On 7/30/2015 6:09 PM, wrote:
rickman wrote:
On 7/30/2015 5:46 PM, John S wrote:
On 7/30/2015 4:28 PM, rickman wrote:
On 7/30/2015 2:01 PM, Dave Platt wrote:
In article , rickman
wrote:

Yes, I read that, but it doesn't really explain this current. Later
they make the statement, "the current on the braid outside side is the
sum of currents other than transmission line currents on the entire
coaxial cable structure". This is pretty clear, but still does not
explain the source, or maybe I should say "why" the current flows on
the
braid and not the antenna.

Don't ask "Why does current flow on the braid?". Ask "What would *stop*
current from flowing on the braid?".

Current flows on *all* paths that have less than an infinite
impedance. That's its nature.

Remember, I = E / R (or, for AC/RF, I = E / Z). "I" (current on the
braid) will be nonzero, if the voltage at that point is nonzero (E !=
0) and the impedance down the braid at that point is not infinite.

The effect of a balun is to place a high "choking" impedance in series
with the outside of the feedline braid, thus "choking off" the current
flow.

I can't say I agree with your "choking" impedance idea. The coax
connects to the balun in the same way it connects to the antenna. The
balun can have no effect on the impedance of the coax shield. Just as
you ask, "What would *stop* current from flowing on the braid?" when
connected to the antenna what will stop the current from flowing on the
braid when connected to the balun?

The balun is an impedance that the RF sees as it starts to travel down
the outside of the coax toward the transmitter. But you know about
common mode currents, I think.

There is something fundamentally wrong with our communications. Are you
saying the balun is *part* of the coax? I have seen baluns made by
wrapping the coax around a core. I have been assuming the balun was a
transformer between the feed line and the antenna.

There are two types of baluns; voltage baluns and current baluns.

A voltage balun is usually a transformer and it forces the output
voltage to be equal.

A current balun is something that increases the impedance of the
outside the shield path. The common forms of choke balun are
simply wrapping the coax into a coil, wrapping the coax into a
coil around a ferrite rod, wrapping the coax into a coil on a
ferrite toroid, or large ferrite beads strung on the coax.

See www.eznec.com/Amateur/Articles/Baluns.pdf

Refer to figure 8. The balun is inserted between the feed line and the
antenna. Clearly this example adds no impedance to the shield of the
feed line. Rather it must present a very low impedance to the flow of
current from the shield to the antenna element.

You can also look at the illustrations in Appendix 1, both the voltage
balun and the current balun. In both cases they show transformers which
must present a low impedance path for the Io current as they call it.

The text here does talk about a construction of the current balun from
coax and the high impedance to current flowing on the outside of the
shield *in the balun*. But when considering the feed line, the balun
provides a low impedance to the current flowing from the inside of the
feed line shield (Ii) which means it will not follow any other path.

--

Rick