"Cecil Moore" wrote in message
...
dansawyeror wrote:
Below is a link to a site that claims to model coax radiation from a
dipole.

http://www.smeter.net/feeding/feedpowr.php

Please note that the third wire to ground creates the unbalance
that causes feedline radiation. You seem to be confusing cause
and effect. The cause of the feedline radiation is the existence
of that third wire, not SWR. All it proves is that feedline
radiation is caused by that third wire path which unbalances
the source currents.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil

How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?
I see this as a situation where the *outer shield* of a transmission line
is conducting current that radiates.
It seems that a "balanced" antenna that is comprised of a single
conductor and a L shaped conductor that includes the outer conductor of the
coax, could be fed with a balanced line for modeling.

Jerry

Jerry Martes wrote:

"Cecil Moore" wrote in message
...

dansawyeror wrote:

Below is a link to a site that claims to model coax radiation from a
dipole.

http://www.smeter.net/feeding/feedpowr.php

Please note that the third wire to ground creates the unbalance
that causes feedline radiation. You seem to be confusing cause
and effect. The cause of the feedline radiation is the existence
of that third wire, not SWR. All it proves is that feedline
radiation is caused by that third wire path which unbalances
the source currents.
--
73, Cecil http://www.qsl.net/w5dxp


Cecil

How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?
I see this as a situation where the *outer shield* of a transmission line
is conducting current that radiates.
It seems that a "balanced" antenna that is comprised of a single
conductor and a L shaped conductor that includes the outer conductor of the
coax, could be fed with a balanced line for modeling.

Jerry


What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

On Wed, 31 Aug 2005 16:17:42 -0500, "David G. Nagel"
wrote:

What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

Please read:

http://www.w2du.com/r2ch21.pdf

David G. Nagel wrote:
What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Is "matched" the correct word to use there? A 50 ohm balanced dipole
is perfectly "matched" to 50 ohm coax but the feedline will likely
radiate. A 50 ohm balanced dipole is not "matched" to 600 ohm balanced
line but with proper attention to details the feedline will like not
radiate much.
--
73, Cecil http://www.qsl.net/w5dxp


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"David G. Nagel" wrote in message
...
Jerry Martes wrote:

"Cecil Moore" wrote in message
...

dansawyeror wrote:

Below is a link to a site that claims to model coax radiation from a
dipole.

http://www.smeter.net/feeding/feedpowr.php

Please note that the third wire to ground creates the unbalance
that causes feedline radiation. You seem to be confusing cause
and effect. The cause of the feedline radiation is the existence
of that third wire, not SWR. All it proves is that feedline
radiation is caused by that third wire path which unbalances
the source currents.
--
73, Cecil http://www.qsl.net/w5dxp


Cecil

How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?
I see this as a situation where the *outer shield* of a transmission
line is conducting current that radiates.
It seems that a "balanced" antenna that is comprised of a single
conductor and a L shaped conductor that includes the outer conductor of
the coax, could be fed with a balanced line for modeling.

Jerry
What I was taught is that in a properly installed antenna system the coax
will not radiate. If the antenna is not properly matched to the coax you
get current flow along the outside of the coax shield.

Dave WD9BDZ

Dave

I'd agree with your statement if "matched" doesnt refer to *impedance*
match.

Jerry

David G. Nagel wrote:

What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

It's too bad people are being taught this. As a number of others have
correctly said, impedance match has nothing to do with whether or not a
coax (or symmetrical twinlead line) radiates. Radiation is due to other,
unrelated causes.

Roy Lewallen, W7EL

David G. Nagel wrote:

SNIPPED

What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

A balanced antenna fed with balanced line, and the line goes away from
the antenna perpendicular [90 degrees angle] for a minimum of 1/4
wavelength is a properly installed antenna system: [line has minimum or
no radiation]

A balanced antenna fed with balanced line, and the line goes away from
the antenna non-perpendicular [other than 90 degrees angle] for a
minimum of 1/4 wavelength is an improperly installed antenna system:
[line will radiate]

A balanced antenna fed with un-balanced line, and the line goes away
from the antenna perpendicular [90 degrees angle] for a minimum of 1/4
wavelength is an improperly installed antenna system: [line will radiate]

A balanced antenna fed with un-balanced line, and the line goes away
from the antenna non-perpendicular [other than 90 degrees angle] for a
minimum of 1/4 wavelength is an improperly installed antenna system:
[line will radiate]

A balanced antenna fed with un-balanced line and a properly installed
balun, and the line goes away from the antenna perpendicular [90 degrees
angle] for a minimum of 1/4 wavelength is a properly installed antenna
system: [line has minimum or no radiation].

