As some of you have heard me hint for about a year, a revolution in small HF
Antennas is unfolding at Applied Research Labs, Univ. of Tx. I have been an
observer of experiments, sounding board, and kibitzer for antenna design.
Excuse my pride and enthusiasm, but my co worker, Dr. Bob Rogers, KM5DR,
really has hit on something of interest to antenna location challenged hams.
Should we call this the Sun City Antenna? Or we might christen this the
"Condo Cone".
The publication is in "IEEE Transaction on Antennas and Propagation", Vol.
49, no. 12, December 2001. I have just received my reprint. The title is
"Folded Conical Helix Antenna" by Justin Dobbins, and Robert L. Rogers. (I
will eventually be testing a HF version of this antenna, on 10 meters.)
The antenna takes the conical monopole to a new twist. A funnel shaped
structure of PVC pipe or wood, or other insulator ribs, holds a spiral
folded dipole like wire arrangement.
The initial paper concerns a low VHF model, however, further experiments
have shrunk a 10m resonant antenna, based on this principle, to a volume of
less than two cubic feet.
It can be further reduced in height over the example in the paper, and
calculations show that it should be possible to operate on 160m with a patio
fitting antenna with 90 per cent efficiency! That was not a misprint, 90
per cent! No loading elements of lumped components, (with attendant
losses), are needed. You do have to use a transmatch to move around the
band.
Materials to make the elements include two differing diameters of wire; even
RG 58 has been used for one element that is coiled into the funnel. The
folding of the element overcomes the fact that an electrically small cone
would have an input resistance much less than 50 ohms at first resonance.
Folding raises this back to 50 ohms. The fold is a shorted transmission
line. This antenna was produced without dielectric loading. However, that
is another method of altering the resonance.
Attempts to calculate the SWR and input impedance with NEC 4 did not give
consistent results. The design is small in terms of wavelength, and this
might be a factor. The folding of the radiators greatly affects these
electrically small antennas. At present, each is a one band antenna.
However, as to performance, a Radio Shack 10 M rig was used to work FM into
the East Coast, (from Austin, TX,) on days of only average propagation with
that version. Good signals were obtained both ways. The Antenna has been
tested over a 2m X 2m square metal ground plane, as well as in an open field
over shallow limestone rock. Increasing the sides of the ground plane did
not materially increase the signal. The Antenna IS doing the radiating.
The pattern of this antenna is much like that of a vertical (elemental
monopole), that is ground mounted. The vertically polarized power level was
22 dB above the horizontally polarized component. The antenna gain is 4.7
dBi, (over isotropic).
This publication is available at most College libraries with an Electrical
Engineering Dept. I would imagine your local library could obtain it on
interlibrary loan. Reprints will be sold by IEEE. Other papers are in
preparation, and I have urged Bob to build one for something like 160M, 80M
or 40M and maybe other bands, and write it up for QST, QEX, or another ham
publication.
The big thing about this approach to a small antenna is that it retains a
resonant length of radiator, while more efficiently using a confined space.
I think this is why the efficiency is so high compared to other "trick"
antennas which are little more than radiating tuned circuits or capacity
hats.
I hope others will be inspired to experiment with this concept. All you
need is a ring of insulator material to form the wide part of the cone,
(such as cut from large diameter PVC pipe), and some spokes or pipe to go
down and in to the narrow part where the feed is placed. Supports for the
spiral of folded transmission line have been successfully fabricated from
fiberglass strips, or other plastic, or even fish line. The antenna is
fitted INSIDE the cone frame, which is an open frame, not closed in at all,
thus minimizing materials. Ham ingenuity can be put to the full test with
arrangements for various bands. It is conceivable that a smaller cone could
be fitted inside a larger, lower band cone, and still have both work. There
are many, as yet, unanswered implementations, and this is why Bob said I
could let the word out now, to encourage further development in the ham
bands.
73,
Stuart Rohre K5KVH
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