A general note on aerials
An aerial or antenna is a device which acts as the mouth and ears of a radio transmitter or receiver respectively. Though we don’t notice any external aerial in many of the commercial radio sets, they in fact have aerials built within the cabinets holding their electronic circuitry. But a amateur radio operator is mainly concerned with an external outdoor antenna without which he can’t expect to radiate radio energy into space from his radio transmitter. Similarly, without an external outdoor antenna, his radio receiver will not be able to pick up the radio waves speeding across the sky. A radio receiver might not need an external outdoor aerial to receive high power radio transmissions, but most amateur radio transmitters use considerably low power (compared to broadcast radio stations) which necessitates the use of outdoor aerials. A low power transmitter with an efficient antenna system or a less sensitive receiver with an efficient antenna system can be made to work beyond imagination!
The aerials are usually made out of metallic rods or wires which are cut into specific lengths. The aerial should not be placed behind any obstruction. Conducting materials such as tin-roof, ferro-concrete and to a lesser extent foliage when wet. The aerial should be as high as practical above the ground and grounded objects such as metal roofs, power or telephone wires etc.
Different types of antenna system used by amateur radio operators.
Different types of antenna system commonly used by amateur radio operators are:
- Horizontal Dipole,
- Inverted -V dipole,
- Yagi beam,
- Ground plan vertical,
- Cubical quad.
The working function of a horizontal dipole antenna.
A horizontal dipole antenna is a resonant antenna which is a half-wavelength long. Resonant circuits are well known in radio engineering as combination of coils and capacitors, which cause a signal gain at certain frequencies. The same is applied to a half-wave dipole antenna.
It consists of two straight wire or rod sections, each 1/4 wave long and positioned in one line (collinear). The antenna is fed in the centre by a coaxial cable having a characteristics impedance of 50 Ohms or 75 Ohms.
The maximum radiation direction is perpendicular to the axis from the middle point. The cause of directional radiation by a resonant half-wave dipole antenna is that the radiation intensity is proportional to the square of the current in the antenna, and in the dipole current is maximum at the middle; hence the maximum radiation line passes through the middle of the antenna perpendicularly.
Why are half-wave dipoles fed at the centre?
Most half-wave dipoles are fed at the centre, because in a half-wave resonant dipole, maximum current point is at the centre of the antenna and this is the minimum voltage point. It is easier to construct transmission lines for low voltage than for high voltage.
The other reason is that in a 1/2 wave dipole, the capacitive reactance and inductive reactance cancel each other (the antenna being resonant), leaving resistance only as net impedance. Under this condition, the antenna impedance is the resistance between any two points equidistant from the centre along the antenna length making it easier to match the transmission line impedance with the antenna impedance.
What is VSWR (Voltage Standing Wave Ratio) ?
When the transmission line does not match the load impedance (antenna impedance), maximum transference of energy to the antenna is not possible. The energy fed down the line is transferred to the antenna only partially; in fact, some is reflected back, forming standing waves on the line. Every half-wave along the line, high-E (Voltage) and Low-I (Current) points appear. Halfway between these two points will be Low-E and High-I points.
The ratio of voltage across the transmission line at the high-E point to that at Low-E points is called the VSWR.
SWR=1:1 or 1
The SWR is also equal to the ratio of the characteristic impedance of the transmission line to the impedance of the antenna (load), or vice versa. For example, if the line has a characteristic impedance of 300 ohms and antenna impedance is 50 ohms, the SWR is 300/50, or 6. A higher SWR indicates a greater mismatch between the transmission line and the antenna.
When the load (antenna) impedance matches the transmission line impedance, there will be no standing waves.
VSWR is greater than one for a mismatched system and equal to one for a perfectly matched system.
When an antenna is excited into oscillation by a RF source, it radiates energy into space acting as a source of power. The antenna, which is the source of power must have an internal resistance or impedance. We have-
Power, P=I2R, Where I=current, R=resistance
So in the case of the antenna, radiation resistance is the ratio of the radiated power to the square of the centre current in the antenna.
Radiation resistance is also defined as a fictitious resistance, which when substituted for the antenna would consume as much power as the antenna radiates.
Radiation resistance is also called ‘Feed-point’ impedance; in the case of a dipole antenna the feed point impedance is nearly 73 Ohms.
Impedance matching is of utmost importance so far as energy transference from the transmitter to the antenna through the transmission line is concerned; because, mismatching will prevent maximum output being radiated, i.e. if the transmission line impedance doesn’t match the antenna feedpoint impedance, a part of the energy fed down the line will be reflected back from the antenna causing standing waves on the line; it makes the system inefficient.
