[TowerTalk] Baluns and SWR
Bill Hider
n3rr@erols.com
Thu, 24 May 2001 15:17:20 +0100
The issue of grounding the coax at the top and bottom of the tower has more
to do with lightning protection than
RF. It's good for both aspects, though, so there's no tradeoff here.
All coax should be grounded to the tower at the top of the tower, as close
to the antenna "rotator loop" as possible; and at the bottom of the tower
and as close to the point where the coax moves away from the tower as
possible (and as low as possible on the tower).
This info is provided in tutorial form on the Polyphaser Website and has
nothing to do with Polyphaser components.
My Website explains in words and pictures how to implement this and has
links to various other related info:
www.erols.com/n3rr
73,
Bill, N3RR
----- Original Message -----
From: N2TK <n2tk@earthlink.net>
To: <W8JI@contesting.com>; <towertalk@contesting.com>
Sent: Thursday, May 24, 2001 3:46 PM
Subject: RE: [TowerTalk] Baluns and SWR
> 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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> for information on our fabulous Trylon Titan self-supporting towers - up
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> 96-feet for less than $2000! at 888-833-3104 <A
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>
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A HREF="http://www.ChampionRadio.com">
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