In article ,
The Natural Philosopher wrote:
Andy Wade wrote:

We've done this one often enough Tony, both here and on
uk.tech.digital-tv. If you connect a coax feeder to a symmetrical
aerial (such as the centre-fed dipole of a typical TV aerial) without
using any form of balun then the outer of the feeder becomes part of
the aerial. If you're transmitting, the outer of the coax will be
'RF-hot' and will radiate, quite possibly causing EMC problems to
low-level parts of the transmitting equipment as well as safety
concerns if high RF power is involved. When receiving, unwanted
signals picked up on this hot feeder will find their way into the
receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

From the mists of my memory I recall a folded dipole has a characteristic
impedance of 300 ohms - and an aerial balun changes this to the UK 75.

--
*Succeed, in spite of management *

Dave Plowman London SW
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