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