6 Monopole antenna is excited by a current source at its base. Directivity is doubled and radiation resistance is half of that of dipole antenna.
7 The best operation: ground is highly conductive (or use counterpoise in case of remote antenna) Shorter than /4 antenna arises highly capacitive input impedances, thus efficiency decreases. Solution: inductive coil or top-hat capacitor Inductive coilTop-hat capacitor
A group of several antenna elements in various configurations (straight lines, circles, triangles, etc.) with proper amplitude and phase relations, main beam direction can be controlled. Improvement of the radiation characteristic can be done over a single-element antenna (broad beam, low directivity) 8
To simplify, 1. All antennas are identical. 2. Current amplitude is the same. 3. The radiation pattern lies in x-y plane 9 From Consider,
Let I 1 = I 0, I 2 = I 0 e j , since r 1 and r 2 >> d/2 for far field, we can assume 1 2 and r 1 r 2 r. 10
11 But the exponential terms cannot be approximated, then
We can write this as F unit = a unit factor or the maximum time-averaged power density for an individual element at F array = array factor = where This depends only on distance d and relative current phase, . We can conclude that the pattern function of an array of identical elements is described by the product of the element factor and the array factor. 12
We will simplify assumptions as follows: 1. The array is linear, evenly spaced along the line. 2. The array is uniform, driven by the same magnitude current source with constant phase difference between adjacent elements. 13 (F array ) max = N 2
Yagiuda (rooftop antenna) 14 Parasitic elements are indirectly driven by current induced in them from the driven element.
Consider power transmission relation between transmitting and receiving antennas where particular antennas are aligned with same polarization. 15 Let P rad1 be P total radiated by antenna 1 have a directivity D max1, With reciprocal property, Therefore, we have
Each variable is independent of one another, so each term has to be constant, we found that 16
Effective area (A e ) is much larger than the physical cross section. More general expressions We can also write 17
Finally, consider P rad = e t P in, P out = e r P rec, and G t = e t D t, G r = e r D r 18 Friis transmission equation Note: Assume - matched impedance condition between the transmitter circuitry/antenna and receiver - antenna polarizations are the same.
Since the receiver is matched, half the received power is dissipated in the load, therefore Without the matching network, 20
A monostatic radar system Some of energy is scattered by target so called ‘the echo signal’ received at the radar antenna.
Let P rad be the radiated power transmitted by the radar antenna, then the radiated power density P 1 (r, , ) at the target at the distance r away is The power scattered by the target is then s = radar cross section (m 2 )
This scattered power results in a radiated power density at the radar antenna of Then By manipulation of these equations, we have or
Radiation patterns for dipole antenna http://www.amanogawa.com/archive/DipoleAnt/DipoleAnt-2.html
Ex1 Suppose a 0.5 dipole transmitting antenna’s power source is 12-V amplitude voltage in series with a 25 source resistance as shown. What is the total power radiated from the antenna with and without an insertion of a matching network? 0.5