Topbanders,
I have noticed from here in the southern cone of South America that the hour
before sunrise, if there is propagation, it is at a low angle of arrival
(i.e., the flag antennas are dominant)....plus or minus 20 minutes from
sunrise, the high TX dipole is the only antenna that has a chance....with
the shielded loop an occasional second. Pre-Sunrise paths can be skewed as
much as 90 degrees in the hour before sun-up.
The past few nights I have been testing a new four square receive array
located on a metal roofed building down the street and across an avenue,
plus two new flag antennas on the roof of the adjacent apartment building.
After some head scratching at the results, here are the conclusions:
All results influenced by Amazon and Pampas thunderstorms...lightning static
is used as a signal source for F/B measurement in most cases.
1. The flags (mounted on opposite edges of the roof, up 55 meters, and
actually mounted so that the wire is just off of the lip of the surrounding
wall and looking down at the street below) are devoid of any high angle
component at all, and exhibit an excellent front to back ratio either phased
in broad-side or fed individually. In modelling the antennas, I was using
height above the roof as the input, with poor soil. The model didn't match
results. I then used height above street level and the model looks more
like what I have observed.
2. The four square array of 3.5 meter bottom loaded verticals at 30 meter
spacing is on a metal roof, 60 meters up. With a simple pair of rotary
switches and relay switched delay lines, they exhibit very good front to
back and have a low angle dominance. The problem is they also look right
into leaky CATV amps on adjoining rooftops and any advantage for receive is
nullified, so they will come down and the rooftop used for a vertical TX
antenna of some sort. The current TX antenna is a half wave flat top at 55
meters, open wire fed.
3. As was the case for VS6DO, and other city dwelling Topbanders, it takes
really good propagation to break the noise barrier in a dense urban
environment. With good front to back and front to side attenuation, you can
knock down about 140 degrees of the incoming city noise, but the remainder
can be a killer in the direction of interest....not to mention mother
nature's habit of thunderstorms on weekends.
4. The arsenal of receive antennas should include a single shielded loop
for lower noise high angle, phased directional low angle, and vertical as
well as horizontal transmit antennas. All antennas being available through
a phasing system to enhance signal and kill local noise sources.
5. All bets are off if conditions are lousy as they have been since I
returned to Argentina.
Thanks for the bandwidth and I am on at sunset and sunrise every day and
overnight on weekends, mostly on 1831.0.
73 de Bob
LU/KY0C
G4VGO
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