Cecil Moore wrote:

Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

I use a choke to reduce common-mode problems in the shack and it does
that apparently by causing reflection of common-mode waves back toward
the antenna which, as you say, wouldn't decrease feedline radiation
between the choke and the antenna.

It isn't really helpful to think of it as "reflecting common-mode waves
back". It is simply Ohm's law: a high series impedance reduces the
amount of current that will flow.

Forcing a current minimum at one location on the feedline (usually right
at the end of the coax) will change the current and voltage distribution
not only on the feedline, but also on the antenna proper. The whole
antenna-feedline system readjusts itself to take account of the fact
that the choke impedance is there. This readjustment will also change
the feedpoint impedance, so it's actually true that a balun will change
the SWR of the antenna - it has become a different system that no longer
involves the feedline.

Reg is right to say that the choke shifts the common-mode voltage and
current standing waves to other places along the feedline. Depending on
the location of the 'victim' equipment (TV etc) relative to those
standing waves, the change can sometimes make RFI problems worse. Even
so, a choke balun right at the end of the coax is almost always the
right place to start. If it doesn't help, it may *still* make sense to
leave it there, and try a second choke somewhere else as well.

A clip-on RF current probe can work wonders in showing you what really
is happening.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:
It isn't really helpful to think of it as "reflecting common-mode waves
back". It is simply Ohm's law: a high series impedance reduces the
amount of current that will flow.

True for a bench circuit. Not necessarily true for a distributed
network. In fact, a choke that makes the outside braid of a
transmission look like an open circuit (best case), makes it look
like a short circuit 1/4WL back from that point. If the choke is
placed at an existing current minimum point, it will have little
effect.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

Reg Edwards wrote:
. . .
Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

Have I upset the apple cart again?

Once again I proudly don the mantle of a Reg's Old Wife.

A common-mode choke, aka "choke balun" or "current balun" can have the
effect Reg mentions, but that's not the only possibility. Depending on
the lengths of the antenna and transmission line and the placement of
the choke, it can reduce the common mode current to a much lower value
everywhere on the line. This is done by the same mechanism as an
insulator reduces the current induced on a guy wire -- by making a
formerly resonant line non-resonant. In stubborn cases, two (or perhaps
very seldom, more) chokes placed about a quarter wave apart are required
to get a low value of common mode current everywhere on the line.

The general principles are easily illustrated by modeling(*). In
practice, the actual path along the feedline from antenna to ground
isn't well known so can't be modeled well, and some experimentation
might be necessary.

(*) Anyone having an EZNEC program, including the demo, can look in the
manual index under "Coaxial Cable, Modeling" for information. To
simulate a choke balun, insert a resistive or inductive load in the wire
which represents the outside of the coax. 500 - 1000 ohms is a
reasonable value to use.

Roy Lewallen, W7EL

On Tue, 11 Jan 2005 12:52:59 -0800, Roy Lewallen
wrote:

Once again I proudly don the mantle of a Reg's Old Wife.

Oh my gawd! Do I detect Kurt Sturba here. GG

Danny

"Cecil Moore" wrote in message
...
Sorry, I don't know the answer. I operate my W2DU choke into close
to a pure resistance that varies between 28 ohms and 90 ohms.

Cecil - I stopped using baluns for dipoles when my neighbor's TV reception
was getting killed every time I went on 10 meters. I took out my W2AU balun,
and no more problem. Whenever I need to match higher or lower antenna
impedances now, I use a 1/4-wave length of the appropriate coax line to get
close to the antenna impedance. Works great, seems impervious to the
weather, and have never found the unbalanced feedline to balanced antenna
(dipoles & full wave loops/quads) was a problem that feeding the coax away
from the antenna in a perpendicular fashion wouldn't solve. Also, two
parallel RG-59 coax lines feed grounded verticals nicely, and a tuner or the
amp pi network can easily do the load match.

ak