In real life, at K1WHS, I seem to remember having problem working rovers with
low antennas (7-10ft) on 6M, and they nearly always had a drop off in front of
them.. I guess what I'm saying is, over a, say, 200 mile path, with the same
rover, there didn't seem to be much difference if he was on flat ground or
overlooking a cliff.. the signal was still a lot weaker than we thought it
should be.. I guess we really need some testing here.. Guess I'll do that in
June and report back...
bill, K1DY
> From: k4gun@comcast.net
> To: jcplatt1@mmm.com; vhfcontesting@contesting.com
> Date: Thu, 6 Mar 2008 21:10:49 +0000
> Subject: Re: [VHFcontesting] A question of altitude
>
> 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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