Bob Bob wrote:
Hi Roy

Mmmmm, yet another base belief falls screaming from the sky! grin

It would be interesting to know how it happened (the misconception that
is). The low level thinking (for me) on a transmission line is that for
a pair of wires, the signal on one side is always 180 degrees out of
phase with the other.

And why would that be? Unfortunately, there's nothing that forces this
to be true. There *is* something that forces it to be true *inside* a
coaxial cable -- the fact that the shield confines the fields created by
the wires. But then there's the outside to worry about. And there's no
such restriction on a twinlead transmission line.

I won't even try to deal with votages on a transmission line, because
you lose any hope of a "ground" reference when you get more than a
fraction of a wavelength away from the Earth. As far as the currents go,
though, there's no reason at all that the currents on the two wires of a
transmission line can't have any possible amplitude and phase
relationship. These are often separated into "common mode" and
"differential mode" or "odd mode" and "even mode" components for
analytical and mathematical convenience. But there are, at the end of
the day, two different currents on the two wires. In a coaxial cable,
the common and differential mode currents are actually physically
separated, the the former being entirely on the outside and the latter
on the inside. If you consider the sum of the current on the outside of
the shield and the current on the inside of the shield to be the "shield
current", you have exactly the same situtation with coax as for twinlead
-- two conductors with two currents that can have any possible
relationship.

Any noise induced by a magnetic field would be in
the same phase hence common mode rejection means it isnt "seen" by a RX.

True only if the receiver inherently has common mode rejection, which
most don't. Take a look at fig. 4 in the balun article. Imagine the
"output stage" being turned around so it's the input stage of a
receiver. Then suppose a noise source comes along and adds 1 mA downward
to both conductors of the cable, which would represent a perfectly
common mode pickup. The radio connector center conductor current would
increase by 1 mA -- which would be delivered directly to the the
receiver. The current on the inside of the radio chassis would increase
by 1 mA, and the current on the outside of the rig would increase by 2
mA (which, not coincidentally, is the total current resulting from the
noise source). The receiver would have a larger input signal as a result
of that noise.

I cant help thinking that unbalanced line is somewhat asymetrical in
that the diameter of the outer conductor would *somehow* have a non
trivial width when it comes to the wavelength of the frequency in use.
Needless to say I havent gone to the extent of plotting magnetic lines
and rereading my base theory stuff. Some things one just has to accept!

I get the *feeling* that balanced feeder is less likely to radiate than
unbalanced.

This becomes a factor only when the dimensions of the transmission line
become a significant part of a wavelength. At that point, you have other
problems maintaining transmission line operation, and up to that point,
other factors are much more important.


Why does one use triax in some situations? (cable damage and inadvertent
DC supply grounding aside)

There are situations in metrology and low level signal work where noise
rejection on the order of 100 dB and better is required. Those require
special shielding and techniques, but only after exceptional balance has
been achieved by other means. The problems most amateurs have with
current imbalance is orders of magnitude greater, and adding another
shield won't help.

You can put a shield around a twinlead line which will guarantee that
the sum of the currents on the two twinlead conductors and the inside of
the shield is zero. But there can be a separate current on the outside,
just like ordinary coax, which can radiate. You don't generally cure
balance problems by adding a shield.


One hopes it is a fair statement to say that any inbalance in the
current on either side of the transmission line (or phase shift ne 180
deg) will result in line radiation.. (However one gets it)

Yes. That's absolutely correct. A line(*), either twinlead or coax, with
equal magnitude and oppositely phased currents won't radiate
significantly. If the currents have any other relationship, they will.
The common mode current is often defined as (I1 + I2)/2, where I1 and I2
are the currents on the two conductors defined as positive in the same
physical direction. The radiation will be the same as if you had two
parallel conductors each carrying this amount of current, both in phase
and in the same direction -- or a single conductor carrrying (I1 + I2)
and no other current. If the currents are equal in magnitude and
opposite in phase, the common mode current is zero and there's no radiation.

(*) The line is assumed to have spacing very small in terms of wavelength.


I have grabbed the ARRL balun PDF from the Eznec site and will see how I
go... Have to pay some bills first!

I was trying to explain why an antenna (folded dipole or qaud) isnt
really a short circuit at the operating frequency, without referring to
terms like Xc, Xl, and resonance. I see I failed! What I was trying to
portray was that for the amount of time and distance (aka wavelength)
that the electrons had to travel around the antenna any instant in time
and at any specifc place would never see a short circuit. Or of you like
AC is very different to DC.

The problem with extremely simplistic analogies is that a thoughtful
person is bound to try and extrapolate to gain further understanding --
just as you're doing here. And the simpler the analogy, the less he can
extrapolate before the analogy falls apart. I don't think any
explanation of AC phenomena can get very far at all without at least a
rudimentary understanding of capacitance and inductance. If there is,
you'd have to be very careful in applying it.


What I really want to know is whether Paul's FM RX is now working!

Cheers Bob

Bob bob wrote:
"The low level thinking (for me) on a transmission line is that for a
pair of wires, the signal on one side is always 180 degrees out of phase
with the other."

That follows for me from the action of the simplest phase inverter, the
center-tapped coil. Feed one end against the center. The other end is
out-of-phase.

The current in opposite wires of a transmission line is flowing in
opposite directions. Hence, the wires are out of phase.

I think a parallel-wire transmission line has a self-balancing tendency.

Two parallel wires form the simplest transmission line, with the power
source at one end and a load at the other.

Distributed along the wires are series resistance, series inductance,
shunt capacitance, and shunt conductance.

Power travels from the source toward the load in the incident wave.
Velocity of the wave and voltage to current ratio in the wires depend on
construction of the line.

The destributed inductance in one parallel wire couples with its mate to
form a mutual reactance. The two wires are inductively coupled. They`re
also capacitively coupled.

With inductive coupling, Lenz`s law says, "the induced current is in
such direction that it opposes the change that produced it". It`s
reactive, pushing back at the imposed current. It flows in the opposite
direction. This is evident in self induction and secondary currents.
Induced current is out of phase.

With current in one wire inducing our of phase current in the opposite
wire of a long transmission line of closely coupled wires, balance
between the wires is enhanced.

Best regards, Richard Harrison, KB5WZI