When you broadcast a signal, the microphone creates a current which is sent to the antenna and creates a magnetic field around the antenna. When a voltage is applied, the magnetic field can then create an electric field. The electric field can then induce a magnetic field which can induce an electric field and so forth. This is an electromagnetic wave and is how radio signals are carried through the air.
A receiving antenna works similarly where the electromagnetic waves in the air create small voltage changes to the antenna. The tuned circuit is what decides if this is the right signal or not and comes after the antenna.
Most antennas will work as transmitting and receiving antennas. This is called antenna reciprocity. Also most antennas will pick up horizontally or vertically polarized signals because in real life, no antenna is perfect. If polarization becomes a problem for you, check out an antenna with circular polarization.
Important Things to Know About Antennas
The three most important things to think about when considering an antenna are directionality, gain and bandwidth. Directional antennas might seem like a hassle, but when used correctly they can help to alleviate unwanted noise and interference from other signals. Be sure to note an antenna radiation pattern when buying so that you can know how to properly direct it.
Gain measures an antenna’s ability to direct or concentrate a radio signal being transmitted. Gain is measured in either dBi (which is in reference to a theoretical isotropic antenna) or dBd (which is in reference to a theoretical dipole radiator).
Knowing what level of gain you need really depends on whats around your transmitting location. The higher the antenna gain, the more concentrated the signal will be so if you live on top of a hill with nothing else around, you could do well with a high gain antenna. If you live in an urban area, you might want a lower gain antenna to be able to get around things.
Bandwidth is the range of frequencies that that antenna will be able to receive or broadcast on.
Finding an antenna that has the right bandwidth for the frequencies you want to broadcast on is easy. If you are wanting to talk on HF and UHF, it might be best to look into getting two separate antennas – most antennas are not broadband.
Types of Antennas
When choosing an antenna, you have to decide how tall you are going to try and make it. Keep in mind that the size of an antenna is related to what frequencies you are trying to send and/or receive. To find the antenna length in meters, you would need to take 300 (this number comes from the speed of light which is how fast radio waves travel) divided by the frequency you are going to use in MHz.
The most basic kind of antenna is a long straight rod (monopole) that sticks straight up into the air. The next most common kind of antenna is a dipole which is what most older tvs us. It looks like a giant “T”. Dipoles are incredibly directional and will pick up signal coming into them at a ninety degree angle.
You may have even seen an antenna that sticks straight up vertically and has several rods lined up horizontally as well. This is called a Yagi antenna. The different patterns of dipoles are concentrating the signal so then it becomes easier to detect. These extra rods are called directors and reflectors.
A loop antenna is a kind of antenna that is shaped like a circle and the circumference of the circle is close to one wavelength of the desired frequency. Loop antennas are popular in radio direction finding at longer wavelengths.
Antennas work by being sensitive to electromagnetic waves in the air. They can pick up the signal and produce a low level voltage that is fed to a tuned circuit. For transmitting, it’s the combination of current and voltage that creates an electromagnetic wave to send out.
The three most important things to think about when looking into antennas is directionality, gain and bandwidth. Remember to note whether your antenna gain is measured in dBi or dBd. Also figuring out how to properly read a radiation pattern and use the directionality of an antenna can significantly improve your HAM radio experience.
There are several types of antennas out there but the most common are monopole, dipole and yagi. The loop antenna is super neat and also worth checking out. There are so many different kinds of antennas and they are super easy to make yourself!