Reg Edwards wrote:

Cec, you took the bait.

So just exercise a teeny bit of your imagination.

Suppose you have a generator directly connected to a load resistance
without any line in between.

Let the generator and load terminals both be spaced apart by the same
distance as the conductors of the non-existent line.

The load carries a current along a length equal to the spacing between
its terminals.

The load, by virtue of its length, possesses radiation resistance.

And so radiation occurs with zero line length.

You've told us about radiation from the connections to the generator and
the termination.

Now tell us about radiation from the line.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

You've told us about radiation from the connections to the generator
and
the termination.

Now tell us about radiation from the line.

=================================

Ian, you are falling into the same sort of trap as old wives who
imagine most radiation comes from the middle 1/3rd of a dipole because
that's where most of the current is.

It is self-misleading to consider the various parts of a radiating
system to be separate components which are capable of radiating
independently of each other. They can't. A system's behaviour must
be treated as a whole.

We have already discussed that the power radiated from a generator +
twin-line + load is a constant and is independent of line length.

Total power radiated is equal to that radiated from a wire having a
length equal to line spacing with a radiation resistance appropriate
to that length. The location of the radiator, insofar as the
far-field is concerned, can be considered to be at the load. The
current which flows in the radiator is the same as that flowing in a
matched load. And the load current is independent of line length.

Mathematically, the only way for the total power radiated to remain
constant and independent of line length is for zero radiation from the
line.

In summary, the system as a whole BEHAVES as if there is NO radiation
from the line itself - only from fictitious very short monopoles (or
dipoles?) at its ends.
----
Reg, G4FGQ

Reg Edwards wrote:
In summary, the system as a whole BEHAVES as if there is NO radiation
from the line itself ...

How about BPL? (The Devil made me do it.)
--
73, Cecil http://www.qsl.net/w5dxp




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Reg Edwards wrote:

You've told us about radiation from the connections to the generator
and
the termination.

Now tell us about radiation from the line.

=================================

Ian, you are falling into the same sort of trap as old wives who
imagine most radiation comes from the middle 1/3rd of a dipole because
that's where most of the current is.

It is self-misleading to consider the various parts of a radiating
system to be separate components which are capable of radiating
independently of each other. They can't.

Actually they can, because that isn't the same as saying...

A system's behaviour must
be treated as a whole.

That is true, of course. Every component of an antenna (or in this case,
a parallel-wire transmission line) interacts with every other component.
The totality of those interactions is what determines how the RF voltage
and current will distribute themselves along the wires.

But once you know the magnitude and phase of the current in each small
segment of the antenna (which need not depend on theory or modeling - in
principle you could go around and measure it) then you have taken
complete account of the interactions. The radiated field from the whole
antenna is then the sum of the fields from the individual components
radiating independently.

However, we weren't originally talking about that...


We have already discussed that the power radiated from a generator +
twin-line + load is a constant and is independent of line length.

No, you have only asserted that.

Total power radiated is equal to that radiated from a wire having a
length equal to line spacing with a radiation resistance appropriate
to that length. The location of the radiator, insofar as the
far-field is concerned, can be considered to be at the load. The
current which flows in the radiator is the same as that flowing in a
matched load. And the load current is independent of line length.

Only if there are no radiative losses from the line itself - and you
have only asserted that, not proved it.

Mathematically, the only way for the total power radiated to remain
constant and independent of line length is for zero radiation from the
line.

Well obviously - but that is a circular argument, based entirely on your
assertion that the power delivered to the load is independent of the
line length.


In summary, the system as a whole BEHAVES as if there is NO radiation
from the line itself - only from fictitious very short monopoles (or
dipoles?) at its ends.

Sorry, but the "behaves as if" argument doesn't wash, because those
short monopoles are real. Since the line spacing is non-zero, those
short transverse sections must always exist, both in practice and in
your circuit model. Each section carries RF current, so it radiates - no
question about that, but it is entirely an end effect. It has nothing
whatever to do with radiation from the main line.

Looking edge-on at the line, we have two conductors carrying equal and
opposite currents, but one is slightly farther away than the other so
their transverse radiated fields do not quite cancel out.

The only question is mathematical: how does the small loss of energy
through radiation translate into a dB/m or dB/wavelength loss along the
transmission line?



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White G3SEK" wrote in message
...
Reg Edwards wrote:

You've told us about radiation from the connections to the
generator
and
the termination.

Now tell us about radiation from the line.

