Tree, I use a parasitic array on 160M. I have 4 134' verticals spaced 85'
from a 130' tower located in the center. Each vertical is in the corner of
a square. The verticals are in the normal directions, NE, NW, SE, and SW
directions. The tower has a shunt wire attached at 60' which dropps to
ground which goes through a LC tuning network. The LC network tunes the
tower for 1.745Mhz (Reflector). Each quarter wave vertical has 3/8
wavelength of coax attached to it. When a vertical is not being used it is
open ended and looks like a reflector; 2/8 wavelength vertical + 3/8
wavelength coax = 5/8 wavelength (Reflector). I also dip the unused
elements for 1.745Mhz. I have 45 quarter and half wave radials under each
of the 5 elements (Tower makes 5). The array works extremely well. Using
the unused elements as reflectors improved the F/B by 7dB. Modeled the
array has a F/B of 21dB, on some signals (esecially low angles) it seems
better. When I switch to the NE I also have a 127' director, spaced 75'
from the NE driven element. It is only grounded when I am using the NE
vertical. It's open circuited in all other directions.
Peter
W5PS
______________________________ Reply Separator _________________________________
Subject: TopBand: Parasitic Array using loaded towers
Author: LarryX Tyree contractor for brentc <larrytx@co.intel.com> at internet
To: <topband@contesting.com>
Date: 12/31/96 2:29 PM
Well - the ongoing saga of "My Two Towers" has gone into a new phase (no
pun intended).
I have been using two towers spaced 105 feet on receive using an ANC-4 to
adjust the phasing. They are in line to 45/225 degrees. Their most
stunning achievements are being able to null out KH6CC almost completely,
and also the same for local stations who might be too close to the DX.
It isn't clear that they really help much on receive into Europe over
the single tower (yes - I am using my transmit antennas on receive -
which I get away with because I am in the country).
Partly because I blew up the ANC-4 by putting 100 watts in one tower and
having it connected to the other one (without protecting it by having it
switch into transmit mode) and because I felt like doing something different,
I have tuned the southwest tower to something near 1730 kHz (as best as I
could). This was done by increasing the gamma match capacitance and
measuring the resonant frequency with an MFJ SWR meter - and with the
transmit antenna disconnected from the coax. If my calculations are
correct, this tower should now act like a parasitic reflector.
There seems to be plenty of coupling between the antennas as the SWR
for either once changes significantly when I connect or disconnect the
other one. This is furthur supported by the ex-resistor in the ANC-4
that really got excited by that 100 watts into the other antenna. The
SWR at the feedpoint is now about 2:1, but in the shack, I guess I
got lucky with the coax lengths and it is flat. Since I am using
hardline - I am not too worried about the mismatch (about a 100 meter
run).
The nice thing about this configuration is that it will work on transmit
as well. The drawback is that it is not adjustable and if it really does
work, it won't be a good antenna for trying to work XV7SW any more. Guess
I need to get that high dipole back up in the tree.
The reflector only has 6 radials on it (elevated about 15 feet off the ground).
It would be easy to add some more in the north-east direction if people think
that might help. The driven element has 20 radials on it.
I might even stick an RF amp-meter on the reflector and see if I can
determine how well it is coupling.
Has anyone taken on any similar projects and can relate their experiences?
The only other vertical parasitic array I have seen is at W5WMU. Pat has a
3 element array on 80 meters that works very well.
Of course this means the band will stink tonight - and I apologize to
everyone in advance for that.
73 Tree N6TR
tree@contesting.com
PS: This is the second time I have fried R2 in the ANC-4. I think this
must be acting like a fuse. It was my own darn fault - not the fault of
the ANC-4.
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