... Quarter Wave Verticals
In order to resonate at a particular frequency, an antenna needs to be at least one half wavelength long. This means that to radiate efficiently at that frequency, an antenna needs to be a certain length (or to look that at that particular length electrically). The following will give an idea of the great variations in length involved for a half wavelength:
At LF, MF and HF frequencies, these lengths may not be a problem for some fixed installations. However, the lower the frequency the greater the difficulties and with some installations, especially mobile communications on ships and vehicles, this is not practical. If a ground plane can be used, however, the required length can be shortened to about half. This is known as quarter wave grounded mode. This type of antenna is also often used for VHF and UHF land mobile communications as the area of the roof of the vehicle is in excess of a quarter wavelength at these frequencies.
On vehicles it is the metal chassis with capacity coupling to ground, and in the open or on buildings, in the absence of a metal roof of sufficient dimensions, metal ground planes or earth mats are used.
At the end of the antenna, the feed point, when at resonance, the voltage is at a minimum and the current at its maximum, giving the antenna a low feed impedance, typically around36.5 ohms, which is around half that of a dipole in free space. Otherwise, the voltage, current and impedance relationship characteristics are the same as those of a half wave antenna.
The performance of the quarter wave grounded vertical antenna will be determined to a great degree by both the type of ground it is connected to and how efficiently that connection is made. It is especially important at HF and MF frequencies for ground systems to have very low resistance. Soil conductivity can vary considerably and where it is low, the reflected wave from the ground may be greatly attenuated. In the case of antennas at sea, losses will be minimal as sea water is a very good conductor of electricity – that is provided the connection to the sea is good.
In the case of land base stations, earth mats or ground planes are used. Although the shape and size of the earth mat is not particularly critical, it should extend for equal distances in all directions. The radials generally need to be at least 5% longer than the driven element. At sea, the ground plane is normally provided by connection to the metal hull, or, if the hull is not metal, to a metal earth plate mounted on the hull permanently beneath the water line. On vehicles there needs to be connection to a metal chassis, making sure that the chassis is metal and not plastic in the vicinity of the antenna. Mounting the antenna other than in the centre of the ground plane will have some affect on the radiation characteristics, as is often the case on ships and may be on vehicles.
Because the quarter wave grounded vertical antenna has a low impedance at the feed point at resonance, performance will also be affected by impedance matching (more usually to 50 ohms). If the antenna does not match the transmission line perfectly, wave interference is set up at the junction of the line and the antenna, and a portion of the energy is reflected by this mismatch point back down the transmission line towards the transmitter. Some VHF ground plane designs feature radials that are bent downwards from the horizontal to raise the feed impedance.
High VSWR indicates mismatch, which means increased power losses and less radiated signal, making it impossible for your transmitter to perform as well as it should. The greater the mismatch, the greater amount of energy is reflected. And the longer the cable run, the more significant the power losses will be. The worst case is when the forward and reflected waves are equal in strength, indicating a short circuit or open connection in the antenna system.
For VHF, the low impedance presented to the feeder by the antenna can also be raised by using an impedance matching element in the antenna, usually in the form of a tapped coil housed in the base of the antenna, or by using a folded vertical element.
For correct operation, quarter wave grounded verticals at HF and below are dependent on antenna tuning units or ATUs. ATUs make it possible to tune over a wide frequency band and provide two necessary functions.
Firstly, because a range of frequencies is normally required and in each case the antenna needs to look like a different length electrically, the antenna must be tuned or made resonant at the frequency required. (If the antenna is too short, inductance is added; if it is too long, series capacitance is added to arrive at the correct electrical length.) Secondly, the feed line impedance needs to be matched to 50 ohms.
ATUs can also provide a certain amount of filtering to eliminate interference on frequencies higher and lower than the one you are using. This can be very useful if you are faced with interference within the band, which with a broadband antenna would affect performance throughout the band. Similar matching networks are used in other types of equipment, such as linear amplifiers, to transform impedances.
It should be remembered, however, that the best performance is achieved when an antenna is naturally resonant at the frequency you are using. An ATU requires power to tune and the greater the amount of adjustment it has to make, the more power is used up. For example, if an antenna is naturally resonant at around 20 MHz and the required frequency is 2 MHz, the antenna length must be adjusted electrically from approximately 3.65 metres (12ft) to 36 metres (118 ft). This results in power lost to radiation of the signal. If you use a tuned antenna, however, you will still need an ATU for impedance matching or there will be considerable losses on the feed line due to standing waves.
In the case of antennas that are physically short for the frequencies to be used, it is possible to minimise losses in the ATU by incorporating centre loading coils to make antennas look longer electrically than they actually are, thus maximising the power available for radiation. When an antenna is to be used over a range of frequencies, choosing the highest of regularly used frequencies for loading can make a significant difference to performance on the lower frequencies.
The diagram above shows the type of skywave radiation that can be expected from a quarter wave vertical antenna. The outer lighter area is theoretical, the inner darker area is what results in practice. This is what happens when either a radial earth system or an earth plate is used.
In the vertical plane, 1/4 wave vertical antennas exhibit low angle radiation providing longer distance communications. In the horizontal plane, the radiation is omnidirectional. This makes them a popular choice for the lower frequencies up to VHF and beyond, especially for mobile applications for vehicles and vessels at sea, where directionality obviously would restrict performance.
Up to around 4 or 5 MHz, ¼ wave vertical antennas have a useful ground wave, which follows the curvature of the earth, as well as a sky wave which is refracted from the ionosphere for long distances. They also have vertical polarisation, and so are very suitable for MF broadcast communications that use the ground wave.
Usually you can count on a range of around 200-240 km (125-150 miles). After this the ground wave ceases to be of great use as attenuation increases with frequency very rapidly and is very high at night. At 2 MHz the surface path normally has a maximum range of about 500km (300 miles). The usual minimum range is about 80km (50 miles). However, under ideal conditions (at midday during winter with low noise conditions) it is possible to communicate on 2 MHz over distances of about 900km (575 miles) by using a 100W transmitter with an efficient antenna/ earth system.
There is one disadvantage, however. Between where the ground wave ends and the first sky wave comes in, there is an area of no communications, known as the Skip Zone. The extent of the Skip Zone depends on the angle the signal leaves the antenna. Higher angles mean a smaller skip zone and lower angles a greater skip zone. Because quarter wave vertical antennas have low take of angles, the size of this zone is greater than you get with horizontal antennas, which have medium take off angles, and slanted wires, which have high take off angles.
One way to have your cake and eat it is to have an antenna that can be laid down – that is one that can operate both vertically and horizontally, like the Moonraker type 23L/D. When laid down, the antenna becomes horizontally polarised and the radiation characteristics change radically, providing medium and high angles for close in NVIS and medium distance communications. To achieve the NVIS pattern the antenna needs to be elevated above the ground plane not more than 6 metres (20ft). This type of operation lends itself to shipboard operations, where the ground plane is the metal deck or the sea.