I disagree with this advice, Tim. Such a choke is strongly inductive
below about 30 MHz, so its common mode Z will be cancelled, at least in
part, by the capacitance of a feedline shorter than a quarter wave. A
multiturn choke wound on one or more #31 cores is predominantly
resistive, so performance is far less affected by the impedance of the
common mode circuit.
73, Jim K9YC
On Fri,9/23/2016 12:36 PM, Tim Duffy wrote:
Hello Jeff,
I like using the W0IYH style of choke in this application. It has several K
ohms of CMI and goes right in series with the RG-11 line. In a 4 square the
W0IYH choke must use 75 ohm coax and it also becomes part of the phasing
line calculation for the 90 degree lines (typically cut for 3.650 MHz).
You can see it here:
https://www.dxengineering.com/parts/com-cfc-75
73,
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jeff
AC0C
Sent: Friday, September 23, 2016 3:25 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 4 square for 80
Tim, you mention below that the elevated radials and the control box must be
choked.
What's the general advice on the choke for each end of the line?
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Tim Duffy
Sent: Thursday, September 22, 2016 4:12 PM
To: cqtestk4xs@aol.com ; towertalk@contesting.com
Subject: Re: [TowerTalk] 4 square for 80
Hello Bill,
I want to clarify some of your notes below, as many guys are reading your
post and might be confused. We need to insure we get this right.
T top elements (loaded) will not have a feedpoint impedance of 36 ohms +/-
j0. The hybrid coupler four square boxes are predicated on FULL size 1/4
wavelength elements - so DXE does not recommend using loaded elements. The
system will not work properly.
I listened to your call with Rod, K8AAV here at DXE to make sure we have
this right. Rod said that you SHOULD NOT use raised radials as they are hard
to get working correctly. They MUST be tuned to work and there is a process
for this. You also need to install chokes when using elevated RG-11 feed
lines at the element feedpoints and at the Comtek box; otherwise there is
significant pattern distortion. They can work - but it takes lots of
patience and we recommend traditional ground mounted radial systems for best
performance.
The tower in the middle of your 4 square will interact with your proposed
system. How much or how little depends on many factors. Rod reviewed the
detuning approach with you - which is almost impossible for you to install
with a rotating tower.
Having engineered 4 square antenna systems (successfully) for over 25 years
- I was the advising engineer to Rod on your project. I was very careful to
make sure Rod had all of his facts correct before giving you advice.
You asked for advice on your project and we gave you the straight engineered
scoop - with tons of experience. We do not want you to go through lots of
work only to find out a system you proposed does not work correctly. You
might get various other guidance from others, my advice is be careful.
Trying to make everything work properly on one tower like this is very hard
and there is no free lunch. Only properly engineered systems have a high
probability of working correctly.
Please feel free to call or email me directly to discuss your options.
Very 73,
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Bill
via TowerTalk
Sent: Thursday, September 22, 2016 4:00 PM
To: TOWERTALK@contesting.com
Subject: [TowerTalk] 4 square for 80
This will be my first foray into the world of 4 square xmit antennas.
Plans are to make the elements from wire, suspended from the tower by rope
at about 80 feet. Due to the slope of the ropes from the tower to the
ground, the elements might wind up a little short. Plans were to T-top them
running the wire parallel to the rope. The elements might be around 60 feet
so it would not be that hard to T top them to make up the extra length
needed for 1/4 wave.
I had planned to do the following: Have an elevated ground system a 1/4
wave radials running from the base of each vertical. Height of the system
would be 10 feet. Radial system would consist of 16 radials for each
vertical using the Comtek 4 square system to control it.
In talking with the guys at DX Engineering about the planned setup I was
given the not so good news.
First, I was told you CAN NOT use an elevated ground system with the 4
square.
Second, I was told the tower would interfere with the system. They said I
would have to cage the tower to make it "invisible" to the system. Caging
the tower is impossible since it is a rotating tower and the cage would wrap
on the stacks mounted on the tower.
Was he right about all of this?
I can not put up another tower due to tower restrictions (one tower
allowed), so the only way I can get enough height to suspend the wires is to
use the tower I have. The county thinks anything 60 feet high constitutes
another tower, so I can't make aluminum verticals. No trees around either
for supports.
I really am in a bind for 80 if all this is true. An inverted Vee at 80
feet is far inferior to the 4 square I had planned.
Ideas and thoughts on this?
Bill K4XS/KH7XS
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