Helical Antenna

1. Helical Antenna

2. Broad Spectrum of Types
• Helical
• Log Periodic
• Biconical
• Sleeve
• Spiral

3. Definition
• A helical antenna is an antenna consisting of a
conducting wire wound in the form of a helix.
• In most cases, helical antennas are mounted over a
ground plane.
• Provide circularly polarized wave
• Its polarization and radiation properties depend on
– Diameter
– Pitch
– No. of turns
– Wavelength
– Spacing between the helical loops

4. Helical Antenna
• Directional
• Circularly Polarized
– Polarization changes
with time
• Both high gain and wide
band

5. Geometry
D= diameter of helix
C= circumference of helix
Lo= length of one turn =
α= pitch angle =
S= spacing between turns
N= number of turns
L= length of helix
2a or d= diameter of
conductor

6. Helical Antenna
• (a) Basic geometry
• (b) axial-mode type yielding an endfire pattern and
• (c) omnidirectional mode yielding a broad-side pattern

7. Associated dimensions
• D - Diameter of the helix; (and, C = πD: Circumference of the
helix)
• s - Spacing between turns (centre-to-centre)
• L - Length of a single turn
• A - Axial length of the helix (= N × s, with N = Number of turns)
• d - Diameter of the helix conductor
• The other dimensions of importance are:
– the distance of the helix from
– the ground plane and the diameter of the (finite-sized) ground-plane.
– The pitch angle (α) of the helix is equal to [arctan(s/πD)]. Angle
between a line tangent to the helix wire and the plane normal to the
helix axis.
α = tan-1 (s/πD)

8. Operating modes
• 1. Transmission mode
– EM wave propagating along its length
• 2. Radiation mode
– specific far-field pattern
– normal/broad-side type (R0 mode)
– axial/end-fire type (R1mode)

9. Normal (broadside) mode
• The field maximum occurs in a direction
normal to the helix axis
• The radiation field will be circularly polarized.
• This normal mode occurs if the dimensions of
the helix are small compared to a wavelength
• It bears neither a wideband nor a high
efficiency capability.

10. Normal Mode
• Radiation pattern similar to
linear dipole
• The dimensions of the helix
are small compared to the
wavelength
• Narrow in bandwidth
• Radiation efficiency is small
• Rarely used
Antenna Theory, Constantine A. Balanis

11. Axial (or end-fire) mode
• The field will be maximum in the direction of
the helix axis and it will be circularly (or near-
circularly) polarised
• It occurs whenever the helix circumference is
of the order of a wavelength.
• It persists over a fairly wide frequency range

12. Axial Mode
• Circular Polarization
– ¾<C/λ<4/3
– C/λ=1:near optimum
– S= λ/4
• Half-Power Beam width: 50 x 50 degrees
• Directivity:
• Typical Gain: 10dB
• Bandwidth: 52%
• Frequency limit: 100MHZ to 3GHz
NS
C
2
3
52
3
2
15

S
C
N

13. Transmission mode - To-mode
• Describe how an electromagnetic wave propagates along
an infinitely long helix.
• In the lowest transmission mode (To-mode), a helix has
adjacent regions of positive and negative charge
separated by many turns.
• It occurs when the length L of one turn of the helix is
small as compared to the wavelength λ (that is, L << λ).

14. Transmission mode- To-mode
• Important mode on the helix used in the
travelling-wave devices such as the travelling
wave tube.
• In a traveling-wave tube, this transmission
mode field is deliberately excited to interact
with an electron.

15. Transmission mode- T1-mode
• When the helix circumference, expressed in free-
space wavelength, is of the order of one wavelength
(that is, as C/λ → 1)
• T1-mode. For small pitch angles, it has regions of
adjacent positive and negative charge separated
approximately by half a turn.

16. Transmission mode- T2 andT3-mode
• Higher-order transmission modes
• For small pitch angles, the approximate charge
distribution around the helix for such modes are
shown in Figure.

17. Radiation modes R0 mode
• For a short helix (NL << λ), the current may be
assumed to be of uniform magnitude and in
phase along the helix, and it may further be
assumed that a T0-mode is propagating.
• Helix radiates with the maximum field in a
direction normal to the helix axis as shown in
Figure(a)- designated as R0-mode

18. Radiation patterns of helices
(a) Normal mode (R0) (b) axial mode (R1) and (c) a
multilobed pattern

19. Radiation modes R1 mode
• With C/λ is of the order of a wavelength and as a T1-
transmission mode propagates along the helix, the
corresponding radiation from the helix is referred to
as the R1-mode
• The field maximum in the axial direction as shown in
Figure (b)
• A helix radiating in this axial mode is usually called a
helical beam antenna.
• Figure (c) shows a corresponding multi lobed pattern
for a helix of large dimensions.

20. Helix Applications
• Space Telemetry Applications of satellites,
space probes, and ballistic missiles
– Signals have undergone Faraday rotation
• Directional applications

21. Early Helix – 2 x 10 Turns
1989

22. 2 x 15 Turn 70 cm Helix
1991

23. AO-13 Antennas
About 1994

24. AO-40 Antennas
2001

25. 13 and 23 cm Helix Arrays
2003

26. Gain & Directivity
• An antenna may be very directive i.e. exhibit a
narrow forward beam width but due to the
configuration of the side lobes and/or degree
of losses, provide higher or lower forward gain

27. Kraus 12.5 cm-Increased Turns
6.5 turns 12 turns
26 turns 52 turns