Electrically, a half wave vertical dipole is not a lot different than a
quarter wave vertical fed against radials or a counterpoise, but it
requires a LOT more height to put it up. I just modeled two different
antennas in EZNEC+ as a comparison (both over medium ground):
80m Vertical Dipole = 133' of #12 wire with bottom end 10' off the
ground, oriented vertically and fed at the center
The maximum takeoff lobe is 0.24 dbi at 16 degrees elevation. The lobe
is "squashed" flat, with the pattern being -6.4 dbi at 40 degrees
elevation. The feedpoint impedance is essentially fixed at 90 ohms at
3.58 MHz, and the SWR is less than 1.8:1 from 3.5 MHZ to 3.7 MHz.
80m Ground Plane = 72' of #12 wire with bottom end 10' off the
ground, oriented vertically and fed against two 58' #12 horizontal wire
radials also 10' off the ground and oriented opposite each other
The maximum takeoff lobe is 0.42 dbi at 23 degrees elevation. The lobe
is broad vertically, being 0 dbi at 16 degrees elevation but only -1.0
dbi at 40 degrees elevation. The feedpoint impedance can be tweaked by
adjusting the ratio of the length of the vertical portion versus the
length of the radials (essentially raising the impedance over that of a
classic quarter wave vertical by feeding it slightly "off center"), and
I quit messing with it once I got it to 44 ohms at 3.58 MHz with the
dimensions noted above. The SWR was less than 2.1:1 from 3.5 MHz to 3.7
MHz.
In my opinion, there is not enough benefit from a vertical dipole to
warrant the extra height requirement, especially given the need to run
the feedline horizontally away from the middle of the antenna ... unless
of course the elevated radials are a game killer. Even then, you can
get almost the same performance with just one elevated radial if you
have room for that.
More importantly, there are ways to simulate an elevated radial without
running one 60 feet away from the antenna. Tom, N6BT, designed and sold
several self supporting loaded vertical dipoles that spiraled both ends
of the dipole to save space. They were called the ZR series, but I
don't remember if he ever made one for 80m.
N6BT has since described a similar technique that he calls the "VOR"
method (Vertical, Open Ring) for wrapping a single "radial" (actually
more of a counterpoise) in an open-ended loop around the base of a
vertical quarter wave antenna. It has the advantage that you can
lengthen or shorten the end of the looped wire to shift the usable
frequency, and by making the counterpoise loop "thicker" (a cage, or
even just two parallel wires) you can broaden the bandwidth that way as
well. Tom has supported his VOR loops with posts, but it's also
possible to support them with insulated line suspended from the same
structure as the vertical portion. The only requirement is that you
have enough space to make a single loop roughly a quarter wavelength
long around the base of the antenna. On 80m the loop would be spaced
less than 10 feet from the base of the vertical.
Just some thoughts. The EZNEC models I used are very simple and in my
opinion realistic, but of course any actual antenna could deviate from
the numbers I quoted above. And ANY vertical antenna is going to be
held hostage to the actual ground conductivity within several
wavelengths of the antenna.
73,
Dave AB7E
On 10/20/2020 9:42 AM, k7lxc--- via TowerTalk wrote:
Howdy, TowerTalkians -
Tnx for all your comments on the Greyline Performance antenna and company. Seems to be a bunch of dissension to
say the least. So what other manufacturers of vertical dipoles would be worth doing business with? I
still think the antenna fits the bill for the installation but am looking for options. Tnx! Cheers,Steve
K7LXC
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