Anteena Basic

7/27/2019 Anteena Basic

1/34

Measurable antenna parameters

1. Simple antennas

2. Antenna feed considerations

3. Gain, Antenna factor, Directivity

4. Radiation pattern and E-field polarisations

5. E-field zones for antenna radiation


2/34

1. Simple antennas


3/34

Ideal radiator

Isotropic radiator is the most simpletheoretical radiator

This radiates all power (without loss)equally in every direction

Impossible to build a perfect isotropicradiator, but useful as reference


4/34

Half-wave dipole

The most simple wire antenna In practice the actual length required for

resonance is about 95% of/2

MHzMHz ffL

5.142

95.02

300

==


5/34

Radiation resistanceRadiation resistance is the real part of the

complex antenna impedance

The power radiated into space is modelled

as an apparent resistive loss

For half-wave dipole radiation resistance is

approximately 72, and will have zeroimaginary component


6/34

Pyramidal horn antenna The most simple waveguide antenna

The flange matches the wave impedance in thewaveguide to the impedance of free-space


7/34

Function of antennas

Transform the electromagnetic energy in aguided wave system into radiated energy.

Designed to optimise this power transfer in

the frequency range of interest.

The radiation will often be required to have

a specified directionality.


8/34

2. Antenna feed considerations


9/34

Baluns (balanced - unbalanced)

Balanced

Unbalanced (coax cable)

With an unbalanced feed the element currents are asymmetric,

and the far-field pattern will also be asymmetric. The balun

rectifies this and acts as a transformer between the balanced

antenna and an unbalanced line. Baluns also match thetransmission line impedance to the antenna impedance.


10/34

Inversion test data (bicone)

-4

-3

-2

-1

0

1

2

3

4

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Frequency (MHz)

dB


11/34

Mismatch parameters

BalunVin

Vout

+

= 1

1

VSWR0

0

ZZ

ZZ

Vin

Vout

A

A

+

==

The reflection coefficient, , is a complex voltage ratio,which may be expressed in terms of the antenna andtransmission line impedances (ZA and Z0 respectively).VSWR is the ratio of the peak to trough amplitudes of the

standing wave generated by the two voltages on the line.


12/34

Return Loss and Mismatch Loss

The reflection coefficient is often expressed as a dB value, and

this quantity is called Return Loss (RL below). Note that

losses are usually expressed as a positive number in dB.

=

120)( 10LogdBRL

The reflection coefficient allows us to work out what fraction

of power the antenna is absorbing (accepted power). The

reciprocal is called the Z0

Mismatch Loss (M below).

210

1

110)(

= LogdBM


13/34

3. Gain, Antenna factor,

Directivity


14/34

Converting to dB

To obtain dB values, the logarithm of voltage ratios are

multiplied by 20, and those of power ratios are multiplied by 10.

( )[ ]linearAFLogmdBAF 1020)/( =

( )[ ]linearGainLogdBGain 1010)( =


15/34

Receiving antenna

E

50

V


16/34

Antenna Factor

Antenna factor is a quantity which directly converts the measured

voltage at the antenna output, measured by a 50 receiver, to theE-field at the elements of the antenna. It is implicitly assumed

that the E-field is incident along the boresight direction of the

antenna. In linear terms antenna factor is simply:

VEAF =


17/34

Use of antenna factor

The measured voltage in (dBV) given by the receiver iscorrected for the transmission loss of the connecting coax cable,

and is then added to the antenna factor to get E-field strength.

( ) ( ) )/(/ mdBAFVdBVmVdBE += Note how the units of the measured voltage relate directly to

the units of the measured E-field. If the voltage were given in

(dBmV) then the E-field would be in units of (dBmV/m).


18/34

E-field expression using AF

Antenna factor describes the receiving properties

of antennas. Using the idea of reciprocity we can

derive an expression which gives the field strengthat distance (r) from the antenna when transmitting.

05

2)/(

Z

P

rAF

FmVE

Tx

M

=

Where: P = Input power (into Z0 antenna feed);

Z0 = Characteristic impedance of system;

FM = Frequency (MHz).


