The destructions that came with my Comtek phasing box(s) suggest that each
element be cut about 5% long, the array erected and then observe the
frequency that the minimum amount of power is dumped into the dummy load
that is connected to the 5th port for each direction. Then you allow a
little (insert not politically correct nationality here) windage for the
differences in frequency, determine a frequency you want to operate at and
adjust all of the elements to this new length, keeping all of them the same
length. And then you just live with the directional differences.
As I am to lazy to erect a single reference element and make the required
field strength measurements (4 sets of complete measurements, one set for
each beam direction) to determine the efficiency of the array, I assumed
that as my array shows good front to back and front to side ratios I had a
working antenna. I don't necessarily want to be the loudest signal (but
sometimes I am) on the band, I just want to be a little louder than my
neighbor with a dipole.
I would not expect a current balun to change the resonant frequency (0 ohms
reactance) of a dipole. I suggest you use your Autek and check your
impedance transformer/phasing lines, including the baluns, to ensure that
they are electrically 1/4 wavelength (0 ohms reactance, ignore SWR) at your
desired frequency. I found the velocity factor on the 75 ohm coax I used on
my 40m 4 square to be closer to .7 than the .8 the manufacturer specified
and those lines wound up being almost taught between the box and feed
point(s). The v/f on the lines for my 80m 4 square were very close to mfg
specs. It has been my experience that once I measured the V/F of a roll of
cable it remained constant throughout the roll, but different rolls may have
different V/F's.
Even though my 4 squares are physically symmetrical I find the frequency
that the array wants to work at is a little different for each direction.
This was determined by observing the dump power vs frequency in each
direction. Just for yucks, I see perhaps 3 watts to the dummy load at 3505
KHz with 1000 watts driving the array. At 3600 Khz I'm dumping maybe 9
watts. At 3700 60 watts and at 3800 110 watts and that's as high in
frequency as I will go. And the SWR never exceeds 1.2:1 across this range.
To keep track of the health of my array(s) I ignore the SWR and just keep my
eye on the power going into the dummy load.
I'm treading on thin ice here so please don't take the following as gospel.
Something else that may be distorting your observations would be the 1/4
wave (75 ohm) impedance transformer/phasing lines. With a feed point Z of
35 ohms (ground plane) the Z at the other end of the line will be 165 ohms.
Assuming that the phase shifting box was designed to drive a ground plane
array this would require the 0 degree port match 165 ohms, the 90d port
match 82 ohms (two elements in parallel) and the 180d port match 165 ohms.
With a dipole array there won't be much, if any, impedance transformation
(feed point Z of each dipole may be 75 ohms +/-), the same shifting box
should be designed to match 75 ohms, 36 ohms and 75 ohms. It would seem to
me that you would wind up with at least a 2:1 SWR if you used a box designed
to feed ground plane elements. Could it be that we should ignore SWR
readings with 4 squares?
I consider the strong point of my 4 squares as being able to switch away
from interfering sources. Depending on how far the DX is from me I have
observed as much as 30 db F/B. I expect at least 20 Db F/B and F/side on
everything except stations at or near the antipode, which ain't all bad.
de Paul, W8AEF
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of D. Rodman, MD
Sent: Wednesday, November 17, 2004 2:19 AM
To: towertalk
Cc: topband@contesting.com
Subject: [TowerTalk] 80m 4Square Dipole VSWR question
I have a measured different resonance curves and corresponding VSWR for my
80m dipole array depending on which antennas I select. For example, the
antennas are oriented NE, SE, SW, and NW. I have two pairs of differing
VSWR curves and resonance points depending on the antennas selected. As far
as I know, I meticulously measured each feedline and dipole last year when
the antenna was constructed. The opposite antennas break down into two
pairs of VSWR curves. The antennas were designed and cut for 3.65 MHz and
fed with a W2DU balun. The balun actually LOWERED the resonance by 120 kHz,
so the dipoles were shortened accordingly during construction and the
phasing lines were constructed of 75 ohm cable. The feed points were spaced
out from the tower so that the diagonal distance was 95'. The antennas are
arranged as equilaterally as one can be and have been up over
a year. The dump power is running about 20% and I am hoping to get
this to a lower 5-10% range. Here is what I see:
The NE/SW pair have resonance curve at 3.58 and 3.57 MHz using an Autek
meter. The NW/SW pair are resonant at 3.66 and 3.67 MHz. VSWR with the
transceiver show identical measurements with and without amplifier. I had
questioned possible interaction between the tower, nearby antennas with the
4Square so I wanted to use a low power device to measure the VSWR curves.
Here is what I did:
The 4Square box was pretty old and I swapped it with a new replacement and
found the VSWR unchanged. I inspected the baluns and feedpoints when the
box was changed. I saw no obvious defect in the connection or balun or coax
cable. I had thought the problem was solved this weekend, but mistakenly
disconnected my power source from the 4Square and found the changed as soon
as I discovered the problem.
Here are my questions:
1. Have any other users seen this difference in VSWR with a dipole or
vertical array arranged with this opposite pair configuration? What did you
do to evaluate or check the system?
2. What can I do next to troubleshoot the system and get the four antennas
to perform identically?
3. My quick and dirty plan is to take the NE/SW pair and shorten the lower
ends by a foot or so to determine if the curves match better. Does anyone
have a better idea?
I would be interested in your experiences and ideas. Thanks.
David J. Rodman, MD
Assistant Clinical Professor
Department of Ophthalmology
Research Assistant Professor
Department of Chemistry
State University of New York at Buffalo
FAX 716-859-4565
Office 716-854-1137
e-mail: rodman@acsu.buffalo.edu
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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