Long wire antenna for multiband operation

 An ideal long wire antenna is one which is several wavelengths long. But that is not practical for most amateur radio operators. Sometimes it is also called as a random wire antenna because of the random length which is involved, though strictly not correct. In any case, a long wire antenna has to be at least quarter wavelength or better one full wavelength of the operating frequency. But wires at even multiples of quarter wavelengths will have very high feedpoint impedance which cannot be matched with an antenna tuner. Hence it is important to avoid even multiples of operating wavelengths.

Long wire antenna is in fact an end fed wire antenna, typically matched with a tuner. Mounting should be as high as possible, for better performance. This method is often used during portable operations where the antenna can be mounted between branches of tall trees in the wild. Long wire antennas can be mounted horizontally, vertically or in a slope, depending on the local situation.

The long wire antenna needs an external antenna tuner for matching the impedance during transmit mode to avoid damage to the radio. Reception may be fair even without tuner, but it is likely to be better with a tuner and proper impedance matching. Another important requirement for a long wire antenna is a good earthing at the radio end, usually from the tuner, with as short a wire length as possible. This can be achieved fairly well during portable operations. The earthing is not meant for protection from electric shocks as is usual for home wiring. That means you need good earthing even if you are running on batteries as in portable operation. Earthing is for better performance of the antenna system in case of long wire antenna. If earth wire is long, the inductance of the wire will cause RF potential in the metallic parts of the radio to be significant enough to cause RF bites.

If the length of the wire is exactly quarter wavelength of the operating band, the current maximum will be at the level of the radio. This can cause RF in the shack and interfere with the functioning of electrical and electronic equipments nearby. Same occurs if the length is an odd multiple of the operating frequency. Hence it is better to avoid such lengths. Yet these odd multiples and exact quarter wavelengths have low impedance at feed point and are easier to tune, even without an antenna tuner. The advantages of the long wire antenna are the low cost, ease of installation and possibility of multi-band operation with a tuner. That is why it is sometimes preferred in portable operations where setting up other types of antennas may have practical limitations.

Usual radiation pattern of a dipole antenna is perpendicular to its length. But for a long wire antenna which is several wavelengths long, the radiation and pick up of signals tend to become along the line of the wire. A truly long wire antenna has maximum sensitivity and radiation along the wire. It becomes an end fire antenna then. It has been mentioned that when the length is 10 wavelengths, the gain over a dipole (dBd) can be as high as 7.4. When it is five times the wavelength the gain is 4.2 dBd. But it is often impractical to have such very long wire antennas. Some have mounted such long wire antennas along the fence and tried using it. If the fence is quite long, you can have radiation along the line of the fence. Yet a compromise is that the height will be low for an antenna wire along a fence and will affect the radiation efficiency.

Long wire antennas can be terminated using a grounded resistor to prevent formation of standing waves so that it become a truly non-resonant long wire antenna. Two variants of long wire antenna which uses resistors are the beverage antenna and the rhombic antenna. While the beverage antenna connects the non-radio end to the ground through a resistor, rhombic antenna has a resistor between the two limbs of the rhomboid shape, making it a directional antenna. Beverage antenna, also known as wave antenna, is a long wire receiving antenna invented by Harold H. Beverage in 1921, the name being totally unrelated to a beverage! The value of the resistor used for termination is 400 to 800 Ohms, equal in value to the characteristic impedance of the antenna.