Thanks four your answers.

I was forgetting you normally use coax in a unbalanced configuration
where the braid is supposed to be at 0 voltage so only currents matter.

Would all this still hold if you used the coax as a _balanced_
transmission line? (unusual but -I think- possible). In this case
wouldn't voltages develop on the braid that could capacitively couple to
other conductors?

(assuming perfect solid shield, ...)

--
Toni

the voltage on the braid is not zero on the inside, it varies along with the
wave traveling along the inside of the coax. and the currents are exactly
balanced inside the coax also, they have to be or it wouldn't work. this
notion of balanced vs un-balanced transmission lines is totally confusing to
most people, in a proper system, say just with a dummy load on a coax the
currents on the shield exactly balance the current on center conductor. so
why do we go through all this stuff with bal-uns?? and coax chokes?? the
currents are already balanced, so WHY?? come on you gurus out there,
explain this one!


"Toni" wrote in message
...
Thanks four your answers.

I was forgetting you normally use coax in a unbalanced configuration where
the braid is supposed to be at 0 voltage so only currents matter.

Would all this still hold if you used the coax as a _balanced_
transmission line? (unusual but -I think- possible). In this case wouldn't
voltages develop on the braid that could capacitively couple to other
conductors?

(assuming perfect solid shield, ...)

--
Toni

Dave wrote:
so
why do we go through all this stuff with bal-uns?? and coax chokes?? the
currents are already balanced, so WHY?? come on you gurus out there,
explain this one!

Water comes out of a hose whether the hose is leaky or not.
So why ever bother patching or replacing a leaky hose?
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
the voltage on the braid is not zero on the inside, it varies along with the
wave traveling along the inside of the coax. and the currents are exactly
balanced inside the coax also, they have to be or it wouldn't work. this
notion of balanced vs un-balanced transmission lines is totally confusing to
most people, in a proper system, say just with a dummy load on a coax the
currents on the shield exactly balance the current on center conductor. so
why do we go through all this stuff with bal-uns?? and coax chokes?? the
currents are already balanced, so WHY?? come on you gurus out there,
explain this one!

The currents on the inside are always balanced -- they're purely
differential mode. The purpose of baluns or common mode chokes is to
reduce the common mode current which, on coax, flows entirely on the
outside.

If you're driving a dummy load from your transmitter, the common mode
current will be zero in an ideal system and negligible in a real system.
A balun or common mode choke will do nothing in that situation.

See http://eznec.com/Amateur/Articles/Baluns.pdf for a more complete
explanation.

Roy Lewallen, W7EL

Toni wrote:
Thanks four your answers.

I was forgetting you normally use coax in a unbalanced configuration
where the braid is supposed to be at 0 voltage so only currents matter.

How do you define "0 voltage"? Do a groups.google.com search of this
newsgroup and you'll find quite a bit of discussion about the futility
of trying to define a voltage between two distant points in the presence
of a field.

Would all this still hold if you used the coax as a _balanced_
transmission line? (unusual but -I think- possible). In this case
wouldn't voltages develop on the braid that could capacitively couple to
other conductors?

(assuming perfect solid shield, ...)

A coax line is balanced when the common mode current is zero. It's
unbalanced when common mode current exists. See
http://eznec.com/Amateur/Articles/Baluns.pdf for more information.

Voltages don't couple, fields do.

There was also some discussion not too long ago on this group about the
role of current in generating both electric and magnetic fields.

Roy Lewallen, W7EL

Well,
It looks like Dave successfully excited some natural frequencies in the
group for some weekend entertainment. Congratulations.
73,
Glenn AC7ZN

Oh, and by the way, natural frequencies cannot exist without forward,
and reflected...
:-

wrote in message
oups.com...
Oh, and by the way, natural frequencies cannot exist without forward,
and reflected...
:-


If there is reflector (impedance bump) in their way.
If W8JI waves are not reflected or opposed, then they would propagate
merrily into the ethernity and become the law of the RF jungle and other
pagan believers would worship them and praise the radio guru.
Waves have frequencies and sines/cosines, so this is related to antennas,
unless, of course there are those who "know better" :-)
Jus' stirring the pot...

Yuri da BUm

wrote:
It looks like Dave successfully excited some natural frequencies in the
group for some weekend entertainment. Congratulations.

I don't see how there can be any argument. The standing wave
current envelope magnitude pattern pictured in Kraus is a cosine
function where the angle of the cosine function is the number of
degrees away from the feedpoint. If the feedpoint current is 1.0
amps at 0 degrees, the standing wave current magnitude in Kraus'
1/2WL thin-wire dipole has the following pattern.

degrees away
from feedpoint current magnitude
0 1.000 amps
30 0.866 amps
45 0.707 amps
60 0.500 amps
90 0.000 amps

How can anyone defend an argument asserting that standing wave
current magnitude contains no phase angle information? The arc-
cosine of the standing wave current magnitude above *IS* the
number of degrees of phase angle away from the feedpoint. Since
the antenna is an archetypical standing wave antenna, that phase
angle is also the absolute magnitude of the forward and reflected
current phase angles where those phase angles have opposite signs.
--
73, Cecil http://www.qsl.net/w5dxp