Engineering Portable Helical Antennae
Advisor: Farhan Ahmad
Unlike simple dipole antennae, like those on old TVs, which produce linearly polarized light, helical antennae produce circularly polarized light. This type of light is characterized by its capacity to resist degradation over long distances, which makes helical antennae very useful implements in space and deep-sea exploration. A common problem with these antennae, though, is that they can be far too large and bulky to fit onto satellites for quality space scans. This project aims to provide possible remedies to that by constructing an expandable helical antenna.
Project Blog
Post 1: Extendable Helical Antennae: Introduction
A human's most powerful tool is the means by which he or she can communicate with another human. As humans evolved, so did their methods of communication, and nowadays, electromagnetic (EM) waves are widely used for wireless communication for all sorts of devices including satellite communication and weather radar. The tools used to create these EM waves are called antennae, and there are two types of antennae that I will explore throughout this project.
The first is called a simple dipole antenna. It's the type of antenna typically found on old TVs and walkie talkies, resembling a thin, taut wire. The second type is called a helical antenna, and it resembles a spiral or spring. What's cool about helical antennae is that they produce circularly polarized light, as opposed to the linearly polarized light produced by simple dipole antennae. Since circularly polarized light is much less susceptible to degradation over long distances, helical antennae are very useful for satellite communication and deep space exploration.
However, helical antennae have the problem of being very bulky and difficult to fit on satellites, which are required to be light and aerodynamic when being sent to space. That brings us to the goal of this project: engineering an extendable helical antenna and identifying the benefits and drawbacks of using said antenna compared to a regular helical antenna.
I’ve always been very interested in radio, and one of my aspirations is to eventually end up talking on a radio broadcast at least once. Last year, I did research on using EM waves coming out of a monochromatic laser pointer to identify impurities in different solutions. Therefore, I had to do a fair amount of research on how EM waves propagate in both free space and different media, such as air and water. That knowledge became the basis of this project. This year, I am taking classes like multivariate calculus and Fourier series, which are used extensively in radio engineering and the signal analysis, respectively.
I hope to document my progress building antennae and analyzing their performance in this blog. See you soon!
Week 1: Extendable Helical Antennae: Set-up
Hello! If you are reading this, then my senior project idea has interested you! Therefore, to make it easier for you readers to keep track of what's happening, I present the following, rough timeline:
Constructing a simple dipole antenna and analyzing it's output
Constructing a helical antenna and analyzing it's output
Constructing an extendable helical antenna and analyzing it's output
Comparing results
Blog posts will be titled appropriately so it's easy to check the overall progress this project has completed.
Please note that this timeline is subject to change to account for any unforeseen circumstances that may occur during the engineering and analysis process. If any large-scale changes do occur, they will be noted within their respective blog post.
Lastly, I would like to address how the "output" of each antenna will be analyzed. As of now, I am planning on building a receiver for the antennae and using it to measure the distance that the radio waves created by the antenna will travel (gain, area). Since radio waves tend to travel long distances, there may be a drive involved. Additionally, if needed, mathematical simulations will be utilized to show how far specific radio waves from a specific antenna theoretically travel. Once again, the goal of this project is to build an extendable antenna and compare its output to that of a regular helical antenna in hopes of gauging how efficient an extendable antenna is.
See you soon!
Week 2: An Internship!
Exclaimer: This blog will be unrelated to my engineering project, so if you readers are only interested in that, I would recommend to go to the next post.
Hi! Before we officially begin with the extendable helical antenna project, I would like to talk about my internship! This spring, I'm working with the OI Foundation, a nonprofit which aims to raise money used to fund research studies concerning Osteogenesis Imperfecta (OI), a rare bone disease which causes collagen deficiency and lowers the bone density of those who have it.
This builds on the work I did last summer, when I interned at Children's Hospital and worked with the bone clinic to oversee a research study on the drug setrusumab. Setrusumab is a protein that works by inhibiting another protein called sclerostin which, itself, inhibits osteoblast activity. In this way, setrusumab indirectly causes bone growth, which is essential for people with severe OI.
