Education on antennas
Antennas are devices used to launch radio waves. When we say launch, what we mean is that electrical energy is radiated as electromagnetic waves.
- Electromagnetic waves can move and create their own medium (field) to travel through.
- Once created, they do not need any external power source to travel through space.
Animation of a half-wave dipole antenna transmitting
radio waves, showing the electric field lines. The dipole, antenna in the center, consists of two vertical metal rods with an alternating current at its resonant frequency applied at its center from a radio transmitter (not shown) The voltage alternately charges the two ends of the antenna positive (+) and negative (−). Standing waves of current flow up and down the rods. Standing waves can reflect power back into the transmitter which is not a good idea, especially if the transmitter final amplifier is all transistors.
Radio waves are created using an oscillator and usually fed into an amplifier. The amplifier is connected to the antenna via some sort of feed line. These oscillations are then connected to an antenna. See below.
Antennas can be used to transmit or receive radio waves. When the electromagnetic radio waves strike the receiver antenna, it creates a current that can be detected by the receiver. This current can then be rectified to remove the audio information added to the radio waves with a modulator. This rectified or detected modulation can then be fed into an audio amplifier and speaker.
Antennas have a resonant frequency as well. The one thing that determines the resonant frequency is the length.
If a load is connected across the feed Line, current will flow in the load. If the radio wave striking the antenna is not at the resonant frequency of the antenna, current will flow, but at a much reduced level.
Before we get started, you need to understand some terms. These are full, half and quarter wave. When talking about antennas and waves, we are talking about the length of the antenna element as compared to the length of radio wave (wave Length) that will be launched from it.
Radio waves lengths are measured in meters. A 10 meter wave would have a length of 98.43 feet. Most HTs (handy talkies) use a 1/4 wave antenna which are not very efficient.
Cell Tower Antenna
The cell antenna shown is a vertically stacked dipoles which are spaced roughly a wavelength apart, and about a quarter wavelength in front of a metallic back-plane.
Stacking the dipoles adds gain to the system. See below.
Other Types of antennas
There are a huge number of antenna types. One thing that is common in all these types is that they all have a driven element. The driven element is what the feed line (for connecting RF energy) is attached to.
Collecting radio Waves (focusing) or how to add gain to your antenna.
If you just put up a ¼ wave length antenna in the air, you would have to depend on the transmitter to put a large signal into the antenna. If you were listening and sending signals to a satellite orbiting Mars or some distance planet, you will need a high gain antenna to receive the very weak radio waves. A large reflector as shown in the graphic would do that. The reflector essentially focuses the radio waves that it collects. The focus point is at the small low gain antenna at its focal point.
You could write many books on adding gain to an antenna system. Because radio waves go all over the place, a considerable amount of the power is wasted radiation. If you want to focus this energy in a controlled direction, you will need to add elements to the antenna. The graphic below shows a 5 element yagi antenna. A typical Yagi array consists of a Driven element, a Reflector and one or more Directors. The number of directors can be increased to achieve a narrower beam and higher relative gain. The reflector does just what it is named, it reflects radio waves.
The driven element
The driven element is attached to the feed line. The configuration will determine the impedance of the antenna. The folded dipole has a 300 ohm impedance while the vertical antenna is 50 ohms.
One of the problems with antennas is that they have a very small bandwidth. Once the input frequency is moved off the center frequency of the antenna, the SWR increases, and the gain decreases. A solution to this is the following types of antennas.
A log-periodic antenna (LP), also known as a log-periodic array or log-periodic aerial, is a multi-element, directional, antenna designed to operate over a wide band of frequencies. The most common form of log-periodic antenna is the log-periodic dipole array or LPDA, The LPDA consists of a number of half-wave dipole driven elements of gradually increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feed line with alternating phase. Electrically, it simulates a series of two or three-element Yagi antennas connected together, each set tuned to a different frequency. Above from Wikimedia Commons.
There are many types of feed lines with multiple impedance ranges.
The most common are the coaxial types and the twin lead as shown below.
I am sure that is enough. If you are looking for some radio equipment, read my blog
Please return often as I have more on radio communications like setting up a home station and powering it from solar or battery power.
Thanks and Cheers
Contact me below. Ill get back ASAP.
I eat an all plant diet and have for about a year. I am now 77 and have as much energy as I did when I was much younger. This book (PDF) is free. To get this Book go HERE.