[Antennaware] helically wound dipole
Terry Conboy
n6ry at arrl.net
Mon Jun 22 20:02:50 PDT 2009
The approach of using a pair of helically wound monopoles to create a
balanced dipole should work fine. Of course, when you shrink a 66 foot
long dipole for 40m down to 8 feet long, you are definitely trading off
bandwidth and efficiency.
I haven't seen any cookbook calculators for TPI for helical antennas,
but I did a quick model in EZNEC for 40m. After some trial-and-error, I
found that using 11 turns per foot over an 8 foot length (88 turns and a
total of 132.25 feet of wire) of a 6 inch diameter hexagonal helix of
bare #14 copper wire, mounted at 6 feet above "Average" ground resonates
at 7.11 MHz. The 2:1 SWR bandwidth is about 14 kHz and the feed Z is
5.2 ohms. The "gain" straight up is -7.8 dBi.
I was curious where the losses were, so I set the wire to Zero loss and
the gain at the zenith rose to -3.9 dBi (an increase of 3.9 dB, oddly
enough) and the feed Z dropped to 2.1 ohms.
With copper wire loss and changing to Perfect Ground, the gain is -7.3
dBi and the feed Z is 3.1 ohms. With zero loss wire and perfect ground,
the gain is +8.2 dBi and the feed Z is 0.088 ohms. In free space, with
zero loss wire, the gain is 1.8 dBi and the feed Z is 1.23 ohms. (BTW,
in free space with zero loss wire, the average 3D gain is 0.00 dBi,
which is a good check that I probably didn't violate any NEC2 modeling
restrictions.)
Increasing the height of the helical dipole from 6 feet to 10 feet
increases the gain by 3 dB to -4.4 dBi. Of course, this lowers the
coupled ground losses, bringing the feed Z down to 4.3 ohms and the
bandwidth drops to 11 kHz. The resonant frequency also moves up to
7.122 MHz (up 12 kHz). Great fun!
Of course this isn't a finished design, but it gives you some idea of
what performance might be possible. The low feed Z could probably be
matched by tapping across a few turns at the center of the helix to find
50 ohms. This is a pretty low-loss matching arrangement, equivalent to
a beta/hairpin match, and shouldn't affect the bandwidth noticeably.
Adding capacity hats to the ends of the helix can improve both the
efficiency and bandwidth of the antenna, and are well worth considering,
if mechanically feasible.
In a real antenna, the insulating support that the helix is wound on
could add some loss, and its dielectric constant will probably increase
the interwinding capacitance and lower the resonant frequency. (I don't
know any way of modeling this directly in EZNEC.)
If anyone wants a copy of my EZNEC model, drop me a note off-list. FYI,
it has 528 segments, so many versions of EZNEC can't handle it.
However, I have a 20m version with ~39 turns, 6 inch diameter with a
length of ~7.8 feet that only has 233 segments and should work in most
versions. (Gain at 10 feet high at 14.048 MHz is +3.1 dBi at zenith,
feed Z=5.8 ohms, and bandwidth=53 kHz using 58.77 feet of wire total.)
73, Terry N6RY
On 2009-06-19 12:37 PM, Drax Felton wrote:
> I searched Google for "Helically Wound Antennas" and found plenty of
> articles for helically wound monopoles and verticals. Same with the ARRL
> antenna book.
>
> Is there such a thing as a helically wound dipole? I know people have made
> Hamstick dipoles, which are helically wound, correct?
>
> I am considering experimenting with a mobile dipole for 20m or 40m that is
> physically short (1 to 8 ft.), horizontally polarized, and is a balanced
> antenna that is continuously loaded without the use of loading coils other
> than the helix itself. The idea is to work NVIS mobile.
>
> I am not looking for wide band operating range. I have an automatic antenna
> tuner, but I only want to operate on the PSK31 spot frequencies, so self
> resonant is better.
>
> Any ideas?
>
> Where I can find the calculations for the number of turns per unit of
> length?
>
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