A quarter wave vertical antenna is widely used for medium wave broadcasting and mobile radio communications due to its omni-directional radiation pattern. It consists of a quarter wavelength vertical element connected to ground. At the feed point, the voltage is minimum and current is maximum, giving a low feed impedance around 20 ohms. For mobile use, the vehicle body acts as a simulated ground. For fixed stations, radials extending from the base improve performance by simulating a ground plane. Matching elements can also be used to match the antenna's low impedance to transmission lines.

1. Quarter wave vertical antenna
- overview or summary about the basics of the quarter wave vertical and
ground plane antennas that ares widely used for medium wave broadcasting,
mobile radio communications applications and many more.
VERTICAL ANTENNAS INCLUDES
 Vertical Antennas Introduction
 Quarter Wave Vertical
 5/8 Wavelength vertical
 J pole antenna
Vertical antennas are widely used at all frequencies from MF up to VHF and beyond. They exist in a
variety of forms including the quarter wave vertical and ground plane antennas. They possess many
advantages and are widely used for medium wave broadcasting as well as for mobile applications in
areas including private mobile radio.
The reason for this widespread use is the omni-directional radiation pattern that they give in the
horizontal plane. This means that the antennas do not have to be re-orientated to keep the signals
constant as the car moves it position.
Single element vertical antennas posses an omni-directional radiation pattern (in the horizontal
plane). This means that the antennas do not have to be re-orientated when used in mobile
applications as the vehicle moves. This is obviously an essential requirement.
A further advantage is that much of the radiation is at right angles to the antenna element, and as a
result it travels close to the earth's surface where the receiving stations are located. Radiation
directed upwards is wasted in many instances as VHF transmissions are normally not reflected by
the ionosphere.
For medium wave broadcast stations a particular advantage is that the radiation is vertically
polarised. It is found that the vertically polarised transmissions propagate further via the ground
wave that these transmissions use.
Basic element
Like the name suggests the antenna consists of a quarter wavelength vertical element. The antenna
is what is termed "un-balanced" having one connection to the vertical element and using an earth
connection or simulated earth connection to provide an image for the other connection.
The voltage and current waveforms show that at the end the voltage rises to a maximum whereas
the current falls to a minimum. Then at the base of the antenna at the feed point, the voltage is at a
minimum and the current is at its maximum. This gives the antenna a low feed impedance. Typically
this is around 20 ohms.

2. A quarter wave vertical RF antenna design
The ground is obviously an important part of the RF antenna. Many MF and HF installations use a
ground connection for this. These ground systems need to be very effective fort he antenna to
perform satisfactorily. They must obviously have a very low resistance, and often utilise large "mats"
of radials extending out from the base of the antenna to ensure excellent RF performance.
For VHF and UHF installations, height is obviously important and antennas need to be raised to
ensure they are above the nearby obstructions. Also for mobile installations it is clearly not possible
to use a true earth connection. In these cases a simulated earth is used. For mobile applications this
consists of the body of the vehicle. The antenna mounting will normally enable a suitable connection
to be made to the vehicle body, sometimes using a capacitive connection. However it is necessary
to ensure that the vehicle body is metal, and not plastic in the vicinity of the antenna mounting.
For fixed stations a set of radials simulating a ground plane is used. In theory the ground plane
should extend out to infinity, but in practice a number of radials a quarter wavelength long is used.
Typically for many VHF applications four radials is sufficient.
A radial system used with a quarter wave vertical
If the radials are bent downwards from the horizontal then the feed impedance will be raised. A 50
ohm match is achieved when the angle between the ground plane rods and the horizontal is 42
degrees. Another solution is to include an impedance matching element in the antenna. Normally
this is in the form of a tapped coil that can be conveniently housed in the base of the antenna.

3. Folded element
In view of the low impedance presented to the feeder by the RF antenna, methods must be found of
presenting a good match and some have already been outlined. Another is to use a folded element.
In the same way that a folded dipole increases the feed impedance of the antenna, so a folded
vertical element can be used. If the diameter of both sections is the same, then an increase by a
ratio of 4:1 is achieved. This would bring the impedance to 80 ohms and will provide an acceptable
match to 75 ohm feeder. By using a smaller diameter grounded element the feed impedance can be
reduced so that a good match to 50 ohm coax can be achieved.
Summary
The quarter wave vertical antenna is widely used in view of its simplicity and convenience. To
improve on its performance other types of vertical are available. It is also possilbe to use further
verticals and feed them with different phases to provide gain to the overall antenna system.