I recently bought a Comtek 2-element phased vertical array system for 80M.
Each vertical is ¼ wavelength in length and the 2 verticals are spaced ¼
wavelength apart.
I have installed 60-- ¼ wavelength radials in a uniform and symmetrical
pattern around the base of the first vertical. Using my AIM 4170 analyzer,
I adjusted the length of the first vertical until the minimum SWR was at
the frequency I wanted (3.71 MHz). At that frequency, the SWR was 1.4 and
the series resistance at the feedpoint measured 35.7 Ohms with a series
reactance of 6 Ohms. (If you are curious, the resonant frequency was 3.68
MHz.)
Next, I assembled the second vertical, making each section and the overall
length the same as the first. I then erected the second vertical in a SW
direction of the first vertical at a distance from the first vertical of ¼
wavelength. I used a transit to make sure the base of the second vertical
was at the same elevation as the first vertical.
I then installed 60-- ¼ wavelength radials in the same manner as the first
vertical. Where the radials of the 2 antennas overlapped, I cut them and
soldered them to a 3” piece of copper strip that runs at a 90 degree angle
to a reference line connecting the 2 verticals.
To make sure the two verticals behaved the same way electrically, I
connected my AIM analyzer directly to the feed point of the antenna being
tested with 50 feet of coax. (The 50 feet of coax was calibrated out of
the measurements.) In taking these measurements, I lowered one antenna to
a horizontal position and measured the one vertical that was upright. I
then reversed the process. Next I superimposed the scans of both verticals
over a frequency range of 3.0 to 4.5 MHz. The traces for the SWR, Theta,
and the return loss only varied slightly from each other. For example, the
SWR of the first vertical was 1.4 and it was 1.45 for the second vertical.
Next, I located the relay box, that contains the hybrid coupler, between
the 2 verticals and connected the relay box to each antenna with RG-213
that is electrically ¼ wavelength long. I then ran RG-213 from the
transmitter port on the relay box back to my shack. I also ran another
length or RG-213 from the”50 Ohm Load” port back to a 50 Ohm dummy load in
my shack. (All of the RG213 I used was new.) I also ran the control line
from the 3-position control box in my shack to the rely box. Finally, I
checked the continuity of all of the wiring and also made sure the relays
closed or opened in response to the signal from the control box.
After all of this effort and expense, I am very disappointed in the
performance of the array. I can’t measure the RF output in the NE or SW
endfire positions to determine if I am getting any gain, but when I listen
to stations in Europe and switch between the NE and SW positions, I can’t
see or hear any change in the signal strength. So, I question whether
there is any front to back rejection.
I returned the relay box to the vendor. They checked it and said it was
working correctly.
I have used my AIM analyzer to scan the feedline at my shack when the array
is in an endfire position. From 3.5 to 4.0 MHz, the SWR slopes downward
from 2.5 to 1.5. There is no resonant point, and Theta starts at -30
degrees and curves upward to -5 degrees.
I increased the frequency scan range and found a distinct null in the SWR
at 3.4 MHz. At that frequency, Theta is 0 and the return loss is at the
bottom of a very deep “V” trace and measured -36dB.
Based on these measurements, it seems to me the mutual coupling between the
2 verticals has moved the minimum SWR and the resonant point of the array,
in an end fire configuration, to a point well below the 80M band. *QUESTION
1*: Am I misreading these results?
After reading and rereading the chapter in Low Band Dxing on Arrays, the
advice is to use 75 Ohm, instead of 50 Ohm, coax for the ¼ wavelength
feeders. So, *QUESTION 2*: should I try RG11 with PL-259 connectors? (I
use a 1.5KW amp on 80M and the maximum voltage rating for RG11 is 300V. Am
I safe using RG11 or should I try LM-400-75 that has a 2,000V rating?)
*
*
*QUESTION 3*: Also, is it OK to put 50 Ohm connectors (PL-259’s) on 75 Ohm
coax?
*QUESTION 4:* What other tests or measurements should I try to improve the
performance of this array? (I do have a dual trace scope, if that would be
useful).
I’m sorry this post has gotten so long, but I wanted to explain everything
I did and the measurements I made.
Thanks in advance for any help you can give me.
Wayne, KK6BT
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Topband reflector - topband@contesting.com
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