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.
While both of these convey good information, neither totally considers
all the parameters which may be important to your situation. Neither of
these numbers accounts for the forward lobe being very flat (low gain in
the elevation direction and wide in the azimuth direction) or being
skinny in the azimuth direction and fat in the elevation direction.
Neither number will change as long as the gain at the elevation angle of
the "desired" signal doesn't change. This does affect the signal to
noise ratio of the received signal but neither accounts for it. So
neither is perfect.
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.
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.
Jerry, K4SAV
Scott McClements wrote:
>Hi,
>
>Since I am in the process of designing a new directional antenna
>(w/NEC2), I have started to think about which measure(s) of rearward
>rejection I would maximize on:
>
>1. Front-to-back - Difference in gain in main lobe direction and 180
>degrees opposite
>2. Worst case front-to-back - Difference in gain between main lobe and
>highest gain in the quadrants opposite the main lobe.
>3. Front-to-rear - Difference in gain between main lobe and average gain
>in the two quadrants opposite the main lobe.
>
>I can easily understand the pros (and cons) of high front-to-back.
>But I am trying to understand which is more desirable for a
>directional antenna that will be used on 14Mhz-30Mhz - a high worst
>case F/B or high F/R. What would be the practical difference between
>the two?
>
>In calculating F/R, to maximize performance, would it make more sense
>to average gain over a smaller range, say +/- 45 degrees opposite the
>main lobe (instead of +/- 90 degrees)?
>
>-Scott, WU2X
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