[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 

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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