Georgek5kg at aol.com Georgek5kg at aol.com
Thu Dec 13 15:36:20 EST 2001

The WT4I Cabrillo Tools software will check for 10 min rule violations...geo

George I. Wagner, K5KG
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>From Leigh S. Jones" <kr6x at kr6x.com  Thu Feb  1 02:58:14 2001
From: Leigh S. Jones" <kr6x at kr6x.com (Leigh S. Jones)
Date: Wed, 31 Jan 2001 18:58:14 -0800
Subject: [CQ-Contest] Air wound coax balun
References: <001b01c08bb9$85ebaf00$310b0f97 at cad>
Message-ID: <06cf01c08bfa$d31dbc00$ede3c23f at kr6x.org>

I'd like to venture an opinion on some of the issues in your e-mail, with
the qualifier that these are not definitive answers, and may be prone to
some error.

WA2SRQ's balun article was interesting in that it provided a few guidelines
regarding winding of coaxial baluns that appear to be useful.  He concludes
that electrically, the balun will be more effective at higher frequencies
(i.e., 10 and 15 meters) if the stray capacitance from turn to turn is
reduced, and he offers that reducing the diameter of the coil and increasing
its length will improve the stray capacitance.  This is useful information.
The suggestion of a coaxial balun wound on a form made from PVC pipe is
useful also.

1) Isn't 4 inch diameter dangerous for rg213 when running full legal US
power on 28?
      I found on some data sheet that the minimum bending radius is 100mm, so
      minimum diameter should be 200mm, equals to approx 8 inches, which is
      double of the test run by WA2SRQ.

RG213 is a solid dielectric coaxial cable with good aging properties.  Solid
dielectric coaxial cable tends to be stiff by comparison to foam dielectric
types, having the disadvantage of naturally higher dielectric loss and the
advantage of stability and high voltage insulating properties.  Stability
here means that the insulation does not compress under bending pressures
resulting in characteristic impedance variations.  Just this moment I
performed an experiment, and found that at ~ +25 degrees C I could easily
bend the coaxial cable's center conductor around a 4 inch diameter coffee
cup without any indication that the flexibility limits of the insulating
layer were being exceeded.  I would suggest that the bending radius limit of
100mm should be considered a "working" limit, in other words it is
applicable to the diameter, for instance, of the stress relief turns around
the mast at the top of the tower rather than fixed radius bends.  Amateur
radio operators who build their own baluns usually have the advantage of
being able to test them before putting them into service.  If you can build
it, and it tests OK at your desired Italian legal power limit, then you
should be confident it will continue to perform.  Experimentally, RG213
looks like a safe choice to me, but the WA2SRQ measurements should be useful
for almost any RG8-like cable.

2)  In the message it's wrote that balun have to be inductive, to suppress
       unwanted current flowing on the outside of the coax shield, then that
       6 turns on a 4 inches pipe are enough from 14 to 35 MHz: on 28 the
       is not inductive, but capacitive: is the balun still effective at that
       frequency or do I have to use it only when the magnitude is approx 500
ohm or
       higher and the phase is approaching +90 degrees?

Perhaps WA2SRQ hasn't really written what he intended to say here.  I
personally don't believe that the balun has to be inductive, but it should
be high impedance.  Actually, the phase reversal observed in his
measurements occurs at a good frequency for operation.  At this frequency,
the 50 ohm current path in the interior of the coaxial cable presents a very
attractive current path by comparison to the outside of the shield, and this
is the intention of a choke type balun.  But the 4-inch pipe/6 turn coil,
with the resonance at 24 MHz, presents a fairly high impedance from 18 to 29
MHz.  The WA2SRQ measurements seem to show that a few more turns would be
needed for optimum performance at 14 MHz.  The 4-inch pipe/6 turn coil looks
like a good compromise for a triband yagi.

3) If my antenna have an impedance around 100 Ohm (2 el. quad), do I have
      to try for different diameters/turns numbers to achieve the high enough
      reactance? Or simply compare the reactance to 50 Ohm, the coax

Try to achieve the higher reactance.

4) Between 23 and 24 MHz phase crosses 0: so on that frequency it's an L-C
      resonant circuit. What happens if I use it exactly on that frequency?
      Is there the same choking effect? So, why look to the phase of the

See question #2.  The phase reversal occurs at an optimum frequency of

5) Talking about power loss, it's wrote that the test air-wound baluns, at
      their resonant frequency, are all about 15KOhm: at 14, 21 or 28 MHz
      are much less of course: how can I calculate the loss power?

The loss is all in the coaxial cable.  See the manufacturers data sheet.  I
should point out that I've often measured the loss of lengths of coaxial
cable, and it has usually been less than the figure shown in the
manufacturers data sheet.  The 6 turns around the 4" pipe will be about 7-8
feet of cable.  The loss should be negligible.

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