On 15 Sep, 19:16, wrote:
I have built several coaxial dipole antennas in the past with great
success. What I like about this antenna is it is broadbanded. If you
design it using the center of the band of your interest, for the most
part you can cover the entire band without retuning. The problem I
have always had with this is the antenna is somewhat fragile. I have
used plexiglass squares to secure the center of the antenna and taped
nylon fishing line to the legs for more strengh. Any one with more
ideas? These are mostly useful in the HF range. You can find the info
on designing these athttp://www.amateurradios.info and other places
on the net.

73's
Bill w5grx

BILL
The best way of using coax with antennas is by stripping the PVC and
soaking the braid
in polyurothane so the clock wise windings are insulated from the
counter wound windings
By shorting one end of the braiding you can feed the other end i.e.
the two different windings
2where the actual span is somewhat less than a 1/2 wave ( calculate
the true wire length)
The same can be accomplished by using somewhat less than a full wave
length folded back on itself
and directly feed the ends. This arrangement takes the fragility away
from the span by not feeding at the center
as well as using the center wire of the coax for mechanical strength.
This method comes in quite usefull when the need is for a 160 meter
antenna where you can wind the coax cross wise fashion so the ends for
feeding finish together.
This comes out at around 60 turns on a 2 foot former. This method
gives more gain over 1/2 wave designs in a similar way that a quad or
loop of one wavelength exceeds others. You can also use a single wire
instead of coax for cheapness if you wish but ofcourse you like coax
but this way you can join scraps of coax and solder them together
without bothering with individual strands. This method by the way does
not require a ground plane so you may leave the barrel on the ground
or place it on top of the tower for rotation purposes. You can have a
lot of fun with this antenna if you can also rotate the axis of the
barrel by useing two rotators or a combination satelite version.
Note thatusing a full wave length antenna you achieve tha same
gains( ~ 3db) that a quad obtains over the standard fractional WL
length radiator(~2db when using single wire). Feed impedance is around
50/60 ohms resistive and is quite broad banded. The above is based
around Gausswian formulae that also conforms with Maxwellian laws so
you can use suitable modeling programs such as AO to derive all
specifications. The above is subject to possible U.S. patents
acceptance at this time
Enjoy
Regards
Art
KB9MZ....XG