Helical Antenna

Helical Antenna
A
Project Report
Submitted
in partial fulfillment
for the award of the Degree of
Master of Technology
in Department of Digital Communication Engineering



Submitted By:
Rakesh Kumar Bazad
 


ACKNOWLEDGEMENT

It is great opportunity to express my sincere thanks to all who have contributed to do this project through their support, encouragement and guidance.






Rakesh Kumar Bazad

Introduction :

A helical antenna is a specialized antenna that emits and responds electromagnetic fields with rotating (circular)polarization. These antennas are commonly used at earth-based stations in satellite communications systems. This type of antenna is designed for use with an unbalanced feed line such as coaxial cable.
To the casual observer, a helical antenna appears as one or more "springs" or helixes mounted against a flat reflecting screen. The length of the helical element is one wavelength or greater. The reflector is a circular or square metal mesh or sheet whose cross dimension (diameter or edge) measures at least 3/4 wavelength. The helical element has a radius of 1/8 to 1/4 wavelength, and a pitch of 1/4 to 1/2 wavelength. The minimum dimensions depend on the lowest frequency at which the antenna is to be used. If the helix or reflector is too small (the frequency is too low), the efficiency is severely degraded. Maximum radiation and response occur along the axis of the helix.
Helical antennas are commonly connected together in so-called bays of two,four, or occasionally more elements with a common reflector. The entire assembly can be rotated in the horizontal (azimuth) and vertical (elevation) planes, so the system can be aimed toward a particular satellite. If the satellite is not in a geostationary orbit, the azimuth and elevation rotators can be operated by a computerized robot that is programmed to follow the course of the satellite across the sky.
Helical antennas have long been popular in applications from VHF to
microwaves requiring circular polarization, since they have the unique
property of naturally providing circularly polarized radiation. One area that
takes advantage of this property is satellite communications. Where more
gain is required than can be provided by a helical antenna alone, a helical
antenna can also be used as a feed for a parabolic dish for higher gains

History :
John Kraus, W8JK, is the originator of the helical-beam antenna; as he puts
it1, “which I devised in 1946”. His 1950 book, Antennas2, is the classic
source of information. The recent third edition3, Antennas for All
Applications, has significant additional information.
Normal Mode Helical Antenna :
Radiating at 90 degrees from the axis of the helix this design is efficient as a practical reduced-length radiator when compared with the operation of other types such as base-loaded, top-loaded or centre-loaded whips. They are typically used for applications where reduced size is a critical operational factor.
These simple and practical "Helical" were primarily designed to replace very large antennas. Their reduced size is therefore most suitable for Mobile and Portable High-frequency (HF) communications in the 1 MHz to 30 MHz operating range.
An effect of this type of concertinaed 'reduced size 1/4 wave' is that the matching impedance is changed from the nominal 50 ohms to between 25 to 35 ohms base impedance. This does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wave at the frequency of operation.
Another example of the type as used in mobile communications is "spaced constant turn" in which two or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance andinductance for the radiating element at a particular resonant frequency.
Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s. Multi-frequency versions with plug-in taps have become the mainstay for multi-band Single-sideband modulation (SSB) HF communications.
Most examples were wound with copper wire using a fibreglass rod as a former. This flexible radiator is then covered with heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna.

Axial Mode Helical Antenna :
In the axial mode, the helix dimensions are at or above the wavelength of operation. The antenna produces radio waves with circular polarisation. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.
In radio transmission, circular polarisation is often used where the relative orientation of the transmitting and receiving antennas cannot be easily controlled, such as in animal tracking and spacecraft communications, or where the polarisation of the signal may change, so end-fire helical antennas are frequently used for these applications. Since large helices are difficult to build and unwieldy to steer and aim, the design is commonly employed only at higher frequencies, ranging from VHF up to microwave.
The helix in the antenna can twist in two possible directions: right-handed or left-handed, as defined by the right hand rule. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the radio waves: a left-handed helix radiates left-circularly-polarised radio waves, a right-handed helix radiates right-circularly-polarised radio waves. Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarised signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarised antennas suffer a severe loss of gain when receiving right-circularly-polarised signals, and vice versa.
The dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency. In axial-mode operation, the spacing between the coils should be approximately one-quarter of the wavelength (λ/4), and the diameter of the coils should be approximately the wavelength divided by pi (λ/π). The length of the coil determines how directional the antenna will be as well as its gain; longer antennas will be more sensitive in the direction in which they point.

Parameter :
The parameters of the helix antenna are defined below.
• D - Diameter of a turn on the helix antenna.
• C - Circumference of a turn on the helix antenna (C=pi*D).
• S - Vertical separation between turns for helical antenna.
• - pitch angle, which controls how far the helix antenna grows in the z-direction per turn, and is given by
• N - Number of turns on the helix antenna.
• H- total height – H=NS
 
Helical feed antennas :
A parabolic dish reflector typically requires a feed antenna with a rather large beamwidth, 90º or more. From the beamwidth formula above, only a short helix of a few turns is needed. Figure 2 shows the radiation pattern provided by a typical short helix, 4 turns with a 12.5º pitch and a ground plane of 0.94λ diameter. The calculated dish efficiency with this helix as a feed is very good, about 77%, at a center frequency of 2.4 GHz, with best f/D around 0.69, just about right for an offset-fed dish. Thus, we might expect a real efficiency >60% feeding a reasonably sized (>10λ) offset dish.
Deep dishes :
All the calculated helical feeds are only suitable for shallow dishes or offsetfed
dishes, with f/D > 0.5, while most prime-focus dishes are deeper, with f/D = 0.4 or smaller. For shallow dishes, a different form of helix is needed.

