Antennas.

What is an antenna?
An antenna, often referred to as an aerial, is a device for receiving or transmitting radio waves. This involves converting electromagnetic energy into electrical energy (receiving) or vice-versa (transmitting).

Efficiency
Energy is neither created or lost in the system, it is converted from one form of energy into another form of energy. An efficient antenna will convert more of that energy into electromagnetic or electrical energy.
Any DC resistance will convert some power into heat. This energy is not helping to transmit or receive signal, and so is just wasted. For this reason, the conductor used should have a low resistance, as should all connections and joints. As all connections can exhibit some resistance, they should be kept to the minimum required.

Length.
For an antenna to work efficiently, it should be the correct length for the frequency being used. This length is worked out by dividing the speed of light by the frequency being used. If it is to cover a band of frequencies, you take the centre frequency (add top and bottom frequencies, and divide by two).
This will give you the wavelength, and the antenna is usually half of this wavelength - after considering other factors, such as width (below), which affect the actual length required.
As it is not always practical to make an antenna of such a length, some of this length may be contained within a length of coiled section. Any such shortening of an antenna will reduce the effectiveness of the antenna, but that may be the only practical solution.

Width.
The width of the conductor used will have an effect on efficiency and bandwidth. Thinner conductors will have a higher resistance than thicker ones - so will waste more power.
The width of the conductor also affects the length it needs to be for resonance at the frequency it is for.

Impedance.
This is an important characteristic of the antenna, as it should match that of the radio, TV, or transmitter being used. The characteristic impedance is set by the antenna design and, as some antennas need tuning, refers to a correctly tuned antenna.

Tuning.
Although some antennas, such as TV types, are pre-tuned and need no adjustment, many antennas for transmitters need to be tuned for the frequency being used. This tuning usually involves adjusting the length while using a meter to check for standing waves.

Mounting.
As nearby objects may waste power (converting the energy into another form, such as heat) or even reflect it back, mounting position may make a big difference. Also, some wire antennas may use a material, such as fibreglass, to stiffen them. Such materials may also have an effect on the efficiency.
Wherever possible, antennas should be mounted clear of other objects which may stop some of the signal from getting where you want it. A higher position above roof tops is best, although not always practical.