Ahh... I think I get it. So to make sure I have this, let me restate it. On
level ground, a 5 degree angle of radiation would actually be measured about
about 148 feet from the base of the antenna.
http://www.webmath.com/cgi-bin/rtri.cgi?c=&e=40&b=13&d=5&a=%3F
That being the case, it would seem that if I park fairly close to a drop of
some sort, I can get the low angle radiation pattern that I would if I had a
higher antenna but on level ground. That argues pretty strongly to forget
about the complexity of a telescoping mast and in favor of finding a mildly
elevated operating position.
This is why I keep asking questions here. I can't tell you how much all the
responses have helped. If nothing else, it gets me thinking in the right
direction.
Steve
-------------- Original message --------------
From: jcplatt1@mmm.com
> 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?
>
>
>
> 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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