One possibility is a backfire helix9, with a small loop instead of a ground plane – the loop is smaller in diameter than the helix diameter, like a director on a loop-Yagi. The radiation peak is toward the end with loop, and the beam is broader than a helix with ground plane. Calculated efficiency is 80% for an f/D = 0.33.

Efficiency remains high at other frequencies, while best f/D decreases with increasing frequency, as shown in Figure 13. Thus, it might be possible to match the reflector f/D by dimensioning the helix for a different center frequency. The circular polarization of the backfire helix is reversed from the polarization sense of the same helix with a larger ground plane, radiating forward.


Mechanical considerations:

In most cases, a helix made of copper or aluminum wire is not selfsupporting, particularly in New England weather. Many helical antenna photographs show a support in the center: one version has a metal center pole with periodic supports for the helix. Another variation winds the wire, like a flat tape, on a dielectric support. Kraus says the dielectric shifts the operating bandwidth to lower frequencies, so that a smaller helix is needed for a given frequency.




Feed impedance :

A typical helical antenna has an input impedance of around 140 ohms. Kraus gives a nominal impedance of Z = 140Cwith axial feed. This is a resistive impedance only at one frequency, probably near the center frequency. Matching the impedance to 50 ohms over a broad bandwidth would be more difficult than simply matching it well for a ham band. A simple quarter-wave matching section with a Zo ~ 84 ohms should do the trick for a single band.
The matching section10 is often part of the helix: a quarter-wave of wire close to the ground plane before the first turn starts. It could also be on the other side of the ground plane, to separate impedance matching from the radiating element.



Polarization :

Circular polarization has two possible senses: right-hand (RHCP) and lefthand
(LHCP). Since a helix cannot switch polarization, it is important to get it right: by the IEEE definition3, RHCP results when the helix is wound as though it were to fit in the threads of a large screw with normal right-hand threads. Note that the classical optics definition of polarization is opposite to the IEEE definition.

More important for a feed is that the sense of the polarization reverses on reflection, so that for a dish to radiate RHCP polarization requires a feed with LHCP. For EME, reflection from the moon also reverses circular polarization, so that the echo returns with polarization reversed from the transmitted polarization. A helical feed used for EME would not be able to receive its own echoes because of cross-polarization loss.
 
Benefit of using macro

• Save time – no redrawing
• Portable – cut and paste
• Optimetrix ready
• Tweaking dimension using pulled down menu – edit
parameters
• Saves data of all iterations
• Easy adaptation of designs onto new chassis- reoptimizing

QUADRIFILAR HELIX ANTENNA
Quadrifilar helix antennas have the unique attribute of producing a circularly polarized conical beam while maintaining a relatively slender and compact form factor.These antennas are becoming increasingly popular and have been produced in substantial volumes for such applications as global positioning receivers, handheld satellite phones and radio-communication terminals. The quadrifilar helix is relatively insensitive to a conducting element placed within its core, provided that the diameter of the internal element is reasonably less than that of the host quadrifilar. Generally, the diameter ratio between the two antennas should not exceed 1:3;however, short sections (¾ λ/4) of the internal conductor resulting in a diameter ratio of 1:2 are permissible if properly located. This leads to the present topic, in which a λ/4 choke is placed within a quadrifilar helix of twice the diameter for the purpose of creating a coaxial monopole.
Quadrifilar Helix Test Model
Designing the quadrifilar helix is a straightforward procedure; however, special care must be taken to ensure that the structure is properly excited for optimum performance. The main complexity of these antennas lies within the feed/matching network design. After selecting the helix parameters that give the desired beam coverage, the mutual input impedance of each winding must be characterized so that an appropriate feeding and matching network can be implemented.3 The feeding network provides an equal distribution of RF signal power to the four helix windings in quadrature phase rotation so as to excite the proper beam mode. The matching network is a set of four reactive impedance transformers, one for each winding.


The geometry for the quadrifilar helix under consideration is shown in Figure 1. The helices have a winding sense for left-hand circular polarization, and an angular pitch for an elevation coverage of 25° to zenith. The selected quadrifilar dimensions are 2" pitch, 0.75" diameter and 2 turns for a developed length of 4".
THE MONOPOLE ANTENNA
The monopole antenna used here is actually a form of dipole antenna whose reflective element is a quarter-wave choke. The design consists of a 4" length of UT85 semi-rigid coaxial line terminated with a 50 Ω SMA connector on one end, and has a section of the outer conductor removed at the other end to expose the center conductor. This forms a sort of rudimentary dipole element. A choke is used to sheath the outer conductor to prevent currents from flowing along its length and producing undesirable radiation.

Equations:
G= 10.8 + 10*log10( (C/lambda)2*N*(S/lambda) ) (Note1)
Z= 150/sqrt(C/lambda) Ohm
D= lambda /PI
S= C/4
HPBW= 52/( (C/lambda)*sqrt(N*(S/lambda)) ), Half power beamwidth.
BWFN= 115/( (C/lambda)*sqrt(N*(S/lambda)) ), Beamwidth first nulls.
Ae= D*lambda2/(4*PI)
Where C is circumference, which is normally chose to be close to one wavelength.
 
Application of Helical Antenna

1. Used in GPS System
2. Used in medical Service
3. Used in Satellite Communication
 
Refrence

1. www.google.co.in
2. www.yahoo.com

Posted in: Antenna