Ron wrote:
Can someone explain how a transmission line starts radiating as the
separation between the center conductor and ground plane becomes greater
and greater. Assume you out start with a wire over an infinite copper
ground plane that forms a 50 ohm Zo transmission line. Then increase the
distance between the wire and the ground plane until the wire becomes an
end fed antenna. What happens to cause radiation to begin?

Electrons shed excess energy by emitting photons. If those
photons are absorbed by electrons, they don't radiate. If
they are not absorbed, they radiate (at the speed of light).
This is one area where quantum electrodynamics is actually
easier to understand, from a conceptual standpoint, than
Maxwell's equations.
--
73, Cecil http://www.qsl.net/w5dxp



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A slightly different way of looking at it from what Gary wrote...but
quite similar.

It's better to think of a two-wire transmission line, probably. If
you want to think of the ground plane, just realize that it's
identical to the situation with two conductors driven out of phase:
you can insert the ground plane without any effect on the fields at
all.

Then each wire does radiate, but to the extent that their currents are
coincident in space and in opposite directions, those radiations
cancel. Net field at any point in space is the linear combination of
all the fields arriving at that point, at that instant in time. As
the wires become more separated, the radiations observed at a distance
no longer cancel. You're not the same distance from each wire, and
more importantly, the phase you see differs. Consider what you see if
the wires are separated by half a wavelength, and you are in the plane
the wires are in...and what you see if you are in a plane
perpendicular to the plane the wires are in and passing between them.
If you observe the fields close to one of the wires, of course the
cancellation is not good there, either, though that's energy
propagating in the direction of the line. Note that there's no
radiation from coaxial line, so long as the net currents in the inner
and outer are exactly out of phase and the current distribution is
uniform around the outer conductor (assuming the conductors are
exactly coaxial), even if the outer conductor is not very thick.

I'll (once again) recommend the "Antennas" chapter of King, Mimno and
Wing, "Transmission Lines, Antennas and Waveguides." You'll find it
in the antennas chapter rather than the transmission lines chapter
because it's radiation rather than energy propagation along the line,
I suppose. The introductory material in that chapter bears on this
topic, and later in the chapter there's very specific mention of
radiation from transmission lines, including what seem some
non-intuitive results about amount of radiation versus line length.

Cheers,
Tom

Ron wrote in message .com...
Can someone explain how a transmission line starts radiating as the separation
between the center conductor and ground plane becomes greater and greater.
Assume you out start with a wire over an infinite copper ground plane that forms
a 50 ohm Zo transmission line. Then increase the distance between the wire and
the ground plane until the wire becomes an end fed antenna. What happens to
cause radiation to begin?

Ron

According to Kraus' "Antennas" 92nd edition, chapter 2)....

An electric charge traveling at a uniform radiation along a straight wire
does not radiate.

When the charge reaches the end of a wire and reverse direction, it
undergoes acceleration (and deceleration) and radiates.

AN electric charge moving at uniform velocity along a curved or bent wire
is 'accelerated' and radiates.

An electric charge moving back and forth in simple harmonic motion (that is,
sine wave) has periodic acceleration and radiates.

If you have two parallel wires, one carrying a positive charge and the other
carrying a negative charge, it will not radiate.

If the two wires are bent away from each other, the charges radiate.

Kraus goes into more detail on all of this. Get yourself a copy of the book
or find one at the library. If they do not have one, they can get one on
interlibrary loan.

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net
"Ron" wrote in message
. com...
Can someone explain how a transmission line starts radiating as the
separation
between the center conductor and ground plane becomes greater and greater.
Assume you out start with a wire over an infinite copper ground plane that
forms
a 50 ohm Zo transmission line. Then increase the distance between the wire
and
the ground plane until the wire becomes an end fed antenna. What happens
to
cause radiation to begin?

Ron

JLB wrote:
An electric charge traveling at a uniform radiation along a straight wire
does not radiate.

Should that be "uniform velocity"?
--
73, Cecil http://www.qsl.net/w5dxp



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oops. You're right. Uniform velocity.

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net
"Cecil Moore" wrote in message
...
JLB wrote:
An electric charge traveling at a uniform radiation along a straight
wire
does not radiate.

Should that be "uniform velocity"?
--
73, Cecil http://www.qsl.net/w5dxp



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It is misleading to think that a feedline can radiate seperately from an
antenna.

When a feedline becomes unbalanced with respect to an antenna it becomes
PART of yet another antenna - which radiates.

The two TOGETHER form a resulting radiation pattern which is different to
either.
----
Reg, G4FGQ

Ron Wrote:
"What happens to cause radiation to begin?"

Ronold W.P. King wrote in "Transmission Lines, Antennas, and Wave
Guides, by King, Mimno, and Wing on page 224:
"Any closed loop of wire that is not confined to the near zone
(shielded) and that does not carry equal and opposite currents very
close together (adjacent and perfectly balanced though opposite)
radiates at least a fraction of the power supplied at its terminals."

Radiation is a phenomenon. Energy tries to spread out from where it is
to elsewhere. Unless it encounters barriers it just continues to spread.

Electromagnetic wave radiation is sustained by perpetual regeneration
from mutual stimulation. The electric wave contains all the energy for
an instant. The growth and decline of the electric field creates a
magnetic field which in turn creates an electric field and so on. These
fields can exchange their energies in a vacuum devoid of electrons or
electricity. They continue through the vacuum at the speed of light. The
waves go so far so fast that much energy escapes the attraction of its
source to recall it.

Terman says in his 1937 edition on page 700:
"A concentric transmission line radiates no energy under ordinary
conditions because the outer conductor acts as a substantially perfect
shield.

The power radiated from a non-resonant two-wire line:

Power radiated=160Isqd(pi a/lambda)sqd watts

I= line current

a/lambda= spacing in wavelengths""

In a resonant line, radiation is increased by about the ratio of peak
current to load current.

Best regards, Richard Harrison, KB5WZI