Mismatching a transmission line to an antenna results in the line at the transmitter end appearing to have either inductive reactance (Xi) or capacitive reactance (Xc), which will detune the inductance-capacitance (LC) circuit to which it is coupled; mismatching should be avoided so that the final stage of the RF amplifier is not detuned.
In many of the commercial wireless equipment, mismatching should be strictly avoided to prevent damage to the circuitry.
Current fed antenna:
There are many methods of feeding energy to an antenna. The antenna is said to be current fed when excitation energy from the RF-generator is introduced to the antenna at the point of high circulating currents. The example is a 1/2 wave dipole antenna. In this case, the 1/2 wave antenna is cut in two parts at the midpoint and energy is fed by co-axial transmission line.
In a dipole antenna maximum current flows through the middle point, hence it is current fed antenna with a characteristic feed point impedance of about 73 ohms, which is considerably small as compared to end point impedance of the antenna. Midpoint is the low-voltage point.
Voltage fed antenna:
When the excitation energy from the RF source is introduced at the point of maximum voltage, the antenna is said to be voltage fed antenna. The example is the 1/2 wave unsplitted antenna excited by a resonant R-F line. Voltage changes at this point excite the antenna into oscillation. The impedance at the end of the antenna is high or it is the high impedance point.
Any multiple of a 1/2 wave resonant antenna may be end-fed by using a tuned feeder system leaving one end of the feed-line unconnected. This antenna is also called Zepp (used earlier on Zeppelins) antenna.
- Using the proper transmission line for each particular antenna is a way of achieving impedance matching. For example, a 1/2wave dipole has a midpoint impedance of 73 ohms, so coaxial cable transmission line which has a characteristic impedance of 75 ohms is used to feed the R-F energy into the antenna.
- Delta match: This type of matching procedure is used with an unsplitted 1/2 wave dipole antenna; the dipole being resonant, its capacitive reactance (Xc) and inductive reactance (XL) cancel each other, leaving resistance only as net impedance. Under this condition, the antenna impedance is the resistance between any two points equidistant from the centre and thus transmission lines having characteristic impedance of 300 to 600 ohms can be used by getting two points of the antenna to feed where it offers a feed point impedance equal to transmission line impedance.
To do so, it is essential to spread out the feeders at the antenna end.
The formula used to make this type of matching are :
B= (0.25 x Wavelength)/2; where B is the distance between the two feed point which will offer 600 ohms impedance.
And C=(0.32 x wavelength)/2, where C is the vertical distance upto which spreader should be spread (the inclination).
- Stub Match : A shorted stub of 1/2 wave length or open stub of 1/4 wave can be connected to the splitted dipole. Here the low midpoint impedance of 73 ohms of the dipole is repeated at the close end of the stub; but there are cetain points on the stub which would offer as high as 600 ohms impedance yet matching with 73 ohms feed point.
- Gamma Match: Here the outer sheath of the 75 ohms coaxial cable is connected to the middle point of the unsplitted dipole, while the inner conductor is connected to a point through a capacitor to cancel inductive reactance, so that antenna impedance at the feed point is 75 ohms. A Gamma match is slightly unbalanced.
- T-Match: In this type of impedance matching, two coaxial cables are held side by side and both their outer sheaths are connected to the midpoint of the unsplitted dipole, while two points are choosen on the dipole where inner conductors going parallel to each other (of the coaxial) are connected.
- 1/4 wave transmission line impedance matching device: A 1/4 wave line can act as an impedance matching device between high and low impedance circuits if it has the proper intermediate impedance found from the formula: Z = /Z1.Z2
Where Z1 = antenna feedpoint impedance; Z2 main transmission line impedance. When we want to match a 300 ohms transmission line to a 70 ohms feed point impedance dipole antenna, then the 1/4 wave transmission line connected between both the system should have
Z= / 300 x 70 = 145 ohms
When a half wave dipole antenna consists of one or more parasitic arrays, the antenna becomes a parasitic beam antenna, named as “Yagi” after one of its designers Professor Yagi, Japan.
The antenna consists of mainly three elements, the 1/2 wave splitted dipole driven element, in front of this driven element is the 5% shorter director element, back of the driven element is the 5% longer reflector. All the elements can be assembled on a single conducting boom.
This antenna has the property of beaming radio signals in the direction of the director and very reduced signals in the backward direction.