K4SAV wrote:
> Those are some good questions. Others have addressed this with some
> limited success. W8JI uses a RDF technique for a figure of merit. This
> is convenient because EZNEC can give you all the data to calculate it.
> It has some limitations in the way rearward gain contributes to the
> total. ON4UN tried to improve on that by developing a DMF figure of
> merit which gave more weight to the response in the rearward direction.
> That is more trouble to calculate.
Eric Scace, K3NA(?), had a series of articles in NCQ about doing a very
sophisticated modeling of propagation, likely "other ends" of the paths,
etc, to do this. They're on the web somewhere.
>
> Most noise from thunderstorms comes in a high angle, at least the ones
> which are closest to you, and those are going to be the ones which
> create the most noise. So an important parameter is to reduce high
> angle gain whenever possible. That's high angle gain in ANY azimuth
> direction.
I think that for most horizontally polarized Yagis, the dominant thing
controlling the vertical pattern is the height above the ground,
followed by the ground conductivity. Most Yagis, in free space, have
fairly symmetric main lobes in elevation and azimuth, but that's
combined with the ground reflection effects to give the characteristic
vertical lobe structure.
A bit of modeling helps (HFTA is reasonably good for Hpol antennas, and
faster to get a qualitative feel than NEC)
>
> Another item not considered by these two techniques is something unique
> to a particular station or location. If you are in an area where there
> are lots of noise sources to your rear, front to rear may be very
> important to you, and you will want to sacrifice performance in the
> forward direction to get it. If you have few noise sources to the rear,
> performance in the forward direction will be very important. You have
> to figure this out for yourself. This is applicable for front to side
> rejection also. Some receiving antennas have nulls at 90 degrees from
> forward, other don't.
I'd almost go so far as to say that interfering sources (which could be
not just noise, but strong other signals from other hams) probably are
the primary driver for what you want the pattern to look like. If
you're trying to dig out a weak DX in one direction, and there's strong
signals coming from some other azimuth, that's what you want to suppress.
The problem is in quantifying this in some meaningful way. Tom W8JI,
and Greg W8WWV, and others, have come up with generic figures of merit,
but they don't take into account the site and user specific things.
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