Antenna



This page created and maintained by Ooi, Sooliam,
SL received formal antenna training from Dr. Gary A. Thiele and Dr. Warren Stutzman of Virginia Tech.

What is an antenna?

An antenna is a radiating and receiving element from which the radio signal, in the form of RF power, is radiated to its surroundings and vice versa. The RF power in Watts on the 50 Ohm transmission line is then distributed into time-varying Electromagnetic field in air. The time-varying field induces time-varying current on a receiving antenna, with very much less energy. The RF current is then fed into the receiver circuit.


The antenna is also considered the EM Interface or air interface between guided wave and radiated wave.

Some people think that an antenna is the last part of the radio you want to put on to the radio. They design the radio to have very good performance on the bench when connected to measuring instruments. When it comes to antenna, they insist that the antenna must be short and should be used for a wide range of frequencies without considering their performance. The radio in turn, suffers market defeat for not having good coverage.

Basic Structure of an antenna

The most basic form of an antenna is a dipole antenna consists of two equal-length straight conductors aligned on the same axis and driven by a source in between them. The total length of the two conductor is half the wavelength of the frequency of operation. So each conductor is typically a quater of the wavelength long.

In the case of portable radio, the whole radio structure of antenna and radio body forms the equivalent of a dipole. The metal mass of the radio chassis, PCB or housing provided for half of the dipole while the antenna is the other half.

Types of antennas for portable radios

There are many types of antennas. Dipole antennas, wire antennas, patch antennas, horn antennas, log periodic antennas, disc antennas, biconical antennas and so on. But the type of antennas suitable to be used on portable radios are:
    monopole antennas and
    helical antennas.
This is due to the fact that these two designs are:

Monopole Antennas

For portable radio, the metal mass of the radio chassis, PCB or housing provided for half of the dipole while the antenna is the other half. Since one of the "poles" does not appear like a pole, it is considered as though there is only one pole, or monopole , and there is only one antenna. In actual case, the chassis of the radio is also part of the antenna.
The antennas below are Monopole Antennas
  1. Whip antennas
  2. Telescopic antennas
  3. Blade antennas

The telescopic antenna is a length adjustable monopole. By pulling to the right length , the antenna can be tuned to operate at different frequencies. Telescopic antenna are generally used for freq. < 400 MHz.

A whip antenna is a fix length monopole. It is made from a single conductor covered in PVC or polyurethane material. Since the quarter wavelength for freq. below 400 MHz are longer than 15 cm, whip antennas are used for UHF, 800 MHz and 900 MHz only.

Blade antennas refers to flat whip antennas. Blade antennas are usually foldable and rotatable.

Portable Antenna Radiation Characteristics

A dipole antenna radiates RF energy equally most part of its surroundings except the top and the bottom. This is a free space pattern, or as if the antenna is hanging in mid air.

A monopole antenna mounted on a radio however, radiates the most to the sides perpendicular to the antenna axis (normal) and gradually less energy towards the top and the bottom of the radio.

A portable antenna on a radio operating above a ground plane radiates not just to the sides, with peaks and dips from 0 to 90 degrees as a result of ground reflections. At those angles where there are dips, the energy radiated are much less compared to the peaks.

Antenna Resonance Frequency

The resonance frequency is the frequency at which the S11, or return loss is minimum. It has the same characteristics as the series tuned circuit.

Relationship between antenna length and frequency

Wavelength=0.95x299.8/frequency in MHz


BandFrequency(Mhz) Wavelength in mQuarter Wavelength in cm
900MHz9000.3167.91
800MHz8600.3318.28
8060.3538.83
UHF Band II5200.54813.69
4500.3167.91
UHF I4300.66216.56
4030.70717.67
300MHz3680.77419.35
3360.84821.19
VHF1741.63740.92
1362.09452.35
Mid Band883.23680.91
684.188104.71
Low Band505.696142.41
309.494237.34
The length of a quarter wave monopole is than 1/4 of the wavelength. The table above gives some frequently used frequencies and their wavelengths.
Notice that at some lower frequencies, the length of quarter wave length antennas are no longer practical.
By winding the quarter wave length into a spring, or helix, the length of a monopole can be reduced.

Helical antennas

It is found by experiments that to make a helical antenna can be made to resonate at the desired frequency by winding a conductor of 5/8 l into a helix. The capacitance formed between the turns of the helix load the antenna such that it resonates like a quarter wave monopole.