An un-balanced antenna fed with balanced line is an improperly installed
antenna and the line will radiate.

NEED I CONTINUE??

Ham op wrote:

A balanced antenna fed with balanced line, and the line goes away from
the antenna perpendicular [90 degrees angle] for a minimum of 1/4
wavelength is a properly installed antenna system: [line has minimum or
no radiation]

The line will have minimum or no radiation only if fed at the
transmitter end with a balanced feed. An unbalanced feed will create
unequal currents in the conductors, resulting in radiation. A
symmetrical (e.g., twinlead) line doesn't assure balance (equal and
opposite currents in the two conductors), and an unbalanced line will
radiate, regardless of its physical construction.

A balanced antenna fed with balanced line, and the line goes away from
the antenna non-perpendicular [other than 90 degrees angle] for a
minimum of 1/4 wavelength is an improperly installed antenna system:
[line will radiate]

True for both symmetrical line and coax. Radiation is due to common mode
current induced on the line. It can be reduced by inserting "current
baluns" (common mode chokes) in the line. A couple spaced about a
quarter wavelength apart are usually adequate.

A balanced antenna fed with un-balanced line, and the line goes away
from the antenna perpendicular [90 degrees angle] for a minimum of 1/4
wavelength is an improperly installed antenna system: [line will radiate]

The amount of common mode current and therefore radiation depends on the
length of the path along the outside of the coax to ground. A current
balun (common mode choke) at the feedpoint will reduce the current and
therefore radiation.

A balanced antenna fed with un-balanced line, and the line goes away
from the antenna non-perpendicular [other than 90 degrees angle] for a
minimum of 1/4 wavelength is an improperly installed antenna system:
[line will radiate]

This is due to induced common mode current. The mechanism is identical
to that when a symmetrical line is used.

A balanced antenna fed with un-balanced line and a properly installed
balun, and the line goes away from the antenna perpendicular [90 degrees
angle] for a minimum of 1/4 wavelength is a properly installed antenna
system: [line has minimum or no radiation].

Correct. A current balun at the feedpoint reduces the conducted common
mode current. Induced common mode current is prevented by symmetrical
feedline placement.

An un-balanced antenna fed with balanced line is an improperly installed
antenna and the line will radiate.

Coax feeding an unbalanced antenna will radiate just as much as twinlead
feeding an unbalanced antenna.

NEED I CONTINUE??

A lot of people have learned a list of handy rules like this without
having much of an understanding of where they came from or under what
circumstances they do and don't apply. For anyone interested in learning
more about common mode current, feedline radiation, types of feedlines,
and feedline radiation, I suggest taking a look at
http://eznec.com/Amateur/Articles/Baluns.pdf. It deals only with
conducted common mode current and doesn't discuss induced common mode
current -- I'll try to get together a supplement covering that topic
when I have time. There was some discussion on this newsgroup not long
ago about induced common mode current, but I can't locate the topic
right off. It should be possible to find it with a google search. Be
sure to also look at the work by Walt Maxwell, W2DU referenced at the
end of the balun article.

Roy Lewallen, W7EL

Jerry Martes wrote:
How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?

The physical construction of the coax (ideal version) ensures
that the inside of the coax doesn't radiate because, for
ideal coax, the internal currents are perfectly balanced.
Any unbalance in the currents is forced to the outside shield
by the laws of physics.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...
Jerry Martes wrote:
How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?

The physical construction of the coax (ideal version) ensures
that the inside of the coax doesn't radiate because, for
ideal coax, the internal currents are perfectly balanced.
Any unbalance in the currents is forced to the outside shield
by the laws of physics.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil

I failed to emphisze that any current on the outside of a coaxial
transmission line are out of the bounds of the defination of Coax.

Jerry