Topband: VP8STI Humor, Design engineer trauma
k7tjr at msn.com
Sat Jan 23 15:28:46 EST 2016
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?
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