This spring, I'm working with the OI Foundation to raise money for OI research, and my first meeting with them is this Monday! Expect next week to be back on extendable antennae! See you soon!
Week 4: Building the Simple Dipole Antenna
Hello! This week's blog will be about how to actually build a dipole antenna.
Well, there are actually two types of general antennae: a transmitting antenna and a receiving antenna. If you are making a transmitting antenna, you must build something capable of transducing an oscillating electrical signal (alternating current or AC) into an electromagnetic wave. A dipole antenna utilizes two radiating elements with each of the elements being one-quarter of the wavelength of the electromagnetic wave that antenna would create. Logically, one can assume that the total length of the dipole would be one-half times the wavelength of the antenna's transduced EM wave.
You can imagine the structure of this antenna as a T-Shape, where the left side of the top, horizontal line on the T is one of the elements and the right side is the other element. Elements can be any conducting metal, such as aluminum or copper, but both elements should be made of the same material because the vibration of that material in response to the AC signal is how EM waves are transduced.
It is important to note that there must be a small space in between the two radiating elements so that the signal source can be connected to both halves of the dipole separately. When an AC signal is supplied to this set-up, an electric field and orthogonal magnetic field is produced, and this is the EM wave transduced by the antenna which goes through space and gets intercepted by the receiving antenna. Luckily the same structure of antenna can also receive electromagnetic waves, turning them into an electrical signal which can be used for any type of communication.
See you soon!
Week 5: My Beautiful Vector Network Analyzer
This is going to be a very exciting blog post for me, because I have recently received my vector network analyzer! I've never mentioned it before, so first I'll provide a description of what it does and then I'll describe how I use it.
A vector network analyzer (VNA) is an instrument used to measure S-parameters in order to tune and characterize all sorts of antennas. There are different types of VNAs. The cheapest ones usually only have one port, which limits the types of measurements the VNA can make. The more expensive ones, however, can have 2-4 ports, and for this project, specifically, a 2-port VNA will be used to get the physical measurements needed to compare the dipole, helical, and extendable helical antennas. Why a 2-port VNA, though? Well, 2-port VNAs are able to measure both impedance, which is effectively the electric resistance at an antenna's feed point, but more importantly, they're able to measure antenna gain, which speaks to the antenna's directionality. Only VNAs with 2 ports or higher are able measure gain because they are able to use the S21 parameter, which is required to do so. Usually, high quality VNAs cost thousands of dollars, but I got a steal on this one for about $120:
Impedance of each antenna can be easily measured and will be used to tune the antenna to its proper feed point resistance, allowing for the most effective EM wave transmission. Gain is a measure of an antenna's ability to focus its EM waves in a certain direction relative to some base antenna, and to measure gain, the radiation patterns of each antenna must be analyzed. This is what S21 will be used for. A reference antenna will be connected to one port with the testing antenna connected to the other port. Then, a rotating table will be used in tandem with the VNA setup to measure S21 vs angle, which will show the radiation pattern of the testing antenna. This will lead us to the antenna's gain.
It's also important to note that coaxial cables should be used as they force the magnetic fields outside of them to be zero due to their construction. This will ensure cleaner results.
Week 6: Considerations when Constructing Antennas
Hello!
I am building the dipole and helical antennas right now and am using our newly acquired VNA to measure impedance, which I mentioned in the last blog. Impedance is denoted as a complex number with the form Z = R + jX, where Z is the ratio of voltage to current at the antenna's terminals, with R as its real component, referred to as its resistance, and X as its imaginary component, which is also called reactance. Most transmission lines (coaxial cables) have a resistance of 50 ohms, so if the antenna's impedance is compatible with that, the antenna is resonant, working at maximum efficiency.