=================================

Ian, you are falling into the same sort of trap as old wives who
imagine most radiation comes from the middle 1/3rd of a dipole
because
that's where most of the current is.

It is self-misleading to consider the various parts of a radiating
system to be separate components which are capable of radiating
independently of each other. They can't.

Actually they can, because that isn't the same as saying...

A system's behaviour must
be treated as a whole.

That is true, of course. Every component of an antenna (or in this
case,
a parallel-wire transmission line) interacts with every other
component.
The totality of those interactions is what determines how the RF
voltage
and current will distribute themselves along the wires.

But once you know the magnitude and phase of the current in each
small
segment of the antenna (which need not depend on theory or
modeling - in
principle you could go around and measure it) then you have taken
complete account of the interactions. The radiated field from the
whole
antenna is then the sum of the fields from the individual components
radiating independently.

However, we weren't originally talking about that...


We have already discussed that the power radiated from a generator
+
twin-line + load is a constant and is independent of line length.

No, you have only asserted that.

Total power radiated is equal to that radiated from a wire having a
length equal to line spacing with a radiation resistance
appropriate
to that length. The location of the radiator, insofar as the
far-field is concerned, can be considered to be at the load. The
current which flows in the radiator is the same as that flowing in
a
matched load. And the load current is independent of line length.

Only if there are no radiative losses from the line itself - and you
have only asserted that, not proved it.

Mathematically, the only way for the total power radiated to remain
constant and independent of line length is for zero radiation from
the
line.

Well obviously - but that is a circular argument, based entirely on
your
assertion that the power delivered to the load is independent of the
line length.


In summary, the system as a whole BEHAVES as if there is NO
radiation
from the line itself - only from fictitious very short monopoles
(or
dipoles?) at its ends.

Sorry, but the "behaves as if" argument doesn't wash, because those
short monopoles are real. Since the line spacing is non-zero, those
short transverse sections must always exist, both in practice and in
your circuit model. Each section carries RF current, so it
radiates - no
question about that, but it is entirely an end effect. It has
nothing
whatever to do with radiation from the main line.

Looking edge-on at the line, we have two conductors carrying equal
and
opposite currents, but one is slightly farther away than the other
so
their transverse radiated fields do not quite cancel out.

The only question is mathematical: how does the small loss of energy
through radiation translate into a dB/m or dB/wavelength loss along
the
transmission line?



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian, G3SEK wrote:
"---how does the small loss of energy through radiation translate into
dBm or dB/wavelength loss along a transmission line?"

It is the reverse of a beverage antenna, which is a sort of single-wire
transmission line above the earth in its simple configuration. The
Beverage is a horizontal wire sensitive to vertically polarized waves.
It is working with the wave throughout its travel along its length.

The Beverage is vertically polarized because that is the direction of
the electric field between its conductors, the wire and the earth
beneath the wire.

The direction of the electric field in a parallel-wire transmission line
is from wire to wire. The effective radiator length of this polarization
is the line spacing. This is short compared to the length of the
transmission line in nearly all cases. The radiation is not emerging
from the end of the transmission line. It radiates slightly all along
the line as the wave navigates the line, much as the Beverage gathers
energy slightly as the wave sweeps along its length.

Best regards, Richard Harrison, KB5WZI

"Ian White said -

Looking edge-on at the line, we have two conductors carrying equal
and
opposite currents, but one is slightly farther away than the other
so
their transverse radiated fields do not quite cancel out.

===========================

Ian, Oh yes they do.

Next to each half wavelength of line there is another half wavelength
of line in which the current is in antiphase with it. And so, in the
far field, the fields from adjacent half-wavelengths of line cancel
each other out.

Now you'll say my logic falls down when the line length is an odd
number of half wavelengths. But you must not consider half
wavelengths of line to be behaving independently of each other.
----
Reg, G4FGQ

Reg Edwards wrote:

Ian, Oh yes they do.

Next to each half wavelength of line there is another half wavelength
of line in which the current is in antiphase with it. And so, in the
far field, the fields from adjacent half-wavelengths of line cancel
each other out.
. . .

No, they don't. They cancel only in two directions, directly normal to
the plane containing the wires. Radiation occurs in all other
directions, because the fields don't add in antiphase. An example of an
antenna which uses two closely spaced elements carrying equal
out-of-phase currents is the W8JK.

Roy Lewallen, W7EL

"Roy Lewallen" wrote
Reg Edwards wrote:

Ian, Oh yes they do.

