# UK Amateur Radio ## Foundation Licence 2002
### Feeders & Antennas Part B
### Types of antenna Whilst there are several basic types of basic antenna, their common aim is to convert the RF Signals into Radio Waves. when transmitting, and when receiving collect the radio waves and feed then to the receiver as RF signals.
The size of the aerials will be smaller at UHF than VHF and VHF aerials are smaller than HF aerials. They all do the same thing, convert RF energy in the feeder to radiated RF energy from the antenna.
### Antenna basics The 1/4 wave, 5/8wave and the vertical dipole have a radiation pattern which spreads out all round and are said to be omni-directional. The yagi on the other hand is a directional aerial and has gain due to its ability to focus the signal in a relatively narrow beam. Gain is the apparent increase in output that the aerial gives when compared to a dipole. The higher the gain the more directional. The ERP (Effective Radiated Power) is the result of multiplying the power to the aerial by the gain of the aerial. Power x gain = ERP The aerial must be designed to work at the frequency of the transmitted signal. If the antenna is not properly designed then it will not work effectively and power will be returned from the antenna to the feeder and not radiate. When working on HF if the aerials is not correctly tuned to the frequency of the transmission then it is possible to match the aerial to the Transmitter by the use of an ATU (Aerial Tuner Unit).
### Balanced antenna A centre fed dipole is a balanced antenna. When using a Dipole and feeding HF a balun should be used to link the coax to the aerial. Balanced centre-fed antennas should not be fed by unbalanced feeder without using a balun.
| The balun in the picture is home made being coaxial cable wound around several "sticks" of ferrite rod. The balun is an RF transformer which reduces the mismatch between the 50 and the impedance of the aerial which is about 75 .
### Meaning of SWR The letters SWR stand for Standing Wave Ratio. knowing that in itself is not a lot of use to you but when I can tell you that a meter exists that can measure the SWR then we can get somewhere. When a signal travels up the feeder to the aerial if the aerial is not correctly matched to the transmitter frequency then it reflects power back to the transmitter, hence this power is not available for radiation and the system is inefficient. If the SWR measured at the transmitter is high it shows that a lot of power is being reflected back and this is the fault of the antenna and not the transmitter.
The left hand above picture shows the dial on the SWR meter. It has twin needles one rotates clockwise and one anticlockwise. The forward power is the clockwise rotation and the reflected power is the anticlockwise. Where the two needles cross the red scale is read and gives you the SWR. When operating, you want the maximum forward for least reflected - within the power limitation of the foundation licence. This right hand picture is of a commercially available SWR unit. The picture is a little fuzzy and I will try to put a better one on the site soon.

page page6b
Click here for important Copyright information
Web Space provided by Hostroute.com Ltd
email the Project Co-ordinator