Zack Higgs
Moves 1: How in the World does an Antenna work?
In the previous move I talked about how radio waves are created, but there are many ways of transmitting and receiving a signal. Before researching and exploring how antennas work, I assumed that it was as simple as a piece of wire. I had no clue how they worked. There is a variety of antennas that are used for many different applications, this includes monopole, dipole, circular polarized, helical, scueplaner, and patch antennas. All of these antennas are used for different applications, but all use the same principles of electromagnetic radiation.
Monopole antennas are deceivingly complex, but one of the simplest antennas that are commonly used. The cord used is called coaxial cable, which is also used in receivers for all TVs. It is a special type of cable that has two wires in it, one in the center that does the receiving of the signal. The other is a meshed shielding that absorbs all radiation, the mesh of wire surrounds the inner cable in a braid adhered to some plastic around the signal wire. Without the shielding, the signal you want would be lost in the mess of all the other radiation. At any given place, there is radiation of a variety of frequency from wifi, cell phone towers, microwaves, and a general background radiation. From the picture above you can see a small piece of wire extruding from the end (this is not the gold end but the end with the black heat shrink), that is the inner wire which is exposed to radiation. This does not have any RF (radio frequency) shielding so that it can absorb the desired signal. This is sort of a way of filtering out unwanted frequencies and allowing a single frequency to be received. In move one, I talked about the wavelength of a frequency, which is the distance from one peak to the next. Now in the radio wave range, signals can be from about 100 Khz to 8 Ghz. As the frequency increases the wavelength becomes smaller, so they are inversely proportional. Now a specific frequency should also have a specific wavelength. For example, if you have a wireless router, the transmitter usually broadcasts at 2.4Ghz. This would mean its wavelength is 1.29m.
One other antenna that has been used for long range application is the helical antenna. As seen above, a helical antenna is made up of a large helical wire and a base plate. This antenna is tuned to its frequency, and it is circularly polarized versus the monopole is linearly polarized. Now instead of having a perfect sphere of radiation, a helical antenna will have very high gain. As an antenna's gain increases its range in a given direction increases. But there is a drawback to using a high gain antenna, and that is as the gain increases the angle of radiation decreases. This would mean that as you moved side to side, you would quickly move out of the range of the signal. The gain on this antenna is about 10 decibels, which is average for a helical antennas, and this is a fairly wide beam of radiation compared to some others. The graph above shows degrees on a unit circle with a total of 360º, then as you move out from the origin it shows the gain in db (decibels). This could also be called the signal strength, a good example of this is a wifi’s signal strength. The strength of a normal router is about -50db and as you move closer to the transmitter the strength will increase. This is shown on the x and y-axis of the graph by the -20db and -10db and finally, 10db. When looking at the graph, you can also see that behind the antenna there is also a radiation pattern. This is caused by waves being propagated between the helix and the ground plate, and doing the reverse that the front is doing.
Moving on to the circular polarized antenna. This is similar to the helical antenna. When looking at the picture below the antenna has a shape like a clover leaf, and to me looks awesome! This, unlike a helical antenna, this has almost a gain of 0. Like monopole antenna, this has a circular radiation pattern and is meant for local radiation (1-2 miles). But unlike the monopole antenna its radiation pattern is toroidal (like a donut), so it covers much more in the z plane. This antenna looks to be on the 2.4 Ghz band, just based on its size. A circular polarized antenna should have the following criteria, “The E-field must have two orthogonal (perpendicular) components, The E-field's orthogonal components must have equal magnitude, The orthogonal components must be 90 degrees out of phase”. When looking at the picture there are two orthogonal components, they are equal in magnitude (length). When they say 90º out of phase it refers to the sine wave that is propagated off of the wire, when they are 90º out of phase the sign wave will be shifted forward so that each peak of the wave is offset. But the reason why it is called a circularly polarized antenna is that when the radiation is propagated from the wire it corkscrews by spinning off of the curved wire. The wires can be either spun to the left or right, this is called left or right-hand polarization so that the wave twists right or left off of the antenna. The benefit to having a corkscrewing wave is that often when transmitting a signal, it reflects off various objects and then two or more signals are received resulting in double images. When a twisting wave is reflected off of something it switches from left to right-hand polarization, just like when you look in the mirror you are inverted, the same thing happens with this wave. Because the receiving antenna has the same polarization as the transmitting antenna, the inverted wave will not be able to be received. This makes the signal of higher clarity and reduces the nose, just because only the signal that is wanted can be received.