Helical microstrip antenna with impedance taper

A helical antenna comprised of a helical conductor having one end adapted to be connected to a feedline, a conductive surface contained within but spaced from the helical conductor, the distance of the conductive surface from the helical conductor being predetermined so as to vary the radiation loss from the helical conductor during electromagnetic emission therefrom.

FIELD OF THE INVENTION 
This invention relates to the field of antennas, and in particular to 
helical antennas. 
BACKGROUND TO THE INVENTION 
A conventional helical antenna which has a radiating conductor length which 
is longer than several wavelengths of a signal it is to radiate suffers 
from the problem of rapid decay of current density along its helical 
conductor (radiator). As a result, there is less emission from points on 
the helical conductor progressively distant from its feed end, and the 
gain of the antenna is lower than that which could be achieved if the 
current density was constant along the length of the conductor. 
Techniques have been used to decrease the rate of decay of the current 
density along the antenna, such as end loading of the helical radiator, 
varying the pitch of the helical conductor helix, etc. However these 
techniques provide only minor improvement in the current density profile. 
SUMMARY OF THE INVENTION 
The present invention provides a simple and inexpensive means for 
controlling the radiation loss over the length of the helical antenna, and 
provides means for creating an antenna with uniform current density along 
the helical conductor, or with varying current density with length and/or 
with peripheral direction along the antenna. 
In accordance with the present invention, an internal conductive surface is 
placed within the helix, which varies in distance from the helical 
conductor. For example, with a helical conductor (radiator) supported on a 
cylindrical dielectric tube or by other means, a truncated conical 
conductive surface located coaxially within the tube can result in uniform 
current density along the antenna, and thus maximum radiation efficiency 
of the helical antenna. 
The crossection of either or both of the helical conductor winding and of 
the conductor surface can be circular or some other crossection in order 
to control the current density at any point of the helical conductor, 
which can vary the direction and/or directivity of radiation of the 
antenna. Indeed, by mechanically varying either of these crossections or 
the distance of the conductor surface from the helical conductor at any 
point, the direction of radiation and/or directivity of the antenna can be 
controlled. 
In accordance with an embodiment of the invention, a helical antenna is 
comprised of a helical conductor having one end adapted to be connected to 
a feedline, and a conductive surface contained within but spaced from the 
helical conductor, the distance of the conductive surface from the helical 
conductor being predetermined so as to vary the radiation loss from the 
helical conductor during electromagnetic emission therefrom. 
In accordance with another embodiment of the invention, a helical antenna 
is comprised of a helical conductor wound in the shape of a circular 
crossection cylinder, a conductive surface in the shape of a truncated 
cone contained within the cylinder shaped helical conductor and spaced 
from the helical conductor, the mutual spacing thereof increasing from a 
first to a second end, a ground plane disposed orthogonally to the axis of 
the helical conductor adjacent the first end, and apparatus for connecting 
a feedline to one end of the helical conductor.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with an embodiment of the invention, a helical conductor 1, 
which may be in the form of a conductive strip, is supported on a 
cylindrical dielectric tube 3. However the conductor may be supported by 
any other means, for example insulating arms or ribs protruding from the 
ground plane. In the embodiment shown, the support tube has circular 
crossection. The helical conductor thus is wound in a circular cylindrical 
shape. 
A conductive surface 5 is located spaced from the conductor internally of 
the helical conductor helix. In the embodiment shown the shape of the 
conductive surface is a truncated cone. The bottom end of the antenna as 
shown is a feed end for receiving feed current for radiation of a signal 
from the antenna. 
A ground plane 7 is located with its plane perpendicular to the axis of the 
helical conductor. It is preferred that the helical conductor support 
should be fixed to the ground plane, so as to fix the position of the 
helical conductor relative to the ground plane. The end of the helical 
conductor, designated the feed end, is connected to a feed connector 9 
which is passed through the ground plane. 
The internal conductive surface may be connected to the ground plane. 
As a result of the relative nearness of the conductive surface to the 
helical conductor adjacent its feed end, the radiation loss at that 
location is a minimum. As the distance of the conductive surface from the 
helical conductor increases, the radiation loss from the helical conductor 
increases. Thus with the linear variation of the conductive surface from 
the helical conductor in the right circular cylinder shape shown, the 
normal linear decrease in radiation loss from the helical conductor with 
distance from the feed which would otherwise exist is compensated, 
resulting in equal or near equal radiation contribution from the entire 
helical conductor over its entire length. 
It will be recognized that there may be cases in which it is undesirable to 
have equal radiation over the entire length of the helical conductor. For 
example where there may be an external shielding structure which would 
interfere with a side portion of the radiation lobe of the antenna, it 
might be desirable to skew the radiation lobe away from the shielding 
structure. There may be situations in which it is only possible to use a 
helical antenna and yet directionality may be desired which is different 
from that otherwise possible from the position of the helical antenna. 
Embodiments of the present invention make it possible to skew or otherwise 
control the directionality of the antenna. 
For example, the distance of the internal conductive surface 5 can vary 
from the helical conductor. This can be effected by axial offsetting 
and/or rotating the axis of the internal conductive surface relative to 
the axis of the helical conductor. 
Other or additional ways of varying the distance of the internal conductive 
surface can be to form the internal surface into a different shape than the 
truncated cone shown, or to form the helical conductor into a shape other 
than circularly cylindrical, or both of the above, with or without 
offsetting and/or rotating their mutual axes, for example as shown 
schematically in FIG. 2. 
Where the conductive surface is close to the helical conductor, radiation 
loss is reduced, and where it is distant from the helical conductor it is 
increased. By predetermining this distance, the radiation loss from 
different parts of the helical conductor, and thus the shape of the 
radiation lobe from the antenna can be controlled. 
Indeed, the distance of the internal conductor can be dynamically 
controlled, e.g. by a mechanical system controlled by a switchable relays 
or motors. For example if the internal conductor is flexible, hinged or 
otherwise moveable, an arm controlled from a motor or relay can move the 
internal conductor nearer or farther from the helical conductor, allowing 
dynamic control and changing of the radiation loss and thus the shape of 
the radiation lobe from the antenna, from a remote location. As another 
example, the internal helical conductor can be a flexible conductive sheet 
having one edge fixed and the other edge wound on a central axle, can be 
wound and unwound from the central axle, changing the distance of the 
entire internal conductor from the helical conductor, thus varying the 
length to gain ratio of the antenna or different parts thereof. 
The invention can be uefully implemented as a helical microstrip antenna, 
e.g. for L-Band satellite communications (1525-1660.5 MHz). It can be used 
in fixed and/or portable installations. 
A person understanding this invention may now conceive of alternative 
structures and embodiments or variations of the above. All of those which 
fall within the scope of the claims appended hereto are considered to be 
part of the present invention.