High efficiency helical antenna

An antenna is provided which includes a half wave helical element RF coupled to a monopole element. The monopole element is situated on the axis of the helical element and extends into the helical element a distance sufficient to permit resonant coupling between the helical element and the monopole element. The monopole element is driven by a source of radio frequency energy such that the helical element coupled thereto is excited by such radio frequency energy.

BACKGROUND OF THE INVENTION 
This invention relates in general to antennas for radiating electromagnetic 
signals. More particularly, the invention relates to helical antennas for 
portable radios and other communications equipment. 
One conventional helical antenna is shown in FIG. 1 as antenna 10. Antenna 
10 is a simple quarter wave (.lambda./4) structure consisting of a quarter 
wave helical element 15 coupled to a radio frequency (RF) output 20 
mounted on radio case 25. .lambda. is defined as the wavelength 
corresponding to the desired center frequency of antenna 10. Functionally, 
such a structure may be viewed as an asymmetric dipole in which the 
helical element 15 is one element and radio case 25 is the other element. 
In one typical configuration of the antenna of FIG. 1, helical element 15 
contributes approximately 6 ohms to the impedance of the antenna and radio 
case 25 contributes approximately 44 ohms to the antenna impedance. The 
impedance contributed by radio case 25 includes both the radiation 
resistance of case 25 and the ohmic losses due to RF currents in and on 
case 25. Thus, the overall impedance of a quarter wave helical element 
situated above a radio, such as in the example of antenna 10 above radio 
case 25, is approximately 50 ohms. This 50 ohm antenna impedance is 
conveniently matched with the 50 ohm impedance of radio output 20. In 
this conventional quarter wave helical antenna, there is a direct physical 
connection between helical element 15 and output 20 of the radio. 
Unfortunately, with this approach, relatively high RF currents flow in 
radio case 25. Thus, when the radio user touches the radio case 25 while 
operating the radio, the user dissipates these RF currents so as to 
undesirably decrease the strength of the radiated signal. 
Those skilled in the art appreciate that it is generally desirable to have 
high RF currents in the antenna of a portable radio in order to transmit 
the strongest signal possible. One way to excite such high currents is 
with a resonant half-wave helical antenna 30 as shown in FIG. 2. In 
antenna 30 a quarter wave transmission line transformer 35 is used to 
directly couple the radio RF output 20 to one end of a half wave 
(.lambda./2) resonant element 40. Unfortunately, although high levels of 
RF current are generated in such an antenna, a large RF current is still 
excited in radio case 25. Thus, as in the case of the quarter wave helical 
antenna of FIG. 1, the performance of antenna 30 is degraded when the user 
touches the radio case 25. 
BRIEF SUMMARY OF THE INVENTION 
One object of the present invention is to provide an antenna which performs 
with no significant degradation when the radio user touches the radio on 
which the antenna is mounted. 
Another object of the invention is to provide an antenna which is highly 
efficient. 
Yet another object of the present invention is to provide an antenna having 
relatively compact dimensions. 
In one embodiment of the invention, an antenna is provided which includes a 
helical element exhibiting an electrical length approximately equal to the 
1/2 wavelength corresponding to a selected center frequency. The antenna 
further includes a monopole element having opposed ends. One end of the 
monopole element extends into the helical element to a predetermined 
distance sufficient to cause resonant coupling between the monopole 
element and the helical element. The remaining end of the monopole element 
is adapted to be driven by a source of radio frequency energy. 
The features of the invention believed to be novel are specifically set 
forth in the appended claims. However, the invention itself, both as to 
its structure and method of operation, may best be understood by referring 
to the following description and the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to FIG. 3, one embodiment of the antenna of the present 
invention is shown as antenna 100. Antenna 100 includes a helical element 
110 which exhibits an electrical length approximately equal to one half 
the wavelength corresponding to the desired center frequency for the 
antenna. Although the particular antenna disclosed herein operates in the 
VHF band and exhibits a center frequency of 160 MHz, those skilled in the 
art will appreciate that the dimensions which follow are given for 
purposes of example and may be scaled up or down so that the antenna of 
the invention will operate in other frequency ranges as well. 
