5c.1 Understand that half-wave dipoles (mounted vertically), ground planes and 5/8λ antennas are omni-directional.
If you want to radiate a signal equally in all directions then you need
a vertical antenna such as a vertical dipole, a groundplane or a 5/8λ vertical. These are omnidirectional antennas. The drawing shows the radiation pattern is equal in all directions.
5c.2 Understand that a Yagi antenna is directional and has a gain because of its focussing ability.
A
Yagi antenna puts out more signal
in one direction than other directions. It is described as directional.
The drawing opposite shows the radiation pattern from a Yagi. Not only
do Yagis focus power in one direction, but they also cut out
interference from other directions
5c.3 Recall that ERP is the product of the power to the antenna and its gain.
E.R.P. stands for effective radiated
power. The E.R.P. is equal to the power applied to the antenna
multiplied by the gain of the antenna. 6dB gain is equivalent to multiplying
the power by 4. So, if 5 Watts is applied to an antenna with a 6dB gain
then the ERP = 5 x 4 = 20 Watts. You
do not need to be able to use dBs in the Foundation examination, but
you will come across it again in the Intermediate and Advanced
examinations.
5c.4 Recall that the antenna system must be suitable for the frequency of the transmitted signal. Recall that if an antenna is not correctly designed for the frequency it will not match the transmitter and will not work effectively.
Antennas have to be designed for the
frequency or frequencies they are to transmit on. A properly designed
antenna and its feeder system are usually designed to have an impedance
of 50 Ohms to match the 50 Ohm antenna socket of a transmitter. If they
do not have a 50 Ohm impedance then there will be a mismatch and power
will not be efficiently radiated. This could lead to damage to the power output stage of the transmitter.
The
drawing opposite shows that the impedance is only 50 Ohms on one
frequency. To raise the resonant frequency the antenna would have to be
shortened.To lower the resonant frequency the antenna would have to be
lengthened.
5c.5 Recall that at HF, where an antenna has not been designed for the particular frequency, an ATU (antenna tuning unit) improves the ability of the antenna to accept power from the transmitter. Recall that, when an antenna is not well matched to a transmitter, a matching unit, commonly known as an ATU (antenna tuning unit), is used to ensure that the transmitter can supply energy to the antenna without damage to the transmitter.
Can a mismatched
antenna (i.e. one that is too short or too long) be used? The answer is that things can be improved by
placing an ATU (Antenna Tuning Unit) between the end of the feeder from
the antenna and the antenna socket on the TX/RX. The ATU matches the impedance of the
antenna and feeder to the 50 Ohm impedance of the antenna socket. This
prevents the transmitter power amplifier stage being damaged. There may
be some losses from the system because of standing waves on the feeder, but the antenna will radiate some power.
