I have had some good questions and I will first refer all to the paper in
the IEEE publication.
However, to summarize: the efficiency was measured using the accepted
method for physically small antennas the H. A. Wheeler , Wheeler cap method.
This is a cylinder that fits over the antenna, and you make S parameter
measurements with and without the cap. It is a big aluminum can that
confines the radiated field, and this reflects back on the S parameters.
Electrically small antennas have a kr less than or equal to 1.
The resonance by spiraling the wires is reduced 13 per cent lower than the
resonance of a solid cone of the same dimensions. This first model was
resonant at 111 MHz with 32 MHz bandwidth to the 3 dB points for 2.8 average
SWR.
Min SWR was 1.3 over the bandwidth. This first model has a ht. of 24.1 cm,
diam. over the wide cone opening of 36.8 cm, and r was 30.3 cm, the slant
distance down the cone to the narrow end, where it is fed. ( Remember, 2.54
cm is 1 inch, for us metrically challenged types.)
There were 16 fold wires and all attached to the wire ring at the top,
evenly spaced. 18 ga. was used for all wires save the feed wire which was
no. 12. Fold spacing was 6.35 mm.
I know this is kind of sketchy, but it starts you off with something to
scale. Remember, later models which we built for 10M were smaller, than
just scaling from this prototype.
The difference between vertical and horizontal patterns was computed by
pattern functions in NEC 4.
Field strength measurements in an open field test site have confirmed the
intense near field and the high current of this design.
Thanks for all the interest, other questions should really go to KM5DR, Bob
Rogers.
73,
Stuart K5KVH
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