[CQ-Contest] Air wound coax balun

i4jmy at iol.it i4jmy at iol.it
Thu Feb 1 11:24:39 EST 2001

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 > RG213 is a solid dielectric coaxial cable with good aging
properties.  Solid
 > dielectric coaxial cable tends to be stiff by comparison to foam
 > types, having the disadvantage of naturally higher dielectric loss
and the
 > advantage of stability and high voltage insulating properties.
 > here means that the insulation does not compress under bending
 > resulting in characteristic impedance variations.

Melting point(loss of cristalinity) for compact polyetilene is only a
little above 100° C.
In hot summer days the 213 balun could easily reach relatively high
themperatures (because of sun radiation and a non reflecting color).
If another heating is internally induced and the diameter of the coiled
coax is too small, the inner conductor could start to migrate.
When the balun has to be small in diameter and/or the 213 appear to be
bulky, a good solution is to use PTFE coaxials like RG142 that's
dimensionally similar with RG58.
(Note that with a smaller cable like RG142 weight is less, turns can be
about twice for the same lenght and the inductance get's squared)

Mauri I4JMY

 > Just this moment I
 > performed an experiment, and found that at ~ +25 degrees C I could
 > bend the coaxial cable's center conductor around a 4 inch diameter
 > cup without any indication that the flexibility limits of the
 > 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
 > the mast at the top of the tower rather than fixed radius bends.
 > radio operators who build their own baluns usually have the advantage
 > being able to test them before putting them into service.  If you can
 > it, and it tests OK at your desired Italian legal power limit, then
 > should be confident it will continue to perform.  Experimentally,
 > looks like a safe choice to me, but the WA2SRQ measurements should be
 > for almost any RG8-like cable.

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