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Cecil Moore[_2_] · September 5th 08, 12:23 PM posted to rec.radio.amateur.antenna

wrote:
The ideal transmission line is common mode and does not radiate
because the fields cancel as you said earlier. The dipole antenna is
ALSO common mode but the fileds do NOT cancel because the conductors
are physically 180 degrees apart from each other so they cannot
interfere with each other; instead the fields radiate into free space
rather than cancel each other out.

If transmission line currents were common-mode, they would radiate
like crazy and would be an antenna instead of a transmission line.
When one shorts the two wires together and feeds the system Marconi
style against ground, then the currents are common-mode. (Please
switch your news-reader to fixed font). A '+' indicates a connection,
not polarity.

+--------------------
| current reference
(V)
| Differential currents to antenna
+--------------------
Normal Transmission Line mode designed not to radiate

+----+------------------
| | current reference
(V) |
| | Common-mode currents (becomes antenna)
| +------------------
GND Marconi Style feed designed to radiate

If the
currents on a transmission line are differential, how would (COULD)
they be converted to common mode currents on the antenna? We would
need a 180 degree phase shift somewhere.

I'm glad you asked. When we take the last 1/4WL of transmission
line and open it up into a dipole, we have rotated one wire by
-90 degrees and the other wire by +90 degrees. That's a 180 degree
difference. The transmission line currents are 180 degrees out of
phase. The extra 180 degrees of physical rotation subtracts from
the 180 degrees in the transmission line for a total of zero
degrees (in phase) at the antenna feedpoint. This is explained
in detail in "Antenna Theory", by Balanis, 2nd edition, page 18.
I will try to duplicate it here using fixed font ASCII graphics.

In Phase Antenna Currents
--------------------+ +--------------------
| |
| |
+------------------+ |
(V) Differential |
+--------------------+
Transmission Line Currents

Note the out of phase currents in the transmission line results
in in-phase currents in the dipole antenna.
--
73, Cecil http://www.w5dxp.com