I do have very high CMI chokes (0ver 10K ohms) at the feedpoint of each
vertical element and at the HI-Z combiner box for each element. This may
seem extreme, but I am more than pleased with the results. I have been
working on improving the low band RX systems here for many years. I have a
113 ft diameter HI-Z 8 circle. And a 113 ft diagonal HI-Z 4 square. The two
arrays have identical element configurations - it is just one is 8 and the
other 4. The array centers are spaced 500 feet apart and they are broadside
to Europe. The loss and electrical length of the feed lines going to the two
arrays are calibrated so they are the same. We have a custom switch that
allows us to listen to the 8 circle alone, the 4 square alone or put them in
phase or out of phase.
Both arrays use 23 foot vertical elements (DXE AL-24) with two 4 ft ground
rods at the base. I am using older HI-Z element buffers that have been
modified to take power directly - not via the RG6 bias tee coax. Any amount
of moisture in a system that feeds DC and RX RF together can have noise
problems. I am confident in my ability to weatherproof, however I wanted the
best possible result. So there are high CMI chokes in the RF and DC lines -
24 chokes and 24 feedlines (one for RF and one for DC). There are so many
things in the arrays that we cannot control, I try to control as many items
as I can.
I built all new verticals (12) this summer and used weatherproofing at each
element joint to keep any moisture out of the element junctions. I also
replaced the older dual shield RG6 with the newer quad shield RG6. Lastly, I
changed out my older HI-Z combiners for the latest ones that Lee makes.
Surface mount components, many with 1% tolerances and improved F connector
mountings.
W8WWV developed a choke system that we use at the element feed - that makes
these arrays deaf above 4 MHz. That helps keep down the possible mixing with
all of the multi multi TX RF here.
So - how do the HI-Z arrays work? Very well. There is great pattern "snap"
as you rotate the array around, with close to 30 dB of F/B. John, N2NC was
the 160 SSB operator here for CQWW and he told me the arrays are working
very well. I was the 160 meter operator this past weekend and even under
poor conditions I thought the performance was very good. Sunday morning I
worked ZM4T who was not very strong, at 1115 Z I worked JW7QIA, then an
amazing QSO at 1130 Z with RW0A in Zone 18. All using the HI-Z arrays. The
ability to feed these arrays out of phase is impressive as it sometimes
fills in - where a station is not directly at 45 degrees, 135 degrees etc.
The best position for RW0A was 45 degrees - both arrays but fed out of
phase. That put a strong lobe at 15 degrees - perfect! I have worked Zone 18
before on 160, but it was always done in the evening here - around 0130 Z.
No more Beverages at K3LR...
73
Tim K3LR
-----Original Message-----
From: Topband [mailto:topband-bounces+k3lr=k3lr.com@contesting.com] On
Behalf Of Lee STRAHAN
Sent: Tuesday, November 30, 2021 2:35 PM
To: Dennis W0JX; topband@contesting.com
Subject: Re: Topband: Feedline Choke Placement in RX systems
Greetings All,
For the Hi-Z arrays any additional common mode choking for most all
installations would not be needed. You are correct at K3LR there are added
common mode chokes in the element feedlines. I am fairly certain that adding
these helped more with interstation isolation. I have not heard for certain
that K3LR saw any noise reduction in that system due to the addition of
chokes. He has an 8 element version that already included combiner input
common mode isolation. Tim may want to comment on this. All my Hi-Z amps
have had common mode isolation in them since day one. So placing another
choke at the Hi-Z amps seems to be not necessary. In the older designs there
was no common mode isolation on the combiner inputs on the smaller arrays
meaning the three element and the 4 element. The 8 element arrays were
designed with common mode isolation at the combiner inputs making these
systems completely isolated. In the new redesigns I have added common mode
isolation to the inputs of the new 4 e
lement array design just because I thought it certainly won't hurt anything
but possibly could help. The Three element array has so much less combiner
loss and its element feedlines are so short I have not added any common mode
isolation to its inputs. I have not seen any need here in many tests of that
array.
My take on the placement would be to put them closest to the controller
for the home brew W1FV array. From my older W1FV schematics his high
impedance amps have common mode isolation built in so placing added
isolation would be better at the combiner. Building your own high impedance
amps would certainly need common mode isolation.
You are correct Dennis as it depends on the overall system design.
Lee K7TJR
Hi-Z Antennas
-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com@contesting.com> On Behalf Of
Dennis W0JX via Topband
Sent: Tuesday, November 30, 2021 9:48 AM
To: topband@contesting.com; John Kaufmann <john.kaufmann@verizon.net>
Subject: Topband: Feedline Choke Placement in RX systems
K9YC made an interesting comment in his post, saying that the feedline
chokes would be most effective if placed at the antenna end of the feedline.
W1FV's 9 circle YCCC array has them right at the controller.
The older K7TJR combiner utilized no feedline chokes. However, I believe
that Lee's new design has built in common mode protection on the combiner
circuit board.
So what is the best way? Is placement dependent upon the combiner design?
BTW, K3LR is using 2.4 in #31 toroids with at least 24 turns of RG-179 as
feedline chokes in his 8 circle HiZ array and I believe that there are
chokes on both ends of the antenna feedlines. Of course, K3LR operates in a
high intensity, multi-transmitter environment and may need all that choking.
73 Dennis W0JX
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