Different Types of Monopole Antennas
Different Types of Monopole Antennas
Several types of monopole antennas are used for radio transmissions, both amateur and professional. The tall tower antennas often seen near medium wave radio stations are monopole antennas. Unlike an amateur radio tower with a Yagi-Uda directional beam antenna at the top, the tower as such is the radiating element for such broadcast stations. Ground plane for the tower antennas of broadcast stations are formed by a set of cables buried under the ground.
The tall mast radiators of broadcast stations are usually supported by multiple guy wires in several directions, with insulators in between. Red beacon lights are also visible at the top if you happen to see them at night. Other aircraft warning lights are seen along the height of the mast as well. Special RF blocking devices are needed to prevent their power supply lines from conducting the high RF voltages on the antenna down to the power source. These mast radiators are quite different from the microwave towers of the telecom department and mobile phone antenna towers. Those towers have several directional antennas on top, receiving and transmitting signals in various directions, linking them with other towers at a distance as well. Interestingly, some broadcast stations may also allow colocation of antennas for other purposes on their tower!
A monopole antenna which is quite familiar to the Ham Radio operator is the Rubber Ducky used on hand held radios. These are used on other types of walkie-talkies as well. It has a helical coil inside and the outer covering is made of flexible material which gives it the name. Helical nature of the wire gives added inductance to cancel out the capacitive reactance of an electrically short antenna which is typically shorter than the quarter wavelength of the operating frequency, sometimes even just 1/10th of the operating frequency. Though the inductance of the helix matches the impedance, performance in terms of gain and range is less than the standard quarter wavelength antenna. Whip antenna commonly seen on automobiles are also monopole antenna with a helical wire either inside it or outside it and a flexible base material.
Ground plane vertical antennas used for VHF and UHF work are also monopole antennas. Usually they are of quarter wavelength radiating elements and the ground plane is formed by three or four radials. Maximum power in horizontal direction and gain among the ground plane monopoles will be for a 5/8 wavelength vertical antenna useful as a mobile antenna for VHF and UHF. When it is mounted on an automobile metallic body, the ground plane is formed by vehicle body.
Magnetic mount antennas for mobile use also come under the category of monopole antennas. They are popular among Ham Radio operators who do not want to drill the chasis of their vehicle to put a mobile antenna. Drilling the chasis would also affect the resale value of the vehicle. Magnetic mount antennas can also be used as indoor antennas to access a nearby VHF repeater, if you can put them over a tall steel almirah which will act as a ground plane. I could use it this way to access a repeater which was 30 km away. Radiation pattern from all these monopole antennas discussed are vertically polarized. The first monopole antenna was invented by Marconi in 1895 and bears his name as Marconi antenna. In the original description, there was a metal plate at the top which was later found to be unnecessary and removed by Marconi himself.