19/34

Radiated power

P

PA PRRadiation

intensity, I(W/steradians)

Antenna

The power accepted into the antenna, PA, is the power from

the feed cable reduced by the mismatch loss. The total

power radiated into space is the accepted power reduced by

the effect of conduction loss, which is commonly calledradiation efficiency, (0 eff 1).

2

1 = PPA AR PeffP .=


20/34

4. Radiation pattern and E-field

polarisations


21/34

Directivity

The radiation intensity, I, is a function of direction (,), and itis given in units of power per unit solid angle (steradians).

For any given direction (,), the Directivity is given by theradiation intensity in that direction, normalised to the

average intensity, i.e. the level produced if PRwere radiated

uniformly over the entire sphere.

4

),(),(

RP

ID =

Directivity may be expressed as a dB quantity by taking

10*Log10[of the above expression].


22/34

Typical radiation pattern

-35

-30

-25

-20

-15

-10

-5

0

5

10

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

Azimuth ()

Direc

tivity(dB)


23/34

Gains

There are two gain figures which often cause confusion. Thefirst is true gain, G, which is simply Directivity calculatedfrom the accepted power, PA.

4APIG =

The second is realised gain, GR, which is Directivity calculated

from the power in the feed cable, P. Note that G will be smaller

than Directivity, and GRwill be smaller than G.

4P

IG R =


24/34

Gain related to antenna factorIn dB terms antenna factor is related to (boresight) realised gain by

the following:

77.29)()(20)/( = dBGFLogmdBAF RM

77.29

410

*100

2

012

=

ZcLog

Where:FM = Frequency (MHz); GR= Realised gain (dB);0 = Impedance of free-space; c = Speed of light;Z0 = Characteristic impedance of system.

The 1012

factor arises from choosing units of frequency to be MHz.


25/34

Example gain calculation

Let us assume we have measured a +6.5 dB Directivity in some

defined direction, at a frequency of 500 MHz. We want to know

what the antenna factor is. We know that the antenna efficiency (eff)

is 95%, and the magnitude of the reflection coefficient, , is 0.2.

dBLogdBM 18.01

110)(

2

=

=

dB

eff

LogdBefficiency 22.01

10)( ==

Realised gain is therefore: 6.5 0.18 0.22 = 6.1 dB

Antenna factor is: 20*Log(500) 6.1 29.77 = 18.1 dB/m


26/34

Polarisation

E

X

Y

Elliptical Linear

Any plane wave may be expressed as the sum of two orthogonalcomponents (X and Y), and the E-field vector produced willtrace out an ellipse. A linear field is just a special case of this.

The two sinusoidal components can have different magnitudesand relative phases. Typically the radiation pattern will be

split into one plot for each orthogonal component.


27/34

5. E-field zones for antenna

radiation


28/34

E-field zones

In the far-field, many wavelengths from the antenna,

the E-field is inversely proportional to distance, andfree-field plane wave conditions exist.

Proximity to the source complicates the field, and the

ratio of E to H will no longer be 0 (376.73 ). When close to the antenna there will be different path

lengths to each radiating point on the antenna.


29/34

E-field from a Hertzian dipole

h

E

r

( ) ( )

42

)(00 111sin

4rr

re

hIE rtj

+=

I0 = Current = Phase constant (2/)

0 = Free space impedance (376.73) = Angular frequency


30/34

Near field component

-2

-1

0

1

2

3

4

5

6

7

0 0.5 1.0 1.50.16

r = / 2

Distance ()

Magnitude(dB)

Radiation from an aperture


31/34

Radiation from an aperture

(r + )

r

Fresnel

ZonesD

For a given distance, r, the nth Fresnel zone in the plane of the apertureis defined as the region in which: (n-1)/2 < < n/2. When viewedfrom the front the Fresnel zones will be a series of rings.

A commonly used criterion for minimum separation is defined to give < /16 at the edge of the aperture, which gives an approximatelimit of:

2

.2 Dr >


32/34

E-field zones summary

0

+

Azimuth

angle

Reactive

near-field

Radiated

near-field

Radiated far-field

(Fraunhofer region)

2

22D Distance (r)


33/34

Typical antenna designs

Monopoles and loops (up to 100 MHz) Dipoles, Bicones, LPDAs (20 MHz to 2 GHz)

Horns (1 GHz upwards)

Reflector antennas (dipole array, dish)


34/34

Documents

Anteena Basic