On 7/30/2015 6:05 PM, John S wrote:
On 7/30/2015 4:55 PM, 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.


The only thing that will stop the current from flowing on the
outside of
the shield when connected to the balun is if the balun presents a much
lower impedance path for the current than does the shield.

Think of a common mode choke. However it is mechanically implemented,
its purpose is to provide a block to common mode currents and allow
differential currents only.

The only way your suggestion makes sense is if the current actually
comes *from* the antenna and the balun prevents that current from
returning to the feed line.

Actually, your statement is the key. The feed point of the antenna IS
the current source. One end of the current source is applied to the
'hot' side of the antenna and the other end is applied to the other
element plus coax shield. As far as the feed point is concerned it has
one element on one side and two elements on the other side.

But that is now how it was presented. Another post indicated the
current the flows on the inside of the coax shield splits at the antenna
feed point and part flows down the outside of the shield. An easy way
to distinguish the two cases is to remove the antenna element from the
shield. Of course this is no longer a balanced antenna, but still, what
happens to the current on the shield? I assume it still flows on the
shield inside in an amount equal to the center conductor current and
then flows to ground on the outside of the shield?

Yes. Remember that the end of the coax connected to the antenna is now
the generator (the source, as jimp said correctly). If the center of the
coax carries X current, then the inside of the shield carries X current.
It MUST go somewhere. So, with nothing there but the outside of the
shield it runs down the outside of the shield with no 'knowledge' of
what is happening inside the coax. The shield has become the other
element of the antenna whether you like it or not and whether it goes to
ground or not.

The point is that the current flows on the outside of the shield because
the current follows the lowest impedance path it has to follow, same as
in all other cases. If you connect a balun to the end of the coax with
no antenna connection on the side from the shield, there will be no
current flow to the antenna element. So all the inner shield current
will *still* flow back down the outside of the shield showing that there
is no need to "explain" anything about a reflected wave from the antenna
end causing the current to flow down the coax.

--

Rick