Tom, if you are bringing your coax cables down to a coax switch mounted near
the bottom of the tower, would you still recommend bonding the shield of the
coax? If so, do you bond it as close to the balun as possible - the mast of
the beam, or somewhere on the tower, or does the grounded coax switch take
care of this? I am planning on using the Force 12 B-1 baluns.
73
Tony
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Tom Rauch
Sent: Thursday, May 24, 2001 9:56 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Baluns and SWR
> use a coax balun. I can get no consensus about how many windings, how
> many inches for the form, etc. The manuals, antenna books, and TowerTalk
> members suggest 12 turns, 6 turns, 8 turns, 4-6 turns, all on either a 4
> inch form, or a 6 inch form, or air wound and taped. The list of
> suggested turns and methods of windings goes on and on, not to mention
> what type of coax to use: 213, 58, 8U, 8X, teflon, etc.
There is conflicting information because the design is a "loose"
design. Choke baluns work with a broad range of configurations,
they are not critical devices. I prefer a single-layer solenoid and
simply calculate the turns and diameter based on having several
hundred ohms reactance on the lowest frequency.
Roy Lewallen measured some scramble-wound baluns. I would not
use them at high power because of voltage breakdown from turn to
turn.
I use choke-baluns on all of my antennas, unless they are perfectly
balanced antennas fed with balanced line or perfectly UNbalanced
antennas fed with coax.
> Tom: The so called "coax choke" does very little in the high current
> section near the feedpoint. You really need a better feed system instead
> of another "Band Aid." There isn't enough capacity between the too few
> turns to create resonance and an effective Hi-Z choke contrary to what
> many have been led to believe.
> The area in the coax where the coax coil choke really works is in the
> Hi-Voltage Area 1/4 wave lower but that is too far down. This is right at
> the Hi-Voltage area where the capacity is higher between the turns and an
> Effective Hi-Z Choke is formed with the right number of turns.
The optimum place for a series impedance is at a LOW common
mode impedance part of the line. That is generally at the
feedpoint, but there are exceptions.
One exception is when the feedline already presents a high
common mode impedance to the antenna. An example would be if
the feedline was grounded 1/4 wl from the antenna. In that case the
common mode impedance of the feedline would be very high, and a
choke balun's high series impedance would be nearly useless. The
worse case condition would be if the feedline's common mode
impedance was the opposite reactance of the balun. In that case
adding the balun would make the system worse.
Most of us mount yagi antennas on large metal towers, taping the
feedline to the tower leg for a considerable length before the cable
leaves the tower and runs with other cables to the shack. In a
situation like this, the common-mode impedance of the feedline
NEVER becomes very high, and the most effective place for the
choke is at the feedpoint. Even small values of choke reactance
can be effective, because common mode impedance is low.
The safest and best idea is to bond the feedline shield to the tower
on the shack side of the balun at the balun. This insures a low
common mode impedance at the point where the balun is installed.
Checking SWR works in some cases, but also can be a poor way
to check balun effectiveness with many antennas. The "hand test"
requires your body impedance to be comparable to the antenna's
common mode impedance at the feedpoint. If the impedance of
your "body" isn't low compared to the antenna's common mode
impedance (the antenna's common mode impedance is the
impedance of the entire structure or system at the point where your
"hand connection" is applied) you won't see much of a change. It
also requires your "body" to be in the electrically neutral part of the
electric field surrounding the antenna.
Clamp-on meters are ideal testing devices.
I avoid rod-type "current meters" because they respond to all EM
fields, not just direct coupled magnetic fields caused by conductor
current.
When using baluns with beam antennas, if you select a choke-
balun that has several hundred ohms reactance and ground the
feedline just on the shack side of the balun to the tower or boom,
you will be safe without any testing.
73, Tom W8JI
W8JI@contesting.com
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