In this particular embodiment of the invention, helical element 110 
exhibits a pitch of approximately 4 turns per cm and a physical length L1 
which is approximately equal to 13 cm. Those skilled in the art appreciate 
that the pitch and physical length L1 of element 110 can be changed from 
the examples given above and yet still have element 110 resonate at the 
above stated center frequency. Those skilled in the art will also 
appreciate that the pitch and length L1 of element 110 can also be altered 
to cause antenna 100 to resonate frequencies other than the particular 160 
MHz center frequency of this example. 
Antenna 100 further includes a monopole element 120 which exhibits a length 
L2 substantially less than one quarter of the wavelength corresponding to 
the selected center frequency of antenna 100. For example, in the present 
example wherein the center frequency is equal to approximately 160 MHz, 
which corresponds to a wavelength of 187 cm, the length L2 of monopole 120 
is approximately 5 cm. 
Monopole element 120 includes opposed ends 120A and 120B. Monopole end 120A 
is coupled to the center conductor portion 130A of coaxial connector 130. 
The center conductor portion 130A is adapted to be coupled to the RF 
output of a radio. Coaxial connector 130 also includes a ground portion 
130B which is adapted to be coupled to the radio case (not shown in FIG. 
3). Monopole element 120 is situated coaxially with respect to helical 
element 110. The remaining monopole end 120B extends into helical element 
110 a sufficient distance to resonantly coupled thereto. For example, in 
this embodiment of the invention, monopole element 120 extends into 
helical element 110 a distance L3 approximately equal to 1/4 of the 
physical length L1 of helical element 110. That is, L3 is approximately 
equal to 3.25 cm. The term "resonant coupling" as used herein includes 
both capacitive coupling and inductive coupling. 
A cylindrical dielectric spacer 140 is situated over monopole element 120 
as shown in FIG. 3. In this embodiment, spacer 140 is shaped in the form 
of a hollow tube inside of which monopole element 120 is situated. Spacer 
140 is fabricated from low dielectric constant materials such as plastic, 
insulative shrink tubing material, Teflon.TM. material or other similar 
electrically insulative materials. Spacer 140 assures that monopole 
element 120 does not directly contact helical element 110. As seen in FIG. 
3, helical element 110 is wound over a portion of spacer 140 to permit the 
desired coupling between helical element 110 and monopole element 120 as 
described above. 
Helical element 110 is spaced apart from coaxial connector 130 by a length 
L4 sufficiently long to avoid capacitive coupling between helical element 
110 and a radio case (not shown) or other structure into which coaxial 
connector 130 is inserted. In the present example, it was found that for 
antenna 100, a distance L4 of approximately 1.8 cm between helical element 
110 and coaxial connector 130 is sufficient to prevent substantial 
capacitive coupling between helical element 110 and a radio case attached 
to coaxial connector 130. Those skilled in the art will appreciate that 
the value selected for L4 will depend on the frequency selected as the 
center frequency of antenna 100. The actual value selected for L4 may be 
more than or less than the example given as long as the above mentioned 
coupling criteria are met. 
In the example of antenna shown in FIG. 3, the outer diameter L5 of spacer 
140 is approximately equal to 0.6 cm. The thickness (outer diameter minus 
inner diameter) of spacer 140 is approximately equal to 1.5 mm and is 
selected to keep monopole element 120 on the axis of helical element 110. 
It is noted that in FIG. 3, monopole element 120 is on the same axis as 
helical element 110. 
When antenna 100 is connected to the output of a radio via coaxial 
connector 130, substantially smaller RF currents flow in the radio case 
than when many conventional antennas are used. Thus, when the radio user 
touches the radio to which antenna 100 is connected, the user tends to 
absorb less RF current than is the case with conventional antennas. For 
this reason, antenna 100 exhibits comparatively less performance 
degradation when the user touches the radio. 
The foregoing describes an antenna in which performance is not 
significantly degraded when the radio user touches the radio on which the 
antenna is mounted. The antenna exhibits high efficiency and relatively 
compact size. 
While only certain preferred features of the invention have been shown by 
way of illustration, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the present 
claims are intended to cover all such modifications and changes which fall 
within the true spirit of the invention.