If you did not start at the beginning you should read part 1 first.
Antennas are a critical component in a radio device. They will gather the very weak electromagnetic wave that is in the air and convert that into sound for you to hear. They will also emanate the radio signal so that other radios can receive it. Not all antennas are the same, there are several properties that make an antenna suitable for a particular radio.
Some antennas are receive only while others can both receive and transmit. Never use a receive only antenna on a transmitter you can damage the radio. Transmitting antennas are tuned for a specific frequency or group of frequencies. Antennas need to be tuned so they resonate at the frequency they are operating on, if they are not tuned for that frequency they can damage the radio.
Antennas have a certain impedance, in this respect they act like a resistor. Unlike a resistor it is radio impedance and not electrical impedance. You cannot measure their impedance with a standard multimeter. The antenna impedance should match the radio, if they do not then a device that matches impedance should be installed, often called a balun.
Just because the antenna may have the correct connector to attach to a radio does not mean that it is suitable for that radio. The mode that you operate does not matter, it is the frequency and impedance that is important.
Antennas can give you gain. Gain is extra power in one direction at the cost of losing it in another. If you think of a radio transmission coming off an antenna like a balloon, gain would be like flattening the balloon so that it expands horizontally but has less vertical coverage. Alternatively think of a flashlight. It has a parabolic mirror behind the bulb. This mirror provides gain, light goes in one direction more than uniformly in all directions. This is generally desirable since you generally do not talk to people straight up or down but instead on the same or roughly the same level you are on. By increasing the gain in this one direction you get additional range.
Another aspect to antennas is polarization. The two most common types of polarization is horizontal and vertical. If you have a standard stick type antenna (whip) placing it horizontally or vertically will adjust the polarization for you. If you have a transmitter horizontal and a receiver vertical (or vice versa) the receiver will only get 1% of the received signal. You can dramatically increase the range you can receive a signal with by matching the polarization. There is a caveat to this which will be explained later.
Continue to part 5