Keith:
I agree that typical DX propagation more commonly occurs at 25
degrees than 8. However, my situation is untypical. I'm trying to get
single band 80M WAZ. The zones that I am missing, 23, 24, 26, 27, 28,
and 30 have one thing in common. They are all very far from Maine. My
VOACAP simulations of 80 meter openings to these zones indicates that
propagation occurs in the range 6-10 degrees. Thus an antenna that
gives superior performance at 8 degrees is the more promising for
enabling contacts with rare zones.
Some comparative results at 8 degrees on 80m are as follows:
My current setup: -6.3 dBi (Straight dipole 46 feet up running
east-west, with one end bent down to fit on my lot)
Force 12 at 1 foot above ground: -5.8 dBi (A $600+ antenna mounted
low is not noticeably better than what I already have.)
Force 12 at 15 feet above ground: -3.72 dBi (Almost 3 dB better than
I have now. Worth $600? Maybe)
Force 12 at 28 feet above ground: -3.09 dBi (The structure needed to
get that extra 0.6 db might be pretty pricey)
QW Vertical with 128 radials: -2.72 dBi (Gold standard: Hideously
expensive and only marginally better than the high mounted Force 12)
Inverted L kinked to fit on my lot: -1.79 dBi ( Cheap, but beats the
the gold standard by a dB)
Two inverted Ls fed out of phase, configured to fit on my lot: -0.15 dBi
I can build two inverted L dipoles for a lot less than the $600+ that
a Force 12 costs and get 6dB better (towards the Far East) than what
I have now. Providentially, I have tall trees in just the right
places. Admittedly, my solution would not work for everybody.
73,
Steve
AA4AK
At 02:06 PM 3/5/2007, Darwin, Keith wrote:
Good info Steve,
I wonder a couple of things though. First, why 8 degrees for the
takeoff angle? That puts it low enough that you're into the ground
effect suck-out zone fairly heavily. I'd have though something like 25
degrees would be a more representative angle for typical communications
use.
Also, what is the effect on the Force 12 antenna if it is between 1 foot
and 28 feet off the ground? What if it is 15 feet off the ground (i.e.
on my garage roof)?
- Keith N1AS -
- K2 5411.ssb.100 -
-----Original Message-----
From: Stephen W. Kercel
The discussion of verticals has inspired me to do a bit of EZNEC
modeling. I plotted the azimuthal pattern at an 8 degree takeoff angle
for several different 80 meter configurations. In all 3 cases, I've
assumed average ground.
The first case is the classical full size vertical, with a quarter wave
monopole element and 128 quarter wave radials. *snip* the pattern is an
omni pattern with a signal strength at 8 degrees takeoff angle of -2.72
dBi.
The second case is the Force 12 vertical dipole, with no radials. ...
bottom of the antenna elevated 28 feet above the ground. ... The signal
at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. ... an
undetectable quarter dB worse than the ideal full size quarter wave
configuration.
How important is the mounting height? It matters. For the same
configuration except with the bottom 1 foot above the ground, the signal
strength at 8 degrees takeoff angle is -5.8 dBi. This is a quite
noticeable >3db degradation from the full featured quarter wave
configuration.
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