Basics of Antennas
Table of Contents
The purpose of an antenna is to radiate the signal generated by a modem through space. It does this by creating a varying electric/magnetic field. The shape, size, and materials of an antenna and its surroundings all change the behavior of this propagation.
There are many ways to characterize an antenna's radiative properties. These include the gain pattern, the Voltage Standing Wave Ratio (VSWR), the antenna parameters (S11, S12, S21, S22, etc.).
Gain Pattern
The gain of an antenna is measured in directivity, with units of dBi. As we know, decibels are always some sort of ratio. In this case, it is the ratio of the ability of a given antenna to focus energy in a specific direction compared to a theoretical isotropic radiator.
Imagine you have a bare lightbulb, with nothing covering it. It will shine light equally in all directions. This is analogous to an isotropic radiator. Now imagine that you put that lightbulb inside a flashlight. Now, all of a sudden, all of the energy from the lightbulb is redirected in one direction, and can illuminate a smaller area much brighter. This is exactly what antenna gain is. You're not making the bulb any brighter, but you're focusing the energy that is available to you into a smaller spot.
In this analogy, the geometric properties of the flashlight beam would be the gain pattern. Things like how wide the cone of light is and the angle at which the brightness decreases to a certain amount are the benchmarks that are typically used for antennas. The most common one is the Half Power Beamwidth or HPBW.
The HPBW is the angle at which the magnitude of the signal being transmitted reaches -3dB from its max. {INSERT PICTURE EXAMPLE HERE}
Propagation
Antennas radiate by creating electric fields. The best way to do this is to be able to group charge into one side at a time, and have it move cleanly from one end of the antenna to the other. For example, in a $\frac{\lambda}{2}$ dipole antenna, the goal is to generate a voltage standing wave such that at any given moment, the voltage at one end of the dipole is exactly the opposite of the voltage at the other end.
Lets examine this a little closer. Take the case where the right of the antenna is negatively charged, and the left end is positively charged. In this scenario, the electric field lines will create paths traveling from the left end to the right end. Next, if we let the wave oscillate until the charge has switched position, the electric field will point in the exact opposite direction, going from the right to the left. We've just created an oscillating field! Because changing electric fields and changing magnetic fields sustain one another, part of that near-field energy propagates away from the antenna as an electromagnetic wave.
The higher the ability for the antenna to create these standing waves, the better it will be at radiating energy. This perfect standing wave described above would only happen when the antenna is in resonance-- in other words, when the wavelength of the signal exciting the antenna is exactly equal to two times the length of the dipole. If this is not the case, it doesn't mean the antenna can't radiate. It simply means that the resonance will not be as strong, and some of the energy will be lost due to reflections.
VSWR is a measure of how well this resonance is able to take place for a given frequency and antenna. {MORE DETAILS COMING SOON}
Polarity
Polarity is simply a description of the axis in which the electric field is oscillating. The $\frac{\lambda}{2}$ dipole I mentioned earlier is an example of linear polarization. This means that the field is oscillating up and down or side to side. Other types of polarization include dual polarization (both side-to-side and up-down at the same time) and circular polarization (the wave oscillates in a spiral).
For a physical example of what this means, imagine you have a rope, and you are pulling the rope up and down and making a wave. If you simply move it up and down, it will oscillate in that vertical plane, and someone on the other end could measure the vertical position of the rope at a given time to measure the "signal" you are sending. If you moved the rope instead so that it is oscillating side to side, a person on the end would need to measure the horizontal position of the rope to see the signal. If they tried to measure your signal by measuring the up-down motion while you are moving the rope side to side, they will not see anything. This is the same exact way that RF polarization works.
An antenna's polarization typically stems from a combination of geometry and how the antenna is excited. Examples of linear polarization include dipoles, monopoles, and certain Yagi antennas (to name a few). Dual-polarized antennas include turnstile (or crossed dipole) antennas, and circularly polarized antennas include helixes and some horn antennas. Patch antennas can be made to be linear, circular, or cross-pol which is why they are a common choice.
Impedance Matching
Coming soon
Antenna Parameters
Related to VSWR and impedance matching, antenna parameters can give you additional information that can help you design and tune an antenna for optimized radiation.
- $S_{11}$
This parameter describes the
Common Antenna Types
Dipole
The most common type of antenna is the dipole.