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[VHFcontesting] A question of altitude

To: k4gun@comcast.net
Subject: [VHFcontesting] A question of altitude
From: James Duffey <JamesDuffey@comcast.net>
Date: Thu, 6 Mar 2008 19:52:28 -0700
List-post: <vhfcontesting@contesting.com">mailto:vhfcontesting@contesting.com>
Steve - This is a good question and one I have been struggling with as  
well. I am a new, 3 time rover, so we are in much the same boat.

After some consideration, study, a little analysis, and conversations  
with other rovers, I have come to the conclusion that, as a general  
rule of thumb, the minimum height for a rover antenna should be a half  
wavelength over any conducting surfaces a half wave or larger within a  
couple of wavelengths of the antenna.

A half wave is the lowest height above ground that has no overhead  
radiation and starts to concentrate the radiation at low angles. Low  
angle radiation is the key to working troposcatter and most of your  
rover contacts will be by troposcatter at least when sporadic E is not  
in.

The key to troposcatter concentrate as much radiation at low angle as  
possible. This maximizes the volume of atmosphere available to use as  
a scattering volume. Some studies indicate that for every degree the  
angle of radiaiton is lowered, there is a gain in signal to noise  
ratio (not antenna gain) at the other end of about 10 dB. So the  
significant parameter of interest to look at is the angle of  
radiation, and gain at that angle, rather than just straight gain.

While this is fairly easy to achieve on two meters, where a half wave  
is about 40 inches, it is harder on 6 meters where a half wave is 10  
feet on 6 meters, at least if you want to do it in motion. If I recall  
correctly, you have a pickup truck. As a minimum, I suggest that you  
try to have the 6 meter antenna at least 10 feet above the bed and the  
2 meter antenna at least 40 inches above the cab roof.

If you can't achieve this in all directions, park so that the  
direction where the antennas clear the ground is toward the direction  
where you want to have the loudest signal. I have a station wagon and  
the 6 meter antenna is at 14 feet, so I am acutely aware of this.

Drop offs help, but you need to be fairly close to them.

You don't need to use the same strategy in every contest. My gut  
feeling is that the telescoping mast is the way to go, at least in  
September and January where activity is low and the probability of  
sporadic-E low. In June, where there is more activity and 6 meters is  
hopping (not in 2007 though) you may well be able to forego the  
telescoping mast and come out ahead because of the increased activity.

How is the CW going? That is the easiest way to add 10 dB to your  
signal. - Duffey

--
KK6MC
James Duffey
Cedar Crest NM





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