>>Actually it is a little more complex than all of this (isn't life like that)
>>because of the changes in mutual coupling, etc. But for a first
>>approximation (read guess) you can spray two directions knowing that 1/2
>>your power is going in a second direction and that power is having little
>>effect on the half that is going in the first direction.
>
>John and co. - I think you have the essentials correct - especially
>the first part of the above statement and the rest of your posting.
>But I would not take even as a zero-th approximation the second part
>of the above statement. Consider this: First, unless the "stacked"
>antennae are many wavelengths apart each will ALWAYS be in the near
>field of the other. Therefor, you can never consider the pair (or
>mulltiplet) as sepreate antennae. You must evaluate the sum total of
>amplitude and phase of the radiation patterns, which will be very
>different than the scalar sum of the patterns that would sugest that
>you can spray two dirrections equally. This is basic physics, and is
>the way it is. Secondly, as you point out, some arbitrary point a
>very long ways off doesn't "know" there are two antennae at a given
>QTH. This only underscores my first point.
>
>I have not modeled this specifically, but think how the wakes of two boats
>moving in different directions interact on the watter - especially when
>they are just within several boat lengths from each other.
>
>Just my two cents worth - for what it's worth...
>cheers, Chuck Claver, PhD.
> de NJ6D
Chuck, we agree on everything but style! HI Of course you need to do the
modeling to see what the exact results are, and clearly the antennas are in
the near field of each other. But I was trying to make a very simple
(simplistic was the word I used in the original note) description and that
description does fit what is observed by the many CONTEST operators who have
a two stack that can be pointed to spray in multiple directions.
It is fortuitous that for most common spray modes the antennas are pointed
at approximately right angles to each other, thereby minimizing the
interactions. Typically on 40M in the afternoon and evening they would be
pointed (from Colorado) at 35 deg (Eur) and 125 deg (Car). At midnight the
Eur goes to JA (320) and the Car goes to Eur (35). And after Europe drops
out the Eur goes to VK/ZL (230). (The choice of which antenna goes where is
a function of height, angle of radiation, and expected benefits (number of
potential qsos and mults) from one over the other in any given direction.)
Of course this works for Colorado but the pointing directions are somewhat
different for other parts of the country.
To summarize my points.
1) Two Yagis pointing in opposite directions on the same tower aren't
necessarily 180 degrees out of phase. You must also take into account the
spatial position of the phase centers of the two, in addition to the "phase
flip" on the driven elements. More easily visualized when talking about
long (in wavelengths) VHF antennas, where, when they are counter rotated the
driven elements could be multiple wavelengths apart.
2) It is a complex problem for accurate analysis and requires the use of
modeling tools but my description of what is operationally observed is
accurate. On receive, when switching from the two antennas spraying in
different directions to only one antenna there is never any dramatic
increase or decrease in the received signal from the direction of that one
antenna. Only a reduction in the qrn and qrm contribution made by the
pickup of the second antenna.
3) For DX contesting from the US, the major population and multiplier
centers are located in directions such that the antennas are at right angles
to each other and therefore have the least interaction. So it is not very
common in normal operation for the antennas to be pointed at 180 with
respect to each other.
4) Trying to explain things in simple terms, yet hinting at the complex
technical issues underlying the explanation, often results in the
sophiticated reader feeling that the answer is inaccurate or incomplete
(which it is and was intended to be). Any short, five-paragraph answer
without mathematics on a relatively complex antenna topic will always be
incomplete. I just hope the intitial answer as well as the dialog that it
has spawned is illuminating to the intended audience.
73 John W0UN
John Brosnahan
La Salle Research Corp 24115 WCR 40 La Salle, CO 80645 USA
voice 970-284-6602 fax 970-284-0979 email broz@csn.net
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