On 1/21/08, Jim Lux <jimlux@earthlink.net> wrote:
> First off.. Most of the benefit of directional antennas is not in the
> forward gain (as you've probably found, almost anything gets you to
> within 1 dB of the absolute best) but in the management of what the rest
> of the pattern looks like.
*Exactly*. This is why optimizing for gain, to me, isn't so important.
Just as you said, at worst its -1dB down from something that is
optimized for gain (for a given boomlength). However, the "rearward
rejection" can be anywhere from "great" to "totally useless".
So in most operating situations, which form of rearward rejection is
best. I can think of two cases where rearward rejection would help:
1. Interference, QRN or QRM
2. Multipath fading
I think its obvious that if the QRM is coming from a certain
direction, then a some type of deep notch, would be very useful
because you can use that notch to help reduce interference. Ok, what
if there is more than one station interfering?
What about multipath fading? Does deep multipath fading predominantly
come from the direction of the primary station? Or can it equally be
caused by a reflection coming from the rear??? If so, is there a more
likely angle that a reflection might come from (say 180 degrees to the
main signal?)?
> This is especially so
> if you use a modeling tool that lets you put symbols and equations in
> the model, like 4nec2. You can make the end points of the elements some
> function of an angle, and then vary it:
Actually, this is why I started this whole discussion. I spent months
writing my own version of a tool to optimize a cubical quad antenna. I
don't use variables in NEC2, my program works directly on the NEC
input deck and feeds it to a NEC2 engine and reads the output deck.
So now I want to understand my design choices before I start
automated, month long runs of optimization.
-Scott, WU2X
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