Antenna resonance and bandwidth
- overview, summary, tutorial about antenna or aerial resonance and
bandwidth and the impact of RF antenna resonance and bandwidth on radio
communications systems.
Two major factors associated with radio antenna design are
the antenna resonant point or centre operating frequency and the antenna
bandwidth or the frequency range over which the antenna design can operate.
These two factors are naturally very important features of any antenna design
and as such they are mentioned in specifications for particular RF ntennas.
Whether the RF antenna is used for broadcasting, WLAN, cellular
telecommunications, PMR or any other application, the performance of the RF
antenna is paramount, and the antenna resonant frequency and the antenna
bandwidth are of great importance.
Antenna resonance
An RF antenna is a form of tuned circuit consisting of
inductance and capacitance, and as a result it has a resonant frequency. This is
the frequency where the capacitive and inductive reactances cancel each other
out. At this point the RF antenna appears purely resistive, the resistance being
a combination of the loss resistance and the radiation resistance.
Impedance of an RF antenna with frequency
The capacitance and inductance of an RF antenna are
determined by its physical properties and the environment where it is located.
The major feature of the RF antenna design is its dimensions. It is found that
the larger the antenna or more strictly the antenna elements, the lower the
resonant frequency. For example antennas for UHF terrestrial television have
relatively small elements, while those for VHF broadcast sound FM have larger
elements indicating a lower frequency. Antennas for short wave applications are
larger still.
Antenna bandwidth
Most RF antenna designs are operated around the resonant
point. This means that there is only a limited bandwidth over which an RF
antenna design can operate efficiently. Outside this the levels of reactance
rise to levels that may be too high for satisfactory operation. Other
characteristics of the antenna may also be impaired away from the centre
operating frequency.
The antenna bandwidth is particularly important where radio
transmitters are concerned as damage may ccur to the transmitter if the antenna
is operated outside its operating range and the radio transmitter is not
adequately protected. In addition to this the signal radiated by the RF antenna
may be less for a number of reasons.
For receiving purposes the performance of the antenna is less
critical in some respects. It can be operated outside its normal bandwidth
without any fear of damage to the set. Even a random length of wire will pick up
signals, and it may be possible to receive several distant stations. However for
the best reception it is necessary to ensure that the performance of the RF
antenna design is optimum.
Impedance bandwidth
One major feature of an RF antenna that does change with
frequency is its impedance. This in turn can cause the amount of reflected power
to increase. If the antenna is used for transmitting it may be that beyond a
given level of reflected power damage may be caused to either the transmitter or
the feeder, and this is quite likely to be a factor which limits the operating
bandwidth of an antenna. Today most transmitters have some form of SWR
protection circuit that prevents damage by reducing the output power to an
acceptable level as the levels of reflected power increase. This in turn means
that the efficiency of the station is reduced outside a given bandwidth. As far
as receiving is concerned the impedance changes of the antenna are not as
critical as they will mean that the signal transfer from the antenna itself to
the feeder is reduced and in turn the efficiency will fall. For amateur
operation the frequencies below which a maximum SWR figure of 1.5:1 is produced
is often taken as the acceptable bandwidth.
In order to increase the bandwidth of an antenna there are a
number of measures that can be taken. One is the use of thicker conductors.
Another is the actual type of antenna used. For example a folded dipole which is
described fully in Chapter 3 has a wider bandwidth than a non-folded one. In
fact looking at a standard television antenna it is possible to see both of
these features included.
Radiation pattern
Another feature of an antenna that changes with frequency is
its radiation pattern. In the case of a beam it is particularly noticeable. In
particular the front to back ratio will fall off rapidly outside a given
bandwidth, and so will the gain. In an antenna such as a Yagi this is caused by
a reduction in the currents in the parasitic elements as the frequency of
operation is moved away from resonance. For beam antennas such as the Yagi the
radiation pattern bandwidth is defined as the frequency range over which the
gain of the main lobe is within 1 dB of its maximum.
For many beam antennas, especially high gain ones it will be
found that the impedance bandwidth is wider than the radiation pattern
bandwidth, although the two parameters are inter-related in many respects.
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