Making a simple antenna for 40 Meters is
not very difficult. That is, if you have the space. A standard center fed
dipole dipole for 40 Meters needs around 67 Feet of space. But, what
if you only have space for a 20 Meter dipole, 33 Feet? If this is
case, than you have several options.
You could just forget about 40
Meters and work the higher frequency bands, 20 Meters on up.
What? And miss out
on all the fun dodging the the short wave broadcasters in the
evening. You could create a Inverted-V
type of antenna and raise the feedpoint on a mast.
This is a possible
alternative, but for this particular case, you would need a 28 Foot
center mast and the apex angle would be less than optimum.
This may cause some signal cancelation and give you a radiation
pattern that you don't want. You could shorten the dipole
arms to fit the space and use a loading/matching coil in the center.
Item number 3 is what this page is about.
Jact Sobel, W5VM (which is now assigned to Vernon Dyer), had at one time
described a shortened dipole
center fed with a loading/matching coil at the feed point. A drawing
of which is below.
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Initially, this seems to be a different approach
than the shortened dipole designs, detailed on my
Short Dipole page. But it's really not.
If you tilt your head, and cross your eyes a little bit, you might start seeing
it as two coils, very close together. In fact, the coils are so close to the center,
that they touch..
Assuming that the two coils are an equal number
of turns, and that the wires attached to each side are equal in length, the center
of an antenna should be a zero current point. this makes a handy place to tie
your coax shield. You could wrap several turns of wire around the coil in the
center and feed it that way. But I couldn't begin to tell you how many turns to use
or what the feed impedance would be. Each turn of the coil, as you move away
from center, provides you with a different impedance and a possible match.
By attaching the center of your coax to one of the coils turns, you should be
able to find a good 50 Ohm feed point. This then
gets around the balanced to unbalanced conversion effort (balun), that would be required
and you were center feeding or link feeding..
Each element arm is 18 Feet 6 Inches (5.029 M) long.
The loading/matching coils consists of 30 turns of 12 SWG
enamelled copper wire wound on 2.5 inch (63.5 mm) diameter PVC tube
6 inches (152.4 mm) long. The winding pitch should be about
6 turns-per-inch (25.4 mm). Although the picture doesn't show it very
well, the shield of
the 50 coaxial cable is connected to the center of the coil. The coax center
conductor is connected to a point 2 or 3 turns away from the center,
to a point which gives the lowest SWR. This point may take some experimenting,
depending on which section of the band you wish to operate in.
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