Coax choke question
 

I have a half-sloper attached to my tower. It is mostly used for 80 meters
(but also for 160 and 40 in rare cases). It is attached to the top of the
middle section of a 55' crankup tower. The tower has a number coax and other
control lines suspended a foot or so outside the tower. I am in the "far
suburbs" but 40/80/160 still have more noise that I hoped and I suspect some
of the noise is from the neighborhood; there is no specific noise generator
that I can detect.

Question: would a coax choke (5 43-material toroids with 5 loops of coax) at
the feedpoint of the half-sloper do any good? I am a little confused since
the tower (and, I assume, the outside of the coax) is an intentional part of
a half-sloper antenna.

At the bottom of the tower, the coax goes underground for about 50' to the
rig.

Bill - W2WO

Coax choke question
 

On Sep 16, 9:41*am, "Bill Ogden" wrote:
I am a little confused since
the tower (and, I assume, the outside of the coax) is an intentional part of
a half-sloper antenna.

There are two sources of common-mode current. A choke at the resonant
dipole feedpoint reduces the common-mode current conducted onto the
outside coax braid. Unfortunately with the tower being a radiator and
the coax tied to the tower, inducted common-mode is the major problem
and difficult to solve. A choke at the antenna feedpoint will have
little effect on inducted common-mode current. I wonder if you are
frying any earthworms along your buried coax? :-)
--
73, Cecil, w5dxp.com

Coax choke question
 

On 9/16/2010 7:41 AM, Bill Ogden wrote:
I have a half-sloper attached to my tower. It is mostly used for 80 meters
(but also for 160 and 40 in rare cases). It is attached to the top of the
middle section of a 55' crankup tower. The tower has a number coax and other
control lines suspended a foot or so outside the tower. I am in the "far
suburbs" but 40/80/160 still have more noise that I hoped and I suspect some
of the noise is from the neighborhood; there is no specific noise generator
that I can detect.

Question: would a coax choke (5 43-material toroids with 5 loops of coax) at
the feedpoint of the half-sloper do any good? I am a little confused since
the tower (and, I assume, the outside of the coax) is an intentional part of
a half-sloper antenna.

At the bottom of the tower, the coax goes underground for about 50' to the
rig.

Bill - W2WO

You're correct that the feedline and/or tower are part of the antenna.
What you have is a dipole (not necessarily symmetrical), with one side
of the dipole being the half-sloper "antenna" and the other side being
the tower and feedline. The feedline has two transmission line
conductors, which are the center conductor and the inside of the shield.
Whatever current flows out of one must flow into the other. The current
going to the half sloper equals the current flowing down the tower and
outside of the feedline shield. If you effectively choke the tower and
feedline currents, you'll also choke the current to the half sloper.

While you can effectively choke the current on the outside of the
feedline (it would probably take two chokes, one at the top and one at
the bottom), it won't help your noise situation. Whatever nearby
vertically polarized noise might have been picked up by the outside of
the coax will still be picked up by the tower. If the problem is
vertically polarized local noise, the only solution is probably to go
with a fully horizontal antenna with a well-decoupled feedline.

Roy Lewallen, W7EL

Coax choke question
 

On Thu, 16 Sep 2010 10:41:32 -0400, "Bill Ogden"
wrote:

the coax goes underground for about 50' to the
rig.

Hi Bill,

That alone should snub common mode currents (into the shack).

What kind of noise?

73's
Richard Clark, KB7QHC

Coax choke question
 

You're correct that the feedline and/or tower are part of the antenna.
What you have is a dipole (not necessarily symmetrical), with one side of
the dipole being the half-sloper "antenna" and the other side being the
tower and feedline. The feedline has two transmission line conductors,
which are the center conductor and the inside of the shield. Whatever
current flows out of one must flow into the other. The current going to
the half sloper equals the current flowing down the tower and outside of
the feedline shield. If you effectively choke the tower and feedline
currents, you'll also choke the current to the half sloper.

While you can effectively choke the current on the outside of the feedline
(it would probably take two chokes, one at the top and one at the bottom),
it won't help your noise situation. Whatever nearby vertically polarized
noise might have been picked up by the outside of the coax will still be
picked up by the tower. If the problem is vertically polarized local
noise, the only solution is probably to go with a fully horizontal antenna
with a well-decoupled feedline.

Roy Lewallen, W7EL

Thank you for the confirmation. I'll skip the choke for the half-sloper.

I am aware that many hams take a poor view of these antennas. Perhaps I
am a bit lucky because mine seems to meet my minimum requirements ---
especially for transmitting ---
probably because I do not use 80 or 160 very often. (I still need about 25
countries on 80 for 5BDXCC and this is my driver.) I would love to have
a quieter receiving antenna for 80. There is a very interesting scheme in
the
QST I received today; in effect a rotary beam for 80, with variable angle
of reception.

Bill - W2WO

Coax choke question
 

On 9/16/2010 7:41 AM, Bill Ogden wrote:
I have a half-sloper attached to my tower. It is mostly used for 80 meters
(but also for 160 and 40 in rare cases). It is attached to the top of the
middle section of a 55' crankup tower. The tower has a number coax and other
control lines suspended a foot or so outside the tower. I am in the "far
suburbs" but 40/80/160 still have more noise that I hoped and I suspect some
of the noise is from the neighborhood; there is no specific noise generator
that I can detect.

Question: would a coax choke (5 43-material toroids with 5 loops of coax) at
the feedpoint of the half-sloper do any good? I am a little confused since
the tower (and, I assume, the outside of the coax) is an intentional part of
a half-sloper antenna.

At the bottom of the tower, the coax goes underground for about 50' to the
rig.

Bill - W2WO

Might, actually, be better to feed the other end of the sloper, bringing
that end as close to the shack as possible, shortening the coax and
dropping out the associated loss, possibly having a favorable effect on
your noise, etc.

Of course the addition of a suitable counterpoise(s) would be necessary
.... just a thought ... and, of course, a notable amount of work.

Regards,
JS