<snip> For instance, several of the places from which I transmitted in the
last contest were close to drops. Look at a bridge. While the antenna may
be 13' above the road on which the truck is parked, the truck may be 20'
from a ledge that drops an additional 20'. When modeling the antenna would
you use the 13' or the 33'? How close or far from a drop could you be to
take this into account?
<unsnip>
I'll take a swing at this .... someone correct me if I am wrong. In a
simple way, its all about reflections. Think of the RF leaving your
antenna at some given low take off angle, for example 5 degrees up. That
same RF is leaving your antenna at 5 degrees down too, striking the earth
at a distance from your antenna that is proportional to your antenna height
and ground height, and is then being reflected back up from the earth where
it combines with the other RF. So imagine standing at the top of your
antenna and look up a few degrees (in this example, 5 degrees) then look
down the same amount (5 degrees) and see what you see. If when you look
down you see the close in ground at 13', its that ground that is in effect.
On the other hand, if you are close enough to the edge and/or your mast is
high enough so that you can see over the edge and the ground further out,
that makes your antenna 33' "high" and its that ground that is in effect.
Using some trig in your example, you can look down as much as 33 degrees
before you see the closer ground where your antenna is only up at 13'.
That means that for take off angles lower than 33 degrees the effective
ground is the ground that is 20' out and 20' down, so the effective height
is 33'. EZNEC lets you model two ground in such a way and indeed when I
looked at your simple model, there is a step change in the elevation
pattern at 33 degrees. In EZNEC when I use a simple dipole at 13' over
flat ground and then compare this to your example, your example has about a
7 dB advantage at low take off angles (I was looking at 5 degrees). That
closer in ground does have an effect on your SWR as your antenna is
"seeing" all this ground. In summary its good to be on a hill with a long
clear horizon. Even slight rises can be good. You can be back from the
edge depending on the "trig". Take a look at what is out in front of you
from your antennas perspective ... look down a few degrees and see what's
there. Here in MN where the land tends to be flat or sometimes has gentle
rolling I have found that the worst place to operate from is at the bottom
of one of these gentle rolls, bowls, or depressions. In these situations,
although its not obvious like a big hill or something in front of you, when
you look down a few degrees from your rover antenna at say 13', you run
into ground pretty fast.
73, Jon
W0ZQ
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