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Author Topic: 40m helicaly wound antenna ?  (Read 3030 times)
KD5RNN
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Posts: 44




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« on: June 13, 2002, 02:22:21 PM »

I have heard of a 40 meter helicaly wound antenna that has 33' of insulated wire on 5' of pvc. does anyone have the plans for this?? And would this be practictle?

73 KD5RNN Neil
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WB6BYU
Member

Posts: 17353




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« Reply #1 on: June 13, 2002, 03:52:40 PM »

It could work, but it probably won't be resonant on 40m.

I've heard a couple approximate formulas for helically wound
antennas:  the ARRL Antenna Book suggests winding a half-wave
of wire (that would be 65', not 33') if it is evenly wound, and an
article from Australia suggests 3/4 wavelengths of wire if the top
1/3 is close wound and the remainder tapered.  (This approach
increases the radiation resistance, which improves the impedance
match and can also help the efficiency.)

However, there are enough variables (including the length and
diameter of the pipe, size of wire, etc.) that either of these will just
give you a starting point:  the best approach is to check the
antenna with a dip meter to find the actual resonant frequency.

In general, the longer you can make the antenna, the better it will
work.  Also, short helical antenna can develop quite high voltages
at the top end, even at moderate powers.  The ARRL suggested
putting a pie pan or other capacitor on top as a capacity hat (this
also reduces the amount of coil needed), or you can add a short
spike or telescoping whip.  Often the capacity loading (of either
type) is made adjustable to tune the antenna over the band.  If the
antenna is resonated a bit below the band, a variable capacitor in
series with the feedpoint can also be used to tune the antenna.

Short antennas will generally have a very low input impedance
and narrow bandwidth, if it is operating efficiently.  Don't expect a
perfect match to 50 ohms, even at resonance.  For  a base-fed
vertical, I'd suggest tapping the feedline up onto the coil a few
turns above ground - first adjust the antenna to resonance, then
find the tap that gives the lowest SWR.  The two adjustments will
interact somewhat.  For a helical dipole, I would use a link coupling
coil wound over the center of the antenna and connected to your
coax - adjust the number of turns on the coupling coil for best
SWR.

Because of the low radiation resistance of short antennas, you
need to keep the losses low if you want it to be efficient.  This
means reasonably large diameter wire.  Also, don't use the grey
PVC electrical conduit, as some types tend to be lossy.  (I've
never had problems with white PVC pipe, and haven't tried ABS.)

The high-Q of these antennas means that any implementation will
require some experimentation to get it tuned.  But, it can certainly
work:  I have a commercial helical mobile whip for 40m that is 6'
long, wound on a 1/2" diameter fiberglass rod.  I think the upper 1/3
or so of the rod is close-wound (perhaps #16 wire?) and the rest
is basically just a connecting wire to the base.  The whole thing
is covered with heatshrink tubing.  And I can make contacts on it.
Granted, it doesn't work as well as my larger wire antennas, but it
does radiate a signal.
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