Out of curiosity I experimented with some models to see
if I could quantify the behavior.
I started with full-sized elements using 1" diameter tubing,
with 15' spacing between them. I managed a bit over 5 dBi
gain, compared to 2.15 dBi for a single dipole. So let's say
3 dB gain for the close-spaced 2-element beam over a dipole.
Then I shortened the elements to 40' each and added loading
coils in the center of each side. I assigned a Q of 100 to the
coils, which is probably overly optimistic for a Hamstick. With
a bit of tweeking I managed to get a nice pattern, with better
F/B ratio than the full-sized elements - about 20 dB over much
of the rear quadrant. Tuning, however, was critical: a 2% change
in the loading coil dropped the gain by about 3 dB.
An even bigger problem was that the gain dropped to -7.3 dBi,
or about 12 dB lower than the original. For a dipole using the
same components, it was -2.5 dB, so adding the parasitic
element dropped the gain by about 5 dB, rather than adding
gain. It did, however, provide a directive pattern that might
be useful for reducing interference from unwanted directions.
And the situation would be worse if I shortened the elements
to 16', which is about that of a typical Hamstick dipole.
What are the issues? And why is the efficiency so much lower
on 80m than on the higher HF bands?
The main problem is the coil design. Yes, it has a relatively
small diameter for lower wind resistance, but it turns out that
a long, small-diameter coil is not the most efficient winding
geometry. A larger diameter coil with fewer turns, spaced about
one wire diameter apart (rather than close-wound on the former)
gives less loss for the same inductance.
And, of course, the short antenna has a lower radiation resistance,
so any resistance in the circuit reduces efficiency by a greater amount.
But there is another physical factor as well. All of the Hamstick-type
antennas basically use the same dimensions for the fiberglass bottom
section (that serves as a coil former) and for the top whip. Basically
the only difference is how far down the the coil extends from the top
of the bottom section. On 80m, the coil extends most of the length
of the bottom part of the antenna, so, rather than putting the coil
at the center of each side, it is closer to the feedpoint - more like
base loading than the center loading achieved with shorter coils on
the higher bands. That further reduces the radiation resistance,
causing higher currents and higher losses.
I have a Hamstick dipole set for 40m and 80m that was given to
me to try to see if it could be made useful. There were two major
issues: it was overly finicky to tune, and the efficiency was low.
The tuning was such that we could watch the SWR swing up and
down as a branch blew in the wind several feet away. That is
a side effect of using a short antenna, and there isn't much that
can be done about it. But a second problem appeared when we
lowered the antenna and raised it back up again, and the
resonant frequency changed without any change to the elements.
That was because it didn't have an effective balun. As a result,
the way that the extra coax was coiled on the ground affected
the SWR curve of the antenna. No wonder we had fits trying to
tune it!
To improve efficiency, I have picked up a pair of mobile extension
masts, something like 48" or 54", to place between the base of
each mobile whip and the feedpoint. That will make the antenna
longer, and move the loading coil further out on the antenna.
I haven't tried this part yet - I expect I may need to extend the
whips or add a capacity hat. But I wouldn't be surprised if that
provides as much gain (but not directivity) as you would get by
using the proposed 2-element loaded yagi on a short boom.