1. The theory of electromagnetic wave propagation
along an infinite helical conductor using
(a) the Sheath Helix model and
(b) the Tape Helix model
has been applied to the helical aerial a result the following predictions have been made
for she first time, for any pitch angle .

(i) the upper frequency limit of the Endfire
Helical Aerial by means of the Sheath Helix
Model

(ii) the upper and lower frequency limits of the
Endfire Helical Aerial by means of the 'ape
Helix Model.

(iii) the upper and lower frequency limits of the
Broadside Helical Aerial with Coaxial
Conducting Cylinder using the Sheath Helix
model.

2. As a result of the above theoretical investigations it has been predicted and confined
experimentally that the upper frequency limit
of the helical aerial is not independent of
length as had previously been reported. she form
of this variation can be computed and the
theoretical values agree with experiment to an
accuracy or better than 10%.

3. The prediction has been further made and
confirmed that what was previously believed to be
a lower limit of pitch angle of 5° for the Endfire
Helical Aerial does not in fact exist. Satisfactory
experiments have been carried out with pitch
angles as lo',: as 1.8 °, which was the lowest
physically possible at the frequency of operation
used. It is believed now that there is no lower
limit of pitch angle.

4. It has been predicted and confirmed that the
free space circumferential length C equal to
unity is not the centre frequency of operation of
the helical aerial in general. It is the centre
frequency only for short aerials using a medium
helical pitch angle. for low pitch angle helices
Cλ equal to unity is well above the upper frequency
limit of the aerial, and for long helices this
may also be the case even for a medium pitch angle.

5. It has been found that it is not always possible
to neglect the effect of the ground screen on the
radiation pattern of the helix. Specifically
when the ground screen is
(a) several wavelengths in diameter
or (b) constructed of n radial wires
the pattern is profoundly affected, adversely.
Experiment has been limited to n≤8.

6. A Tchebycheff type of current distribution has
been proposed for the helical aerial. It has beer
shown theoretically, using the assumption of a constant amplitude travelling wave, that the
prospect of increased directivity from this
distribution is severely restricted because of the
linking up turns necessary. Reduced side-lobe
level has been obtained however, in agreement with
the above simplified theory, over the small
frequency range determined by the narrow pitch
turns. The measured current distribution shows
large fluctuations by comparison with the simple
uniform helix.

7. General theoretical solutions which in
principle enable the ::base velocity to be
calculated, and hence the frequency limits of the
aerial to be predicted, have been obtained for
the first time for each of the following cases: -
(i) The Sheath and Tape Helix with a Coaxial
Conducting Cylinder
(ii) The Tape Helix wound on a dielectric tube
(iii) The Sheath Helix embedded in a dielectric
medium, with a hollow coaxial tube.
(iv) Two Coaxial Sheath Helices.
Calculations have been carried out only in Case (i)
for the Sheath Helix and approximate calculation
for the Tape Helix.

8. What is believed to be a new type of helical
aerial - he linearly polarised Conora--Round
Helix is proposed. Some measurements have been
made, and the travelling-wave analysis for the
simple helix has been extended to include this
case.

9. A critical review has been made of the
theory that all End-Fire Travelling Wave aerials
intrinsically possess the same pattern bandwidth.