I've had some telephone line caused server problems, and I'm not
sure my post made it to the reflector or anywhere.
From: "Greg Gobleman" <firstname.lastname@example.org>
To: "Tod-ID" <email@example.com>, <firstname.lastname@example.org>,
"Bill Coleman AA4LR" <email@example.com>
Subject: Re: [TowerTalk] K3LR and W0IYH "choke" baluns in the
Date sent: Thu, 10 Jun 1999 20:02:45 -0500
> I also read W2FMI's book and I would have to agree that something isn't
> right about the W2DU type Balun. I experienced heating and a rise in SWR
> when using a KW and an under 2:1 SWR but not flat. It would heat up and
> the standing wave would rise over 2:1. This is not to say that all bead
> Baluns are bad. I had heard good things about the Force 12 version.
> Perhaps it uses a different ferrite material.
Walt's balun is based on good engineering for choking, but if you
look at it closely there is no headroom for power. I suspect Walt
never caught that because he mostly runs low power.
There are certainly many cases where his balun would work OK,
but 73 material or ANY material with high loss tangent is the wrong
material for QRO or for use where the core is involved in handling
any high flux density.
> I built several of the W1JR type of Baluns and have had no problem with
> heating. I have had a problem finding an inexpensive enclosure. I have
> tried using 3" PVC caps and plugs and have about $5 in the enclosure.
> However, I created another problem. Weight of the enclosure and the
> core/coax with connectors is a bit much for a dipole. An inverted V or
> mounting on a beam is not a problem.
There is no need for the criss-crossed winding style, a single layer
solenoid winding measures nearly the same. Some articles and
books tell you any stray C across the balun reduces choking, but
the opposite actually happens. You just have to be careful and not
use such a large winding that the self-resonant frequency of the
balun is lower than 1/2 of the highest operating frequency.
The cheapest balun for a given impedance and power rating is still
an air-wound coil of coax on a PVC drainpipe.
> I have also had excellent success with a coil of coax. When ur lighting
> every florescent tube within a block at 2 AM while on 80m with a flat SWR.
> This will cure it.
If you use multiple turns through a core, the impedance goes up by
the square of the turns increase. If you stick them through a string
of beads, the increase in impedance is linear with length and has
almost nothing to do with bead thickness. An air wound choke is
somewhere between unity ratio and squared impedance as turns
are increased, depending on mutual coupling between turns.
A string of 43 material beads 36 inches long has the same
common mode impedance as a stack of 43 cores 1 inch tall with 6
turns of coax. The string of beads will handle more power, because
it has more surface area exposed directly to cooling air no matter
how thick the beads are (beyond a certain limit).
The more stress the balun has, the lower the ui of the core you
should use. At the feedpoint with high power, a low-ui low-loss-
tangent core is generally best, like air or a 61 material. This is
especially true if the feedline parallels the antenna, or if the
element is off balance, or if the element impedance is high.
In a coaxial line connected the normal way near the shack (like in
the second chokes K3LR uses), a string of 73 material beads
would almost certainly be acceptable no matter what the power
The feedline should be grounded to the tower or another ground as
soon as possible after the balun, only on the side of the balun
closest to the shack if possible.
I use air chokes, or 61 material cores at transmitting antennas. I
use 73 or 75 material cores for receiving and in-the-shack or "down
the cable a distance" isolation.
73, Tom W8JI
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com