On Sep 5, 7:23*am, Cecil Moore wrote:
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

Upon doing independent research, it turns out my theory is exactly the
same as your's but my terminology was reversed. The situation is
summed up in an article by W8ji:

(quote)...the line will not radiate or contain substantial electric or
magnetic fields external to the line area. The lack of external
fields, even at a very small distances, is rooted in two conditions:

1.) All outgoing currents on one conductor are matched by equal
level and exactly opposite phase currents on a return conductor at any
given point along the line. (The reason I said the line will not
radiate) This causes an exactly equal and opposite magnetic field
along each conductor. The opposing magnetic fields caused by equal
currents flowing opposite directions cancel magnetic fields outside
the general area of the two conductors.

2.) All voltages from each conductor of the line to the outside
environment surrounding the line are either contained within a closed
shield, or are exactly equal and opposite an imaginary neutral
reference point representing the environment around the line (balanced
lines) (I discussed this imaginary reference point in some detail as
well). We always have a constant ****differential*** voltage across
the line (between the conductors) and that voltage changes only with
standing wave ratio as we move along the line.

3.) The vector product of differential current flowing in
conductors and voltage between line conductors at any point along the
line always equals the power transmitted in transmission line mode.
(unquote)

So I simply reversed the terminology and I agree that your use of the
term "differential" was correct. I was calling the mirror image
current pattern "common mode" when it should be differential.

But I still shy away from matching impedances with a "choke". I would
match impedances with a balun or RF transformer. I agree that you
must isolate, either mechancially or electrically, the input from
output on a balun. I made the simple mistake of confusing the terms
differential and common mode. Thank you for the correction. And I have
tried using the CM mode marconi style feed (shorting the antenna at
the transceiver input terminals) to try to get my 80m G5RV to radiate
at 2Mhz some years ago, as suggested by MFJ in their instruction
manual, to no avail. I am sure it can radiate quite well at some
frequency however!