FYI. This is off the Force 12 reflector and I found it very interesting
reading.
Cheers,
Steve K7LXC
TOWER TECH
>> Message-ID: <008501c2c3b4$68b4bc80$cf457443@main>
From: "K2KW" <k2kw@prodigy.net>
To: "Bill Brannick" <WmLB@attbi.com>, <force12talk@qth.com>
Subject: [Force 12 Talk] Vertical Installation Height
To: <towertalk@contesting.com>
Date: Fri, 24 Jan 2003 06:25:12 -0800
> I wonder if anyone as done any "real-world" comparisons between installing
> the antenna at ground level (feed point < 12 feet) vs. putting it up on
> the roof (> 20 feet). I see in some of the technical notes from Force 12
> that suggest some effect on the higher frequencies as the distance between
> the feed point and the "dirt" increases. But there are other comments that
> say that antenna efficiency (?) is not effected regardless of the
> installation configuration.
Hi Bill,
While I haven't played with the Sigma 5 specifically, I have installed a few
hundred verticals over the past years (no joking). Here's some theoretical
background on your question:
Efficiency of a vertical antenna does not change with height, but
performance will change. As you noted, all verticals are impacted by their
surroundings: trees, fences, houses, etc. Some will sap energy, some will
distort the pattern from a true omni. The result being that the development
of the wave front is impacted, which may degrade performance. By raising
the vertical you will be less impacted by these items, allowing the wave
front to develop better. BUT - there is ALWAYS a BUT with antennas.
With the vertical mounted on the ground, you get one big elevation lobe.
That means you have equal radiation from around 12 degrees take off (for
average ground) to 30-35 degrees, with useful energy to around 45-50 degrees
take off. These take off angles cover most amateur communications (that's
one of the great things about verticals). If the vertical is mounted over
salt water, you would get equal radiation from around 0.1 to 35 degrees
elevation (better for low angle DX).
As you may be aware with horizontal antennas, if you raise them, the
elevation angles change. The general rule of thumb is that you get one lobe
split for every wavelength in height. Well, the same is true for verticals.
The higher you raise the feedpoint, the more lobe splits you will get.
Let's assume that the Sigma 5 is a full size (physical) on all bands. That
would put the feedpoint slightly above 1/4 wavelength on each band. In
doing so, you only slightly squash your elevation angle on the upper angles,
and get a slight increase in gain (compared to a 1/4 vertical with the
feedpoint on the ground) at the lower angles. The tradeoff is well worth
going to the vertical dipole.
Now raise the base of the vertical dipole to 20', and on 10-15m the
feedpoint is roughly 1.0 to 5/8th wavelength high (respectively). That one
big lobe you had when ground mounted, has split or is starting to split from
1 big lobe (as described above) to 2 lobes. The first lobe is right around
10-20 degrees (for average ground), but the 2nd lobe is up around 50-60
degrees (not very useful). The result is a big null around 25-45 degrees,
which are very useful angles for amateur communications.
Raising the vertical from ground to roof is likely going to be very location
specific in what you see. If the vertical was mounted in heavy clutter on
the ground, you actually may come out ahead with it on the roof. But then
again, it may not. Location of antennas is always a tradeoff. Sometimes
you just need to do things from a realistic point of view to keep the wife
and grandkids happy. But the above was meant to give you a quick blurb on
some of the theoretical differences of ground mounted vs. roof mounted
vertical dipoles.
Kenny K2KW
Leader, Team Vertical >>
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