It may help to think of the antenna as just a device to couple the radio waves coming out of the transceiver at an impedence of typically 50 Ohms to the impedence of free space which is approximately 377 Ohms. This is to ensure radio waves are emitted as efficiently as possible. I should point out that this idea is an absolute heresy to many, but the true technical explanation is rather too complex and mathematical to go into on this forum.
This can be accomplished by having wires of a specific length for a given frequency, or by using capacitors and inductors to electrically alter the apparent length of the wire. The lower the frequency, the longer the wavelength and the more wire you need to make a tuned antenna. The transceiver doesn't care what it is transmitting into so long as it has the right impedence i.e. it is tuned correctly for the frequency in use. An impedence mismatch will effectively reflect power back into the output stages of the transmitter section and cause overheating of components and failure.
Technically there is no reason why the impedence of a transceiver couldn't match the impedence of free space, thus allowing a suitable length of wire to be connected directly to the antenna socket. The problem is that for each frequency used, the wire length would need to be altered. The 50 Ohm impedence is just an industry standard that has evolved so that equipment from different manufacturers can be interconnected without too many problems. It is easier to design electrical circuits to artificially lengthen or shorten a wire than to use a mechanical system to do the same thing. Early aircraft fitted with radio equipment did trail long wire antennas that needed to be wound out after takeoff and retrieved before landing. These wires frequently broke in flight preventing communication between the aircraft and ground stations.
In order to efficiently radiate a signal, the general rule is the bigger the antenna, the bigger the signal you will put out.
Modern transceivers can generate very high power levels and have very sensitive receivers, so the actual antenna size is less relevant now than in the past.
Antennas can also be designed to be directional in that they radiate (or receive) signals in a specific direction or pattern rather than omnidirectionally. That can also increase the efficiency of the system by limiting radiation of signals in directions that are not necessary.
Receivers can be designed to be so sensitive that a metre long antenna will pick up signals from around the globe. The sensitivity is so great that antenna impedence mismatches are virtually irrelevant. The same cannot be said for transmitters. In order to reliably transmit a signal any significant distance you need a large antenna, or a powerful transmitter. That said, very low power equipment is capable of global communication under the right conditions on short wave frequencies. One of the reasons why radio users are required to be licensed according to international regulations because of the potential for causing widespread interference if equipment is operated incorrectly.
Last edited by G0ULI; 20th Aug 2017 at 02:37.