ma and his book. It takes enormous precision in
everything involved with this design including likely impossible electronics,
cabling, ground incontinuities and environmental differences. Not to mention
the narrow beamwidth making the need for 16 elements in this footprint to cover
the entire azimuth. I do not believe that you could make this antenna work
without a mile or better separation from other objects of any kind.
Short answer, 8A is 1.4 dB RDF better by Eznec.
Bear in mind the fact that one can never have enough receiving antennas at any
given time. The ionosphere is a fickle beast at best.
Lee K7TJR Central Oregon
Thanks. Assuming for a minute that it wasn't the location, how might that
antenna have been superior?
73, Mike
www.w0btu.com<http://www.w0btu.com>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|