In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
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.
I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....
No, thats not the point.
It is the point as subsequent posts will show. The only inaccuracy in
that statement is where you are driving a Marconi quarter wave vertical
where it is an unbalanced system!...
I am not talking driving anything.

I am talking about receive only.


You just don't get the fact the whether transmitting or receiving the
theory and operation are identical!...


Not so.

It's like saying that wiring up a motor is no different from wiring up a
microphone.

And what is so very different apart from the voltages and currents?..


And in fact its perfectly sensible to run a microphone earth down the
same line as that returning current from a large electric motors.

Of course it is..




--
Tony Sayer