Hi,
I have a few questions that I have not seen addressed anywhere and I would
appreciate thoughts and comments that any of you are willing to share.
I currently use a 4-square for 80 meters (with 3/16 wavelength spacing –
approx. 52’ between the 63’ long elements and ~ 73.5’ across the “square” of
the array) with three elevated radials for each of the four legs. The feed
points are about 12 feet above ground and the tops of all 4 elements are
attached to ropes that go to the trees through which the pull ropes pass. Two
elements are almost completely vertical but the other two are vertical for ~
50’ and horizontal for the remainder and are attached to the pull ropes with
cable ties. (The elements lengths are shortened ~ 3% for mutual coupling plus
another ~ 2% due to the PVC jacket on the element wire)
Directly behind the 4-square the ground is flat and is considered to be
wetlands. On BOTH sides (running North to South) and in FRONT of the 4-square
(to the North), there is a natural ledge formation (I am on a hill top) that is
~ 12’ feet high and runs for ~ 400’ before dropping back to ground level.
Assume that the feed points in the “front” of the array (to the North) are
about 10’ – 20’ from the ledge on either side.
A simple “picture” of the elements of the array and ledge is as follows:
.
.
.
N
O
R
T
H
LEDGE LEDGE
Continues Continues
L NW NE L
E E
D D
G G
E E
SW SE
FLAT and wetlands ...........................
S
O
U
T
H
.
.
.
.
Assume that the top of the ledge is ~ 12’ high and that the feed points are
also ~ 12’ off of flat ground and are looking at the top of the ledge.
Putting aside questions of the phasing system (I use the Comtek box)
1) Are the minimum wave angles of the transmitted and received signals
primarily determined by the height above ground at the feed point? At the top
of each element? Or some complex calculation of the average height along the
entire length of the vertical legs? I am sure that having part of two elements
horizontal also affects the wave angles.
2) If the feed point is at the same vertical height as the top of the ledge
when the array is pointed towards the ledge, is the minimum wave angle affected?
3) Is the minimum wave angle increased if the feed point of the vertical legs
is below the top of the ledge? Is it a straight geometry calculation – that is
if the feed point were as 0 feet and the direction of the array was aimed
pointed straight at the 12’ high ledge from a distance of 20’, would the
effective wave angle be ~ 35 degrees? Or, in fact, is the minimum wave angle
considerably lower since the broadside of the NW – SE legs are effectively
radiating to the NE and therefore is about an additional 37’ away from the
ledge than the NE element?
4) I would rather have a ground mounted 4-square with a ground screen of about
120 radials and with all four elements being fully vertical, but I am concerned
about effects of the ledge on the wave angles when the array is “pointed” at
the the ledge.
I would appreciate any thoughts/comments that you might have. Thank you in
advance for your interest and cooperation!
73
Bob KQ2M
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