A helical antenna for portable radio is constructed by winding a conductor into a helix . The following symbols are generally used to describe a helical antenna.
D = diameter of helix (center to center)
S = spacing between turns (center to center)
n = number of turns
h = antenna height = nS
C = circumference of helix = PiD
L = length of one turn = Sqrt[ (PiD)^2 +S^2]
a = pitch angle = arctan S/C

TRANSMISSION AND RADIATION MODES OF HELICAL ANTENNAS

The term transmission mode is used to describe the manner in which an electromagnetic wave is propagated along an infinite helix as though the helix constitutes an infinite transmission line or waveguide.
The lowest transmission mode for a helix has adjacent regions of positive and negative charge separated by many turns ,and a substantial axial component of the electric field is present. This mode is designated as the T0 transmission mode . The T0 transmission mode is important when the length of one turn is small compared to the wavelength and is the mode occurring on low frequency inductance. It is also the important transmission mode in the traveling -wave tube. A helix excited in the T0 transmission mode may radiate.

The helical antennas shares the same characteristics as the series tuned circuit. All the circuit elements are determined by the material and dimension of the antenna.

Antenna Impedance

The impedance of an antenna, Za, is then given by the expression:

Za=(Rr+Rl)+j(wLa-1/(wco ))

This series tuned circuit has a resonant frequency f is given by the expression below:

f=1/(2Pi*Sqrt(LaCo))

The resistance R of a helical antenna has two parts, the radiation resistance Rr and the loss resistance RL . The loss resistance, is the resistance by which power delivered to the antenna is dissipated. The radiation resistance, on the other hand, is the resistance by which power is radiated. For example, if the power delivered to an antenna is given by :

P=(Rr+Rl)I^2


where,
P = power delivered to antenna
I^2 Rl =power dissipated as heat
I^2 Rr =power radiated.

RL is determined by the conductivity and the total length of wire used, where as Rr is determined by the physical length and the current distribution in the helix.
Its inductance La depends on the total number of turns, the total length of the helix and the inductance between the turns and the ground. Its capacitance Co is contributed by the capacitance between the turns and the also the capacitance between the turns and the ground.

Antenna Driving point impedance Measurement.

The driving point impedance affects the interaction between the antenna and the transceiver circuits. It is an important factor in the consideration of power transfer, transmitter stability and current drain. The impedance, in the form of VSWR , is also a limiting factor in the usable bandwidth of the antenna. In this case, an impedance of 50 ohm is desired, for matching with the transceiver circuit.

A portable radio is generally used for the measurement. A coaxial cable connects the radio to a Network Analyzer with S-parameter measurement capability. The coaxial cable is terminated at the feeding point of the antenna, in this case, the input/output point of the transceiver circuit. The coaxial cable is then connected to the radio with a balun, or a quaterwave section, on to the transceiver printed circuit board . The PCB is connected to a metal mass heat sink.

The metal mass, together with the PCB ground, acts as the counter poise of the helical antenna in the place of a ground plane. The impedance of the antenna is very sensitive to the ground on which it is mounted and its surroundings. The antenna has to be measure d with the radio held 2 inches from a persons face. This is to simulate the condition in w hich the antenna is actually used. Antenna impedance measured over a copper ground plane is generally lower than that measured on a hand-held radio heat sink. The metal mass of the portable radio is preferred since it is the actual mass on which the antenna operates.
The network analyzer is used to do one port measurement. Impedance is measured as S11 and may be displayed in the forms of either Log Magnitude, SWR or Smith Chart. The resonant frequency of a helical antenna is best indicated in Log Magnitude form, while SWR plot gives a good display of bandwidth. The Smith chart gives a good picture of the impedance and it is best used for matching.
The network analyzer is first calibrated to 50 ohm at the end of the coaxial cable which connects to the transceiver board and not the antenna port. This is to ensure that the transmission line connecting the transceiver circuit to the antenna connector is included in the impedance measurement. This is necessary so that the impedance measured is the actual impedance presented to the transceiver circuit. When a monopole is cut to the quarter wave length, or a helical antenna is cut to the right number of turns, the antennas will exhibit the characteristics of a tuned circuit. It can be measured by monitoring the driving point impedance.

Antenna Gain

Antenna gain is a measurement of how good an antenna radiate or receive power it is given, or subjected to. In the transmit mode, the higher the gain the higher the power an antenna radiates out given the same power level from the transmitter. Using an antenna with higher gain is like transmitting with higher power. In receive mode, the higher the gain the stronger the signal picked up from the field it is subjected to. It is as if the receiver has a better sensitivity. Bottom line, the higher the gain, the further the pair of transceivers can maintain the RF link.

