[VHFcontesting] A question of altitude

[Bill Olson]

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 at comcast.net
> To: jcplatt1 at mmm.com; vhfcontesting at 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 at 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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