Author Topic: Quadcopter Antenna Design and Optimization: Part 1  (Read 7932 times)

Dr_Dankeinspank

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Quadcopter Antenna Design and Optimization: Part 1
« on: February 04, 2014, 10:06:11 PM »
Check out the original article http://cali-bros.com/?p=839.
Antenna_Part1 ANTENNAS:
Part 1
Overview: In this post we will look at the a) type of antenna and b) location of an FPV antenna for a typical quadcopter. We will use an open-source software (NEC2) to determine the performance of many configurations for comparison. We will also do some crude tests to validate our conclusions. The Python programming language is used to rapidly iterate through configurations. Everything is provided here so that you can follow along.
Introduction: First lets learn a little vocabulary that is common in antenna design/analysis:
GAIN -- This is what antennas are all about. Gain is a number that describes how the antenna radiates energy, electromagnetic energy. The gain can vary significantly (orders of magnitude), that is why we typically use units of decibels aka dB. Here is an interesting fact about dB, 3dB is half, therefore an antenna that has 6dB radiates twice the energy as that of a 3dB antenna (but only in one direction). A plot of an antenna's gain is called its radiation pattern. There are a few common plots, gain vs. azimuth, gain vs. elevation, and gain vs. azimuth & elevation. We will take a look at each of these to determine the best antenna configuration for our quadcopter.basic_patterns
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.




 different_frequencies
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.


 wavelength
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.
  • MONOPOLE -- Mono = One, Pole. This is a simple antenna with one element extruding out of a ground plane. The element is isolated from the ground plane by an insolator. For a quadcopter we don't really have a ground plane unless your chassis is conductive, which could be dangerous so close to the high voltage system. We will ignore monopole antennas for this study.
  • DIPOLE -- Di = Two, Poles. This is another simple antenna with two elements. A typical dipole is the 1/2 wave dipole, which means that each element is 1/4 wavelength. Lets equate this to our 900MHz FPV antenna and determine the overall length of the antenna. We already determined that 900MHz has a wavelength of 13.1 inches, so divide that by half and we get 13.1/2 = 6.56 inches. Take a look online at 900MHz dipole antennas, right around the same length, cool. A 1/2 wave dipole antenna is what you will typically find stock on your transmitter and receiver. Why? Well that is because of the radiation pattern. A dipole pattern looks like a doughnut, radiating everywhere but at the poles.
half-wave_dipole_1
The image above shows the radiation pattern (gain vs azimuth & elevation) for a half-wave dipole. Where the maximum gain is at 0.0 degrees elevation and nulls at plus/minus 90.0 degrees elevation. This is typical of the pattern from your transmitter and receiver if using a dipole. Now you have to sit back and think about all the times you have been out flying and reflect on the orientation of your airplane/quadcopter relative to you. A dipole should look like a good choice of antenna for FPV. No matter what direction the quadcopter is flying we have about 2dB gain! 2dB doesn't sound like much, but it can get much worse. Lets look at some directional antennas.
  • YAGI -- These look like a bunch of dipoles in a row, but only one of the dipoles is actually being fed a signal. The rest are there to react to the electromagnetic waves. These interactions of the elements are very complicated, which make them very difficult to design and optimize. I found one on the internet to copy which is designed for 16dB (pg. 15).
 yagi_15el
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:
 yagi_15el_2
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.
  • BIQUAD -- aka "bow-tie" antenna. This is becoming a very popular antenna for ground stations and for good reason. The radiation pattern has a nice wide lobe and they are simple to build. Check out IBCrazy's post on how to build your own here. Lets take a look at the pattern for the 910MHz biquad described on IBCrazy's post.
biquad_gain_1
  • You can see that there is a wide main lobe. So this is another high gain antenna which is more appropriate for our ground station rather than on our quadcopter. For this study we are working to optimize the FVP transmitter antenna on our quadcopter, so a biquad probably isn't the right choice.
Part 1 Wrap Up:
  • You cannot have high gain at all azimuths and elevations
  • A high gain (directional) antenna is best suited for a ground station if you want long range in one direction
  • A dipole antenna is a good choice for a quadcopter FPV transmitter antenna because it radiates in all directions
  • Next post we will look at optimizing the size and location of the dipole antenna on our quadcopter
~Dr. Dankeinspank
« Last Edit: February 04, 2014, 10:46:29 PM by Dr_Dankeinspank »
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