"Bal uhn" or "bayl uhn"?
 

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).

"Bal uhn" or "bayl uhn"?
 

On 7/29/2015 2:32 PM, John S wrote:
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).

I don't follow what skin effect has to do with the issue. The current
flowing on the outside of the shield is the only current flowing in the
shield. What's your point?

--

Rick

"Bal uhn" or "bayl uhn"?
 

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

"Bal uhn" or "bayl uhn"?
 

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?

--

Rick

"Bal uhn" or "bayl uhn"?
 

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.

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.
--
Ian

"Bal uhn" or "bayl uhn"?
 

rickman wrote:
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 energy IN the coax is not carried by either conductor, but in the
field between the conductors.

https://en.wikipedia.org/wiki/Coaxia...al_propagation

Once you connect to coax to something, the outside of the shield looks
like another current path with some impedance of it's own.

Likely easiest to visualize on a vertical antenna as being another radial.

See http://www.eznec.com/miscpage.htm and in particular the article
"Baluns: What They Do and How They Do It".


--
Jim Pennino

"Bal uhn" or "bayl uhn"?
 

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

"Bal uhn" or "bayl uhn"?
 

On 7/29/2015 4:38 PM, wrote:
rickman wrote:
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 energy IN the coax is not carried by either conductor, but in the
field between the conductors.

https://en.wikipedia.org/wiki/Coaxia...al_propagation

Once you connect to coax to something, the outside of the shield looks
like another current path with some impedance of it's own.

Likely easiest to visualize on a vertical antenna as being another radial.

See http://www.eznec.com/miscpage.htm and in particular the article
"Baluns: What They Do and How They Do It".

I'm not at all clear on what you are trying to say. I have no idea why
you are shifting the conversation to the details of the power
transmission. Exactly what was written that you are replying to?

--

Rick

"Bal uhn" or "bayl uhn"?
 

rickman wrote:
On 7/29/2015 4:38 PM, wrote:
rickman wrote:
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 energy IN the coax is not carried by either conductor, but in the
field between the conductors.

https://en.wikipedia.org/wiki/Coaxia...al_propagation

Once you connect to coax to something, the outside of the shield looks
like another current path with some impedance of it's own.

Likely easiest to visualize on a vertical antenna as being another radial.

See http://www.eznec.com/miscpage.htm and in particular the article
"Baluns: What They Do and How They Do It".

I'm not at all clear on what you are trying to say. I have no idea why
you are shifting the conversation to the details of the power
transmission. Exactly what was written that you are replying to?

What I am talking about is the current on the outside of a coax, not
"shifting the conversation".

You asked "What exactly is the source..." and I am providing the
answer to that question.

One thing I left out is that the match and reflections have nothing
to do with the current on the outside of the coax.

Try reading the links I provided if you want more details to the
answer to your question of where does the current on the outside of
the coax come from.



--
Jim Pennino

"Bal uhn" or "bayl uhn"?
 

On Wed, 29 Jul 2015 15:19:29 -0400, rickman wrote:

I am having trouble forming an image of this. What exactly is the
source of the "returning RF"?

This might help:
http://www.antennex.com/w4rnl/col0606/amod100.html
The first few paragraphs are the applicable parts. Quoting a few
tibits:
"Fig. 1 presents one traditional way to portray the situation
at the dipole feedpoint. Its general purpose is to show why
the insertion of a balun is important as a precautionary measure
in dipole construction."

"However, the current from the braid has 2 paths: the right
leg of the dipole in the figure and the outer side of the
coaxial cable braid."

The rest of the article deals with modeling issues and problems.

When modeling a balun, I use three conductors for the coax. The usual
inner and outer conductors, which are assumed to handle only
differential current and therefore do not radiate, and a mysterious
3rd conductor on the outside, which carries all the common mode
current that does the radiating.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558