Topband: RDF

[Tom Rauch]

> I have been hunting for more information on RDF, which Tom Rauch defined
> the other day as "the difference between  average gain and gain in the
> desired direction and angle."  I get the concept, but also recall a
> statement that the RDF could readily be computed using NEC-2.  I can't
> figure out how -- can anyone point me in the right direction?

Later Windows versions of Eznec, as others have explained, calculates
average gain of an antenna when you do a 3d pattern plot.
This "average gain" is the radiation (or reception) efficiency of the array.
You simply find the difference between average gain and gain at the desired
signal direction and angle to calculate RDF.

As W4ZV points out, there are some short term shortfalls in using RDF to
choose antennas. Over time, however, it is by far the best method. The
exception would be if you ALWAYS had grossly dominant noise only from one
direction. The dominant noise would have to be so strong as to exceed
distributed background noise by the null depth ratio between an antenna
selected by RDF compared to a F/R selection in order for a F/R selection to
be valid, and would have to do so with some consistency.

For example, assume noise from a rearward quadrant was 20dB higher than
average noise from all other directions. Once the array had greater than
20dB F/R ratio you could simply quit worrying about looking at F/R averages.
Once the spot noise is down in the average noise, any additional depth is
meaningless. At that point RDF takes over.

Another VERY important thing is when we work DX at local sunset or sunrise,
the rearward area is looking into a zone of poor propagation. I see a 5-10dB
noise drop to the east at sunrise, and a 10-15dB drop to the west near my
sunset. This is because distant noise does not propagate in through the
daylight areas. In this case, F/R is virtually meaningless and probably is
"over considered" even in RDF. Another thing is when we look into an area of
good propagation, noise is enhanced from that direction also. The same
mechanisms that enhance noise propagation enhance signals, so we had better
consider beamwidth (which RDF does).

RDF does not work well for local noise, but than nothing else will either.
That's because Eznec and other programs do patterns at "infinite" distance
and do not show true response along the earth. They have no groundwave. If
your modeling program does not have an input for distance, you can be sure
it ignores groundwave. As such, there isn't an accurate model or method for
those of us limited by local noise sources. RDF is exceptionally good for
comparing similar antennas, such as a single Beverage to phased Beverages.

I think there are very few skywave noise cases where anything but RDF
applies. Even while it is far from perfect, it is the best overall method.
If you have a case like those of us in the SE USA do, where a certain land
mass has frequent thunderstorms (Florida and S Ga), then it might pay to
always be sure to have a deep null over that area. Even so, I would never
pick the antenna exclusively based on the ratio of average gain in the null
area to gain in the desired direction. That would be a serious mistake.

At my QTH, antennas that have a poor 15dB F/B hear just as well or better
than antenna with huge 40dB F/B ratios when the forward BW of the modest F/B
arrays is narrower. The exception is summertime, when thunderstorms are off
the rear. The worse thing you can do, over time, is go for extreme F/R at
the expense of HPBW!

73 Tom