Antenna Gain VS Amplifier Gain

The term Gain is not to be taken the same way as the amplifier gain. The unit of antenna gain is dBi, or some time as simple as just dB. Unlike amplifier gain, a positive gain in dB for the antenna does not mean that you can have higher power out given the power in. The gain of a antenna refers to the gain of power when the energy is concentrated in certain direction, with respect to when it is to be radiated in all direction. A point source radiating energy in all directions has Zero dB gain, or 0 dBi. If we concentrate the radiated energy such that energy only radiate through the top half of the hemisphere surrounding the antenna. The energy radiated will be twice the strength. In this case, the antenna is considered to have double the gain, or 3 dB gain.

LENGTH DEPENDENCE OF HELICAL ANTENNA

The radiation resistance increases with the square of the ratio between the physical height of a helical antenna, he, to the wavelength of the frequency of operation of the antenna.

EFFECTIVE HEIGHT

The effective height of an antenna is the ratio of the induced voltage to the incident field. An antenna subjected to incident electric field E, will have voltage V, induced at its terminal depending on the effective height he.
The effective height of a helical antenna having sinusoidal current is related to the physical height. Low effective height results in low induced voltage, or power, which is translated as low gain.

Gain Efficiency Factor

Since the radiation resistance is low, the factor k will be very small. For example, for an antenna having the total resistance of 50 ohm and radiation resistance of 1 ohm, the factor k will be only 0.2.
Coverage distance- The gain of an antenna determines the coverage distance of the radio it is operated on. Using the Friis formula, the distance of coverage can be calculated given the minimum received signal level to maintain communication link of the radio used. The impact of gain on the distance of coverage is best illustrated by assuming a pair of UHF radios both using the same antenna which has the gain of -1 dBi, and the minimum level of received signal is -100 dBm. Then the coverage distance is 10 km, as shown above, as the reference. If another pair of antennas having the gain of 1 dB lower than the reference set of antennas, the coverage distance, according to Friis transmission formula, will be reduced to 8 km. Subsequently, another pair of radios both using antennas having the gains 2 dB lower than that of the reference antenna, the radios can only maintain a communication link of 6 km. 5 km for antennas with gains 3 dB lower, and so on.
Reduction in size is one of the main causes of gain reduction. The impact is illustrated by translating the effect directly to the reduction in coverage distance. If there exist in the market a 1 Watt radio using whip antennas of 16 cm in length and covering the range of 10 km, then any competition models using 12 cm(75%) antennas for the same power can only cover 7 km, or only 3 km if the antenna is 8 cm(50%) and so on.

ANTENNA GAIN MEASUREMENT

Antenna gain is the most important fundamental property of an antenna. An antenna can have good matching but it will be useless if it does not radiate. Antenna gain is measured as the power gain of the effective radiated power over the radio transmitter output power. The gain of a normal mode helical antenna for portable radios is measured in the direction normal to its axis, which is the direction of its maximum value.

The measurement method is referred to as the gain-transfer method, or the gain comparison method, which is the most commonly employed method for gain measurements. This method requires the use of a gain standard to which the gain of the test antenna is compared.

REFERENCE ANTENNAS
Dipole and biconical antennas are used for the gain measurement. The gain of the antennas had been accurately measured and the data supplied by the manufacturer. The dipole antenna and the biconical antenna, having high degree of dimensional stability and linearly polarized, are commonly accepted as gain standards.
Measurements are conducted on an antenna range. The antenna range is a open-site, ground reflection range. This is the type of range preferable for the frequencies below 1 Ghz because of the difficulties in simulating free-space condition in indoor set ups.
An antenna range is a open area test site used for measuring radiated emissions. The test site is designed to provide repeatability in the measurements by having an open area clear of reflecting objects and a ground of a plane homogeneous surface. In a typical antenna range, a sensing antenna positioning tower, is set up three or ten meters from a rotating turntable.
Figure above shows the plots of the amplitudes measured on the antenna range, for both dipole antenna and the radio antenna. The differences between the amplitudes of the dipole antenna and the radio antennas indicate the differences in gain between the two antennas. The key to this method is that the height of the antenna under test and that of the reference antenna must be exactly the same.

If you wish to learn more about antenna in details, you can refer to the following books.

Recommended References

Henry Jasik and Richard C. Johnson (ed.), Antenna Engineering Handbook,2nd ed., New York: McGraw-Hill, 1984

John D. Kraus,”Antennas”, 2nd ed., New York: McGraw-Hill, 1988

K.Fujimoto, J.R. James, “Mobile Antenna Systems Handbook”, Artech House, Inc.1994