Jim, in my opinion it is not necessary.
Until last summer I would have agreed with you.
After spending about 200 hours in the field measuring this stuff, I did not
find enough cmc on the coax of a dipole, "IF AND ONLY IF" the coax runs 90
degrees away from the antenna, to bother worrying about. You need no choke
or balun.
As soon as you begin to bend the coax and run it at a less than 90 degree
angle, the picture changes.
Then you should place "something" at the feedpoint.
At the feedpoint, I tried several different kinds of baluns and a simple 1:1
Guanella choke.
It made no noticeable difference what was at the feedpint, as long as you
used something.
I have EXACT MEASURED DATA for all of this.
ALL tests were made running 100w from a Ten-Tec Eagle into the antenna
system.
However, I would ALWAYS recommend placing a cmc choke at the house before
the feedline enters the shack.
If the feedline is very long, then as Jim recommends in his "RFI Ham", I
would insert one or more cmc chokes in the coax along the long run.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Sunday, June 01, 2014 8:21 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Trading radios
On 6/1/2014 11:01 AM, Bob McGraw - K4TAX wrote:
> If one chooses to use only one Common Mode Choke or Line Isolator,
> best first placed at the radio.
No, the first choke should be at the antenna feedpoint. W2AJI is exactly
right. A choke at the radio would only matter if the shield of coax
connector was not bonded to the chassis. So far, I've not seen a transceiver
where this mistake was made.
Now, an additional choke either at the radio or at some intermediate point
on the line CAN be useful to the extent that it functions as an "egg
insulator" for the common mode current induced on the line by a nearby
vertical antenna, which makes it a parasitic element of that other antenna,
modifying its pattern.
And there ain't no such thing as a 1:1 common mode choke. A common mode
choke is a straight through section of transmission line wound around a
ferrite core. No transformer action. No turns ratio.
In every radio I've looked at, the bodies are buried in the unbonded
unbalanced jacks, not on the output SO239.
73, Jim K9YC
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