Antennas have three particularly important characteristics, namely directionality, gain, and bandwidth.

Directional

Dipoles have strong directionality: they can receive radio waves at right angles to them. That's why TV antennas must be installed correctly in your home and oriented in the correct direction.

The telescopic antenna on FM radio does not have such obvious directionality, especially in strong signal situations: if you point it straight upwards, it can almost capture good signals from any direction. The ferrite amplitude modulation antenna inside the radio has much stronger directivity. When listening to amplitude modulation programs, you will find that you need to swing the radio left and right until it can receive a very strong signal. After finding the best signal, try rotating the radio 90 degrees and you will find that the signal often disappears almost completely.

The dipole antenna shown in the above figure has strong directivity.

Although high directional antennas may seem troublesome, as long as they are aligned correctly, they can help reduce interference from unwanted sites or near the sites you want to detect. But directionality is not always a good thing.

Think about your phone. You want it to be able to answer calls from any position on the nearest telephone pole, or when it's in your bag, you can receive information no matter which direction it happens to point in, so highly directional antennas are not very useful.

Similarly, GPS receivers also use signals from multiple space satellites to tell you their location. Due to the signals coming from different satellites at different positions in the sky, they come from different directions. Similarly, here, high directional antennas will not be of much help.

Gain

The gain of an antenna is a very professional measurement indicator, but broadly speaking, it can be attributed to the degree to which the antenna enhances the signal. Even without an antenna, the TV will still receive weak signals. This is because the metal casing and other components serve as the basic antenna, they do not focus in any specific direction and will receive a certain signal by default.

If a suitable directional antenna is added, better signals can be obtained. The gain is measured in decibels (dB), and the larger the gain, the better the reception effect. In terms of televisions, complex outdoor antennas (such as a parallel "array" of 10-12 dipoles) can achieve much greater gain than simple dipoles. All outdoor antennas have better performance than indoor antennas, including window antennas and fixed antennas with higher gain and better performance than built-in antennas.

In the above picture, the lunar rover of the Apollo 17 mission (photographed on the moon in December 1972) carries a high gain, foldable parabolic reflector antenna in the upper right corner, resembling an umbrella.

Bandwidth

The bandwidth of an antenna refers to the frequency or wavelength range within which the antenna can effectively operate. The wider the bandwidth, the wider the range of different radio waves you can receive. This is very helpful for programs like television that require receiving many different channels, but it is not as useful for telephone, mobile phone, or satellite communication because in these programs, what you are interested in is only transmitting very specific radio waves on a relatively narrow frequency band.

Mobile devices typically use compact microstrip antennas (directly printed on circuit boards) designed for very limited specific frequencies, so their bandwidth is often very narrow.

(1) An antenna that supplies power to RFID tags inserted into library books. Its internal circuit has no power supply, and all energy comes from the received radio waves, typically at 13.56MHz (according to the international standard ISO18000).

(2) Dipole antenna in PCMCIA wireless internet Wi Fi card. This antenna uses 2.4GHz radio waves with a wavelength of 12.5 centimeters, so its total length is only about 6 centimeters.