Thanks, Jim, for a short and sweet explanation!

--
Dan D.
http://www.ameritech.net/users/ddevillers/start.html


..
"Jim Weir" wrote in message ...
Here's the theory...all antennas are "dipoles". You know, those rabbit ear
things we used to have on top of our TVs when we watched Eisenhower take the
oath of office. Di (two) pole (rod). Two rods. Remember the Frankenstein
(That's Franken SCHTIEN to you, bozo) movies where the Jacob's ladder started in
the middle and slowly progressed to the end, where it disappeared. Did it
really disappear? No, it transformed itself from one form of energy to radiated
energy. Don't believe me? Try and receive an AM radio station with one of
those suckers in the same room.

Now, it comes to pass that almost all of the energy that we put into the rods
where they come close at the center will be transformed to radiated energy when
the rods are an odd multiple of a half-wavelength long. The rods, being equal,
are therefore an odd multiple of a quarter-wavelength long each (with a finagle
factor shortening them up about 5% for such esoterics as velocity factor and end
effect). The shortest dipole is with the odd number 1 as the multiplier, which
is why we usually just say that the dipole ears are a quarter-wave long.

Just like we can trick somebody into thinking we are where we ain't with
mirrors, we can trick one of the dipole ears to think that it has the requisite
other ear by putting a metal "mirror" underneath it. That is, the dipole looks
into the mirror and "sees" what it thinks is the other dipole ear (really just
an image of itself) and is quite happy to radiate...IF the mirror is an odd
multiple of a quarter wave from rod to edge.

Fortunately for us, the mirror doesn't have to be one continuous sheet of
metal...it can be broken up into smaller sheets, rods, tubes, wires, and other
metalmongery. If it is the metal fuselage of a metal aircraft or the metal
framework of a tube'n'rag aircraft, the theory is that SOMEWHERE in that
conglomeration of metal, there will be SOMETHING that is an odd multiple of a
quarter-wave (about 22" at 127 MHz.).

That's the long answer. The short answer is that your mounting plate should be
electrically tied to the metal framework of the airplane at every opportunity...

Jim




Bill French
shared these priceless pearls of wisdom:

-The antenna base plate on my ragwing ( Bellanca Decathlon) is
-approximately 12" long by 8" wide and sits on wooden formers on the
-turtle deck that are centered on the main longeron - Decathlon /
-Citabria triangular fuselage structure - and about 2" above it.
-
-The front and rear ends are flanged downward and there is a metal tab on
-the longeron at each end of the plate. The tabs are about 2" from the
-flange.
-
-Is it necessary to bond both ends of the plate to the fuselage tabs to
-use the steel frame as an adequate ground plane or is just bonding one
-end sufficient?
-
-I'm bonding the plate with a piece of aluminum the same width as the
-plate and then tapering to approximately 3/4" where it joins the tab.
-
-I'm concerned about setting up improper radio signals by having the
-plate just grounded at one end.
-
-Also crawling into the fuselage rear section when you're 5' 11" and 220
-lbs is a real pain so would just like to do this job just once.
-
-Any help greatly appreciated and thanks in advance.



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com