FREQUENCY —

If you were at the beach and two waves crashed within one second, the

waves would have a frequency of 2. Frequency has the units of Hertz aka

Hz. An FPV transmitter might have a frequency of 900MHz, or 900

mega-Hertz, or 900,000,000 Hz, or 900,000,000 cycles per second, which

would all mean the same thing. For this post we will use Gigga-Hertz

aka GHz. So 900MHz = 0.9GHz. Lets take a look at at what different

frequencies look like plotted. I have included the python script so you

can follow along.

import numpy as np # numpy is numerical python
import matplotlib.pylab as plt #matplotlib is a plotting package
deg = np.linspace(-360.0,5*360.0,1000)
wave_1 = np.sin(np.radians(deg)/2) + 2
wave_2 = np.sin(np.radians(deg))
wave_3 = np.sin(np.radians(deg)*2) -2
plt.plot(deg, wave_1)
plt.plot(deg, wave_2)
plt.plot(deg, wave_3)
plt.axis('off')
plt.show()

In

the plot above you see three sin waves representing three different

frequencies. The lowest frequency is in blue and the highest frequency

is in red. Look at the distances between the tops of the waves, this is

the waves wavelength which brings us to our next subject.

WAVELENGTH
— This is just the distance between two crests of waves. For

electromagnetic waves you just divide the speed of light by the

frequency: wavelength = (speed of light)/frequency. We will be using

inches for our unit of wavelength. A wave with frequency of 1 GHz has a

wavelength of 11.8 inches, almost a foot. So lets calculate the

wavelength of our 900MHz FPV antenna. wavelength = 11.8/0.9GHz = 13.1

inches. Just divide 11.8 by your frequency in GHz to find your

wavelength in inches.

import numpy as np
import matplotlib.pylab as plt
deg = np.linspace(0.0,2*360.0,1000)
wave = np.sin(np.radians(deg))
plt.plot(deg, wave)
# Add annotation
plt.annotate(
'', xy=(90, 1), xycoords = 'data',
xytext = (90+360,1), textcoords = 'data',
arrowprops = {'arrowstyle':'<->'})
plt.annotate(
'WAVELENGTH', xy=((360+90)/2, 1), xycoords = 'data',
xytext = (5, 0), textcoords = 'offset points')
plt.axis('off')
plt.show()<br>

POLARIZATION
— Electromagnetic waves (this is what is radiated from the antenna)

have an orientation. There are two common polarizations, horizontal and

vertical. Horizontal means that the electrical field is to your left

and right. Vertical polarization means that the electrical field is up

and down. On your transmitter your dipole antenna is usually pointed

up, this means that you are radiating vertically polarized

electromagnetic waves. It is very important that the antenna on your

ground station is at the same polarization as the antenna on your

quadcopter. Proper placement is also important, with poor placement of

your antenna you can have the waves bouncing off of motors, propellers,

and batteries which change polarization.

Wow, that was starting to get boring. Lets move on to

something more interesting. Lets familiarize ourselves with different

types of common antennas.

The

image above shows the radiation pattern of a 15 element Yagi antenna.

As you can see the antenna is very directional in both elevation (up

& down) and in azimuth (left & right). So if the antenna was

pointing directly at the ground station we are in luck, imagine the

range this baby will get you! But, wait, what if you need to turn

around and come home? Lets look at the radiation pattern another way,

gain vs. azimuth:

Now

we are only getting -6.85dB gain in the side sector, what a bummer.

Well, this is a good lesson on antennas. With antenna gain you cannot

get more gain without loosing it somewhere else. The Yagi antenna might

be better suited to a ground station where you have someone or

something tracking your quadcopter very closely. Lets look at one more

directional antenna.

~Dr. Dankeinspank