If there are errors in construction, such issues with the coax cables, antenna length, or grounding, a simple test can be performed to determine whether the frequency of the waves created by the antenna are too low or too high. One can use a VNA to measure the reactance, and assuming the resistance of the antenna is 50 ohms, if it is positive, the antenna acts like an inductor and resistor in series, showing that the frequency is too high. If it is negative, the antenna acts like a capacitor and resistor in series, showing that the frequency is too high. If the reactance is equal to zero, the antenna will act like a simple resistor, and it is resonant.
If the resistance of the antenna is not 50 ohms, however, a smith chart must be used to measure the ratio of the antenna's impedance to the transmission line's resistance. If the ratio is equal to one, the antenna is resonant.
I am currently using a smith chart and the VNA to tune the antennas as they are built. See you next time!
Week 7: Dipole and Helical Antenna Prototypes
Hello!
It takes a lot of research to begin building antennas, but as of about a week and a half ago, I began construction. In this blog, I will share my design parameters and show pictures of the first antennas I built.
Simple Dipole Antenna
In one of the last few blogs, I talked about the design of a dipole antenna and how its physical length is equal to half the wavelength of the EM wave it transduces. In the realm of antenna building, however, wavelength is almost never used to describe EM waves or anything related to antennas. Instead, frequency is used. Frequency (measured in hertz) is equal to the speed of light c divided by wavelength (in meters), and its units are in inverse seconds. So if a wave has a frequency of 5 hertz, it makes a full oscillation 5 times every second. The equation relating the length of a dipole antenna to the frequency of the wave it transduces is L=142.5/f(MHz), where L is length of the antenna in meters and f is the frequency of the transduced wave (in megahertz). In the last blog, I also mentioned the importance of impedance matching between the antenna and coaxial cable. To do this, a device called a "balun," which performs the required impedance matching, is used. Therefore, I bought a balun rated for up to 50MHz, which led me to construct a simple dipole antenna that operates at 50MHz. It is 2.85 meters in length, where each dipole is 1.425 meters long. A photo is shown below:
This prototype is made of 16 gauge copper wire, where about 3 meters of wire was cut, 2.85 meters was used as the radiating element, and the remaining 15 centimeters was used to fold around the balun, as shown. I then connected two coaxial adapters to the bottom of the balun in series, leading to the blue coaxial cable which was plugged into my VNA for antenna analysis. This blog will be too long if I go over all the antenna analysis that was completed, though, so I will leave you with this cool and mysterious photo:
(HINT: "SWR" is a ratio, and I have mentioned it with another name in one of my last blog posts.)
Helical Antenna
This type of antenna is much, much harder to make than the dipole, but I'm currently working on MK. 2 and it looks much better than the one I'm about to show. In the literature, it is stated that the circumference of the antenna should be about equal to the wavelength of the transduced EM wave. Furthermore, the antenna should have a pitch angle of 12-14 degrees and have more than at least three turns. The first prototype of helical antenna I created has a circumference of 0.273m, a pitch angle of 12 degrees, spacing of 0.058m, and seven turns. This antenna prototype is also made of 16 gauge copper wire. A photo of the antenna when I first made it is shown below:
This is only the radiating element of the antenna. To connect it to the VNA, it must be connected to a coaxial cable, but the balun I used for the dipole antenna is rated for only up to 50MHz, while the helical antenna is rated for 1GHz, which would result in a disaster for my precious equipment if they were connected. Instead, I will utilize a metal strip as an impedance transformer. The impedance of the antenna will be around 140ohms while the coaxial cable's is 50ohms; therefore, a metal strip about one fourth the wavelength of the transduced EM wave will be able to match the impedance of the antenna to the cable. There should also be a conducting surface underneath the antenna.
Next Blog
Expect a description of measurements with the VNA and/or the extendable helical antenna. New prototypes of the dipole and helical antenna are also being created from thicker wire. Below are all the materials I used for this project:
Week 8: Measurements and Further Construction
Last time, I showed a photograph of my NanoVNA measuring something called SWR. This blog will go over what that means, how I will use it, and how I will measure radiation pattern.
