Date: Tue, 5 Jul 2016 10:42:34 -0700
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
On 7/5/16 9:23 AM, Jim Brown wrote:
> On Tue,7/5/2016 7:34 AM, Jim Thomson wrote:
>> IF your sloping half wave dipole is UN balanced...then why insert a
>> CMC at the feed point at all ??
>> I would dump the CMC at the feedpoint, then perhaps use a CMC way
>> downstream , like just
>> before the coax meets the tower..or at the base of the tower..or both.
>
> Nope! The first choke should ALWAYS be at the feedpoint. Additional
> chokes farther down the line can be used to prevent current induced on
> the feedline from causing grief in the shack (like with your rack full
> of un-necessary audio processing gear). :)
>
I can conceive of situations where putting the choke somewhere down the
line might be ok or desirable (from a mechanical standpoint). Maybe not
realistic or practical situations. But if you had a 40 m sloping dipole
that's 20 m long, and your choke were at, say, 2 meters, and the coax
from dipole to choke were at right angles to the antenna, I think it
would work just about as well.
On the other hand, and getting to the "practical" aspect putting it at
he feedpoint, the mass is better supported by the antenna itself.
## Roger, K8RI was talking about the 40 m sloper system u see in the
old arrl books. IE: 5 x half wave slopers, arranged every 72 degs around
a tower....or tree etc. Only one is driven at a time. Each feed line was
aprx
.375 wave long. The un-used coaxs had both their center conductor and braids
floating. Braids were not bonded to each other, nor the metal casing on the
metal
box containing the various relays. The length of the un-used lines made the
ant
look slightly xl at the feedpoint, so the un-used slopers become reflectors.
## I never saw one of those systems ever use a CMC at the feedpoint, and they
all
had 3-4 db gain..and typ 20 db FB. The theory was...a half wave sloping
dipole
is unbalanced to begin with..so deep 6 the CMC at the feedpoint. On those
setups,
at most you might require a CMC on the main feed line coming up the tower.
## I never saw anybody using CMC at the feedpoint of a 1/4 wave sloper
either.
But on 1/4 wave slopers, I always ran the coax down the inside of the tower,
since it
was easy to do, since the tower I used at the time was 33 inches across. ( I
climbed
up the inside of it).
## the yagis at the time all used gamma or omega matches... an no CMCs used.
Jim VE7RF
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