FYP2: basic antenna parameter

Antenna basic

Aperture family: effective and scattering

Lobe family: main, side, back, grating刺耳

Beam width: directivity, gain

Antennas are three-dimensional and live in world of beam area, steradians, square degrees, and solid angle. Antennas have impedance (self and mutual). They couple to all of space and have a T measured in K. Antennas have polarization: linear, elliptical, and circular.

Basic Antenna Parameters

Radio antenna: structure associated with the region of the transition between a guided wave and free space wave, or vice versa. Antenna converted electrons to photons or vs.

(Photon = quantum unit of EM energy equal to hf, where h=Planck’s constant=6.63x10-34 Js, f=frequency, hz)

All antenna type involve same basic principle that radiation us produced by accelerated (or decelerated) charge.

Basic eq:

dIL= Qdv

I=time charging current, , L=length of current element, m
Q=charge,C,   v=acceleration of the charges, ms-2

Thus, time changing current radiates and accelerated charge radiates. For steady state harmonic variation, we usually focus on current. For transient or pulses, we focus on charge. The radiation is perpendicular to the acceleration, and the radiated power is proportional to the square of dIL or Qdv.

An antenna is a transition device, or transducer传感器，between a guided wave and a free space wave, or vs.

From the circuit point of view从电路的角度来看, the antenna appear to the transmission lines as resistance,Rr , called the radiation resistance. It’s not related to any resistance in the antenna itself but is a resistance coupled from space to the antenna terminals.

In the transmitting case, the radiated power is absorbed by objects at a distance: trees, buildings, the ground, the sky and other antennas. In the receiving case, passive radiation from distant objects or active radiation from other antennas raises the apparent表面上 temperature of Rr.

For lossless antennas, this temperature has nothing to do with the physical temperature of the antenna itself but is related to the temperature of distant objects that the antenna “looking at”. In this sense, a receiving antenna (and its associated receiver) may be regarded as a remote-sensing远程传感temperature measuring device.

 Pattern

Rr and T are simple scalar quantities. The radiation pattern, on the other hand, are three dimensional quantities involving the variation of field or power (proportional to field squared) as a function of the spherical coordinates θ and phi.

The completely specify the radiation pattern with the respect to field intensity and polarization requires three patterns:

1) the θ component of the electric field as a function of the angles θ and phi (vm-1)

2) the phi component of the electric field as a function of the angles θ and the phi (vm-1)

3) the phases of these fields as a function of the angles θ and phi (rad or deg)

Three-dimensional field pattern of a directional antenna with maximum radiation in z-direction at θ=0º. Most of the radiation is contained in a main beam (lobe) accompanied by radiation also in minor lobes (side and back). Between the lobes are nulls where the field goes to zero. The radiation in any direction is specified by the angles θ and phi. The direction of the point P is at the angles θ=30° and phi=85º.

Any field pattern can be presented in three-dimensional spherical coordinates, as in Fig.2-3, or by plane cuts through the main lobes. Two such cuts at right angles, called the principal plane patterns (as in the xz and yz planes on Fig.2-3) may be required but if the pattern is symmetrical around the z axis, one cut is sufficient.

Figures 2-4a and 2-4b are principal plane field and power patterns in polar coordinates. The same pattern is presented in Fig.2-44c in rectangular coordinates on a logarithmic, or decibel, scale which gives the minor lobe levels in more detail.

The angular beam width at the half-power level or half-power beam width (HPBW) (or -3dB) and the beam width between first nulls (FNBM) as shown in Fig.2-4, are important pattern parameters.

Normalized field pattern and Power Pattern

NFP: E(θ,phi)n= E(θ,phi)/E(θ,phi)max (dimensionless)

The half-power level occurs at those angles θ and phi for which E(θ,phi)n = 1/surd 2= 0.707.

Pattern may also be expressed in terms of the power per unit area. Normalizing this power with respect to its maximum value yields a normalized power pattern as a function of angle which is a dimensionless number with a maximum value of unity.

NPP: P(θ,phi)= S(θ,phi)/S(θ,phi)max

Where

S(θ,phi) = Poynting vector

The decibel level is given by

dB = (10 log Pn (θ,phi)

Two dimensional field, power and decibel plots of the 3-D antenna pattern of Fig.2-3/ taking a slice薄片through the middle of the 3-dimensional pattern of Fig.2-3 results in the 2-dimensional pattern at (a). It is a field pattern (proportional to the electric field E in V/m) with normalized relative En (θ) =1 at θ=0º. The half power beam width (HPBW) = 40º measured at the E=0.707 level.

The pattern at (b) is power slot of (a) (proportional to E2) with relative power Pn=1 at θ=0º and with HPBW =40º as before and measured at the Pn=0.5 level.

A decibel (dB) plot of (a) is shown at (c) with HPBW = 40º as before and measured at the -3dB level. The first lobes are shown at the -9dB and second side lobes at -13dB. Decibel plots are useful for showing minor lobe levels.

reference: antenna for all application third edition (John D. Kraus & Ronald J. Marhefka)