On 7/29/2015 4:05 PM, Ian Jackson wrote:
In message , rickman writes
On 7/29/2015 3:14 PM, Ian Jackson wrote:
In message , John S
writes
On 7/29/2015 1:16 PM, rickman wrote:

Perhaps someone can explain the issue of current in the coax shield.
Current gives rise to a magnetic field. But the current in the inner
conductor is opposite and would create a magnetic field that would
cancel the field of the outer conductor, no?

What am I missing?


Skin effect. The currents on the inside of the shield and on the
outside of the shield see different things. They each have no idea
what the other is doing.

As for magnetic field, I must step aside. I can only report what the
gurus say (nothing that I've found).

Even though the coax shield is grounded at the shack end, both halves of
the antenna get fed push-pull (in anti-phase) with the RF signal flowing
on the outer skin of the inner conductor and the inner skin of the
shield.

However, at the antenna end, the returning RF on the shield side of the
antenna doesn't know that it should stay on the inside of the shield.
Because of the skin effect, it happily makes for the outside, whence it
flows back to shack, and through the shack grounding connections.

I am having trouble forming an image of this. What exactly is the
source of the "returning RF"? Is this reflected RF at the impedance
mismatch at the feedpoint? If so, the situation being discussed has
no impedance mismatch, so no returning RF. Is the returning RF from
the signal being radiated from the antenna inducing current in the
shield? If so, doesn't the inner conductor also pick up the radiated
signal?

The RF (which is, of course, an AC signal) doesn't just flow out of the
top end of the coax and into the two halves of the antenna. The fact
that the antenna has a standing wave on it means that some RF is
bouncing off the far ends of the antenna, and back to (and into) the top
end of the coax.

So you are saying that with a perfect match to an antenna with a real
only impedance (the stated condition for this discussion) there will
still be a reflected wave on the feed line?


There is no reason why the returning RF current on the shield leg of the
antenna should want to flow back on the inside of the shield - in fact,
a combination of the Faraday shield effect and the skin effect
encourages it to take the easy route on outside of the shield.

I'm not at all clear on the location of current flow on the shield, but
what about the current flow on the inner conductor? If the antenna
reflects a balanced signal back into the cable isn't there also a
current in the inner conductor which will create an opposing magnetic
field? Maybe that is not the issue as some are talking about the
problems created by the voltage drop to ground on the shield.

--

Rick