In message , John S
writes
On 7/29/2015 1:16 PM, rickman wrote:
On 7/29/2015 1:42 PM, John S wrote:
On 7/29/2015 11:56 AM, Wayne wrote:

Lots of questions. A large area for investigation.

The reason for my balun question (other than to generate meaningful
technical banter on the newsgroup) is that some years ago in the age of
sliderules, a widely known and respected antenna guru told me that a
balun was unnecessary at resonance ( j=0 ).

I lost touch with him and don't know if his views changed over the
years.

I disagree with him.

Please see Roy Lewallen's (W7EL, author of EZNEC) site for some good
reading. He is a superb writer of easy to understand technical tidbits.
He has some balun stuff among other stuff.

http://eznec.com/misc/

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.

If the shack ground connections are not very short - or imperfect - the
supposedly grounded equipment is hot RF-wise. Furthermore, on
transmission the shield outer current radiates, and if it is in close
proximity to any susceptible domestic equipment, it can cause
interference problems to it. And because things will be reciprocal on
receive, the shield won't act too well as a shield to nasty RF
interference being emitted my nearby domestic equipment.
--
Ian