Joe,
Sounds like you have a winner for an antenna. However the antenna you
describe is NOT a folded dipole. It would be a folded dipole if you did not
cut out the section of open wire line opposite your feedline connection.
Without the section cut out your antenna would be resonant on 40 meters and
would be a great performer on 40 meters, but I have no idea how it would work
on other bands. Your antenna is a LINEAR LOADED DIPOLE which will be resonant
around 5 or 6 MHZ. It should load well on 40 and 80 for sure since it will be
resonant around 5 to 6 MHZ. The losses on linear loaded antennas can be very
small, so on 80 meters I suspect it will perform nearly as well as a full size
dipole, perhaps within a db or so. If you could reach the center of the dipole
and jumper the cut out section, possibly, but not for sure, you might have
slightly better performance on 40 meters operating in the folded dipole mode.
You could set up a field strength meter with a horizontal antenna several
hundred feet away and do comparison readings.......the Z would be very
different so you would need to re tune.
Some "limited space" folks take your good idea one step further and make a 3
wire (or more) linear loaded antenna element. Your dipole might only be around
50 feet in length if made with 3 wires. However, there is no such thing as a
free lunch. Shorter antennas have less gain. My personal "rule of thumb" is
that a well designed antenna can be shortened up to APPROXIMATELY 50% with only
minor loss of gain, but further shortening usually results in rapidly
decreasing performance. Shorter antennas do suffer from greatly decreased
bandwidth, however if you you use a tuner that should not be a significant
factor.
I know many ELECRAFTERS will take exception to my 50% "rule of thumb",
however non technical folks might find it a useful tool. At least be VERY
suspicious of very short antennas that claim great results. Likewise antennas
that are AT LEAST half size MIGHT come close in performance to a full size
antenna. The 25 ft wire most of use on our KX1's with fantastic results is a
short antenna on 40 meters, but still is very acceptable compromize since it is
about 75% the size of a full lenght quarter wave antenna and also is an easy
match for the tuner. I believe my 50% rule is OK for most simple antennas. A
yagi would need to be much larger than 50% size for acceptable performance.
I find it is very interesting to model antennas with an antenna modeling
program. Many hams use either the EZNEC or Nittany Scientific NEC-Win Plus+
programs.
Either of these NEC 2 core programs do a very acceptable job of modeling most,
but not all, of the antennas hams typically use. ARRL offers an Antenna
Modeling on line course, or you can buy the ARRL course textbook and work
through it yourself. You do not need to be an engineer or scientist to use
these courses, however you should have a good grasp of algebra and perhaps trig
and AT LEAST 100 hours of time to complete the course.
Rick KL7CW Palmer, Alaska
KX1 # 798
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