Well, if I understand Carl's proposed antenna - he is proposing enough
vertical height for two 1/4 wave ground plane antennas, one above the other.
In that case I would elect to use a vertical dipole. That would place the
current maximum at least a 1/4 wave above ground - reducing the ground
losses - and no radials needed. Depending on frequency, Carl's 6-12' spacing
from the tower is really tight, and much would depend on the electrical
height of the tower. Doesn't sound like a very attractive radiator.
I agree with Tom, regarding the "stacking gain" of the quad. Quads can have
other advantages, like less sensitivity to precipitation static, improved
bandwidth, reduced turning radias, and more attractive driving point
impedances, less corona discharge, but the "stacking gain" at 1/4 wave
spacing is really not significant.
Regards,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Friday, September 06, 2013 10:01 AM
To: topband@contesting.com
Subject: Re: Topband: Are stacked verticals feasible?
> If I am reading the question correctly, aren't we talking about
> something that is done at VHF/UHF with great regularity? Stacked
> vertical elements, stacked vertically polarized beams and all manner
> of stacked vertical "anything" are done there all of the time to avoid
> cross polarization loss when the other stations (especially mobile) are
the main users.
>
Stacking compresses beamwidth in the plane of the stacking. It's nothing but
a collinear antenna placed vertical.
Stacking gain depends on individual element directivity and spacing between
radiation areas (which are the current maximum areas).
Much of the stuff with VHF or UHF Ham antennas is just a gimmick with
completely false gain claims. This is because Hams have a false idea that
two antennas have 3 dB more gain than one antenna. If we really look at it,
spacing has to be pretty wide (typically almost 3/4 wave) with broad pattern
antennas like verticals to get near 3 dB, and that would be with zero
feedline loss in the stack. It takes a commercial 150 MHz antenna about 20
feet to make 5 dBd gain. It takes a Ham manufacturer less than ten feet to
make 6 dB gain. Someone is clearly misleading people, and I doubt it is the
commercial people.
Directional antennas like Yagi's are even worse. The more directive each
stacked cell is, the wider spacing has to be to get near 3 dB gain. In
practice, peak stacking gain is rarely over 2 dB. This is especially true if
ground gain already compresses the pattern in the same plane as stacking. My
40M stack of two 3-element full size Yagis, spaced optimally with a height
limitation of 200-feet, only has about 2 dB stacking gain. That's a lot of
work for 2 dB. Adding a third antenna, even going over 300 feet limit, adds
even less gain.
What mostly makes my 40 meter system work is location and propagation, not
the big antennas on a 200 ft tower. Because I'm in a rural location, I can
hear and work DX that people with very similar antennas just 20 miles away
near populated areas have no hope at all of hearing. I could probably outdo
a Yagi stack located in a nearby city area with a regular dipole.
Now imagine those quad people who "think" two half size Yagi's stacked 1/4
wave apart (that's all a quad is) have 2 dB gain! The truth is, the gain is
zero to 1 dB depending on height.
Gain is all about the spacing between high current areas, and the initial
pattern. But results are mostly all about location and local environment.
> So understanding that it is done at those frequencies, the answer to
> the original question of "can it be done," so to speak, is a resounding
YES.
> I just don't have any idea how you could extrapolate that to MF (160
> meters)...... It would be a monstrously tall structure..... he he he.
> Actually, I have a set of stacked vertical beams that I use for a
> point-to-point link with a marginal repeater from my cabin up in the
> high country on the Mogollon Rim in AZ...... It is an incredibly
> effective antenna that was much less so with a single vertical
> beam..... Hopefully I didn't just waste everyone's time by
> misinterpreting the question..... :)
> :)
>
The system described can be done, but the gain would be near zero. The gain
could also easily be negative, and with the described scenario, would never
be noticeably more than just a regular old vertical dipole. It's a
complicated picture, especially when at VHF with multipath. Things often are
not what we imagine.
73 Tom
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Topband Reflector
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Topband Reflector
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