Next to each half wavelength of line there is another half
wavelength
of line in which the current is in antiphase with it. And so, in
the
far field, the fields from adjacent half-wavelengths of line
cancel
each other out.
. . .

No, they don't. They cancel only in two directions, directly normal
to
the plane containing the wires. Radiation occurs in all other
directions, because the fields don't add in antiphase. An example of
an
antenna which uses two closely spaced elements carrying equal
out-of-phase currents is the W8JK.

Roy Lewallen, W7EL

=================================

Roy, I've never head of a W8JK. You are confusing the issue.

The problem is concerned with a LONG balanced transmission line and
its terminations which form part of the whole radiating system. And as
we can agree it is incorrect to consider parts of the system in
isolation.

To simplify the questions, wthout loss of rigor, it is best to
consider the line itself as being lossless with matched terminations.

I have stated that power radiated from the system is independent of
line length and nobody has disagreed. Indeed, a radiating power
calculating formula from reputable authors (of which I was unaware)
has confirmed this.

The power radiated from the system is identical to that radiated from
a monopole or short dipole, of length equal to wire spacing, with a
current equal to the current which flows in the terminations (ie., the
load). The terminations actually exist.

Radiated power = Load current-squared times calculated radiation
resistance.

That is obviously true down even to zero line length.

The implication is that radiation occurs only from the termination(s)
and that no radiation occus from the line. But, I repeat, we must NOT
consider the parts in isolation as do old wives.

You have stated that radiation from the line itself (in isolation)
must exist in the plane of the wires because of the finite spacing
between the line wires.

But we must consider only the far field. Not that in the immediate
vicinity of the line and its termination.

I suspect that the radiation pattern of a LONG-line SYSTEM converges
towards that from a monopole located in the position of the load.

Many of us are curious to acquire an idea of what the radiation
pattern looks like.

You are familiar with programs which produce far-field radiation
patterns. Do you know of a program which accurately produces the
radiation pattern of a very long close-spaced, zero resistance, pair
of wires terminated with a wire of length equal to wire spacing and
including a load resistance equal to Zo.

Patterns, of course, will change with frequency. It will be necessary
to statistically analyse results. Or just look at them from a common
sense point of view.

From a practical engineering viewpoint it is quite sufficient to know
what I innocently stated in the first place - the minute amount of
power lost is the load's radiation resistance times load current
squared and is independent of line length.
----
Reg, G4FGQ

Reg Edwards wrote:

"Roy Lewallen" wrote
Reg Edwards wrote:

Ian, Oh yes they do.

Next to each half wavelength of line there is another half
wavelength
of line in which the current is in antiphase with it. And so, in
the
far field, the fields from adjacent half-wavelengths of line
cancel
each other out.
. . .

No, they don't. They cancel only in two directions, directly normal
to
the plane containing the wires. Radiation occurs in all other
directions, because the fields don't add in antiphase. An example of
an
antenna which uses two closely spaced elements carrying equal
out-of-phase currents is the W8JK.

Roy Lewallen, W7EL

=================================

Roy, I've never head of a W8JK. You are confusing the issue.

The problem is concerned with a LONG balanced transmission line and its
terminations which form part of the whole radiating system. And as we
can agree it is incorrect to consider parts of the system in isolation.

To simplify the questions, wthout loss of rigor, it is best to consider
the line itself as being lossless with matched terminations.

I have stated that power radiated from the system is independent of
line length and nobody has disagreed.

Here on the back row, there's always been a hand raised in disagreement
on that point.

No argument that it's very, very small. But exactly zero - definitely
not.

Indeed, a radiating power calculating formula from reputable authors
(of which I was unaware) has confirmed this.

If you mean the Sterba reference, then please re-read it. All the
endorsements of the formula that you quote are peppered with caveats
such as "an approximation" and "providing that operations are confined
to wavelengths other than those within the ultra-short-wave region."

This is for the very good reason that some small amount of transverse
radiation does exist. Transverse radiation in the plane of the line is
small because the vector components of radiation from the two parallel
lines are equal in magnitude and almost exactly opposite in phase - but
never exactly opposite.

I am probably the only person in this discussion who has actually USED
parallel-wire lines "within the ultra-short-wave region". If you can
maintain good balance, the losses due to transverse radiation are
negligibly small for engineering purposes.

But to claim they are exactly zero is a physical absurdity... and I'll
always disagree with those.

(Sorry, I'll have to be out of this discussion again for a while.)

--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek