Transmission line radiation
 

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

Can someone explain how a transmission line starts radiating as the
separation
between the center conductor and ground plane becomes greater and greater.

Ron, A transmission line, by definition does not have a center conductor, and
a ground plane that it reacts with. A transmission line is two close parallel
conductors carrying equal current, but 180 out of phase, so the EM fields
created by each conductor cancels. A transmission line does not radiate.
If you start increasing the spacing between the conductors, then the EM
fields cannot cancel, and according to Maxwell and others, radiation occurs,
you have an antenna. The most efficient radiator is a 1/2 wave dipole, almost
100%.
73 Gary N4AST

On Tue, 20 Apr 2004 21:39:16 GMT, 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?

Ron

One of the pest explanations I know of can be found at:

http://home.iag.net/~w2du/Reflection...nProblem. pdf

73
Danny, K6MHE

Ron, when the conductors in a transmission line are close, much much
less than a wavelength, the currents, which are out of phase, create
plus and minus EM fields that effectively cancel in all directions.

As the spacing becomes greater and becomes a significant portion of a
wavelength, the fields cancel in some directions but the spacing causes
the phase to add in other directions. Under this class of conditions the
line starts to become more like an antenna than a transmission line.

As a point of further confusion, coaxial cables that have woven
overbraid have a leakage inductance that allows some of the rf to leak
out. So, even coax has some radiation due to leakage.



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.

SNIP

On Tue, 20 Apr 2004 21:39:16 GMT, 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?

Ron

Hi Ron,

How it starts? You left the hose running. It comes out the end
because there is nothing there to keep it in.

This is not to say it comes out with a gush, however. A very short
protrusion of wire (the teensy radiator above this ground plane) has a
very, very small radiation resistance. In comparison to its Ohmic
loss, it can be so short that the wire itself simply turns it to heat
(or reflects the power back to the source due to the massive
mismatch). Result, not much gets out.

Increase the length of the wire, and the radiation resistance rises
faster than the Ohmic loss (the whole point of radiation is to load it
into THIS resistance and not the Ohmic loss of the copper). There may
still be a mismatch, but with the Standing Waves residing along the
length of the wire (now of sufficient length so that radiation
resistance dominates) the power eventually (within microseconds) gets
radiated. Result, enough gets out.

Increase the length of wire to wavelength proportions (typically a
quarter or more), and the radiation resistance easily eclipses Ohmic
loss. Result, most of the power is transferred out.

Let's fantasize some numbers:
1 inch radiator offers 0.001 Ohm loss and 0.000001 Ohm radiation
resistance. If you could tune this mismatch, you've got yourself a
cigar lighter. At night, folks would see you before they could hear
you.
1 foot radiator offers 0.01 Ohm loss and 0.0001 Ohm radiation
resistance. If you could tune this mismatch, you've got yourself an
infrared lightstick. You might be heard AND seen at the same
distance.
10 foot radiator offers 0.1 Ohm loss and 0.1 Ohm radiation resistance.
If you could tune this mismatch, you've got yourself a baseboard
heater suitable for a mobile 80M shoot-out.
100 foot radiator offers 1 Ohm loss and 30 Ohms radiation resistance.
Now you are talking DX.
Caution, these numbers are spun from whole cloth, but it illustrates
how important the relationships are.

Length counts in relation to wavelength. To strain a metaphor, how
warm and comfy would you feel on a cold winter night with an electric
blanket with a one inch element? Increase the length, spread the
load, and the best purpose is achieved with both.

73's
Richard Clark, KB7QHC

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

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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Dan Richardson wrote:
One of the pest explanations I know of can be found at:

Just who are you calling a pest?



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Oh Sheeeeeee! Those typos are going to kill me. F
Danny

On Tue, 20 Apr 2004 20:01:58 -0500, Cecil Moore
wrote:

Dan Richardson wrote:
One of the pest explanations I know of can be found at:

Just who are you calling a pest?



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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