SWR is short for standard wave ratio, and it is calculated by finding the ratio of the impedances of the antenna and the transmission line. Conceptually, however, SWR informs an experimenter how much the transmitting wave is sent back as a reflection. If the SWR is equal to one, all of the energy used to transmit a wave is put into said wave, but if it is greater than one, there is some amount of reflection. A SWR of 1-1.5 is the golden interval for an antenna builder; anything more than 3 is considered unacceptable as most of the wave is reflected back. I will use this measure to show that the antennas being built are acceptable to put under test for radiation pattern.
An important consideration is what happens when you touch the antenna. The human body introduces a huge capacitance to the antenna where a large proportion of the current will travel to. To the NanoVNA measuring the SWR, the impedance of the antenna spikes, and therefore, the SWR spikes too.
The other important consideration is the radiation pattern, which is essential to comparing the helical and extendable helical antenna. It is measured by plotting S21 vs angle on a polar graphical plane. S21 represents the gain of an antenna in decibels, and if it is plotted as an antenna is rotated 360 degrees, a radiation pattern is created, which shows the directivity of the antenna. This, and SWR, is what will be mainly used to compare the helical and extendable helical antenna.
Week 9: The End of Part One - Simulations
Hello!
I've been busy since the last blog was posted and have completed my senior project! However, there are still two blogs left after this, so in this blog I will talk about my SWR and radiation pattern simulations, and in the next two, I will talk about my measured results and engineering problems, respectively.
There is a software called MMANA-GALBasic that's used to model radiation patterns, peak gains, and SWR based on the shape of antenna inputed into said software. Specifically, you start off with an empty XYZ coordinate plane and must use linear 'wires' to model any antenna you desire to create. Below is a screenshot of the input system:
For my helical antenna made of 8-gauge copper wire, I used 100 wires of the same length, but starting and stopping at different coordinates on the 3D plane, to model my helical antenna (7 turns, 12 degree pitch angle, 0.087m diameter). Then, I include a voltage source and a small inductive load at the beginning of my first wire which represent my NanoVNA. The resulting structure is what you see below:
After creating the structure in the software, I run calculations on it to reveal the simulated radiation pattern and SWR of the antenna. My simulations for this 8-gauge helical antenna are below:
Pay attention to the "Ga = 10.4 dBi" and "Gmax - Ga = 0.0 dBi" notations on the top center of the first screenshot. These are the gain numbers. Since Gmax - Ga is equal to zero, the top point (where the red dot is located) is the peak gain of the antenna, and at that point, said peak gain is 10.4 dBi. This software also says that the SWR of the antenna is equal to 2.8, but in real testing this number was lower, which will be explained later.
These were the simulations for only the 8-Gauge helical antenna, but simulations were also done on the 12-gauge helical antenna as well as the 8 and 12-gauge dipole antennas. They are all below, starting with the 12-gauge helical antenna:
8-Gauge Dipole Antenna:
12-Gauge Dipole Antenna:
See you soon!
Week 10: The End Part Two - Measured Results
Hello!
This is part two of three in the finale of the extendable helical antenna blog series. Today, I will go over the measured results of SWR and S21 vs angle via NanoVNA.
Measured results of the dipole antennas are below:
10 different measurements of the same two antennas were taken, and their averages were plotted on the bar graph above. The 8-gauge dipole had a statistically significant lower average SWR value than the 12-gauge at the 90% confidence level (p=0.0806); however, this was due to three high outliers in the 12-gauge measurements. The majority of other measurements were around 1.3.
Measured results of the helical antennas are below:
10 different measurements of the same two antennas were taken, and their averages were plotted on the bar graph above. The 8-gauge dipole had a statistically significant lower average SWR value than the 12-gauge at the 90% confidence level (p=0.0996); however, this was due to two very high outliers in the 12-gauge measurements. The majority of other measurements were around 1.4.
Helical antennas have higher directivity than dipole antennas, so their peak gains are far greater. However, similar to the dipole antennas, the thinner the wire, the greater the SWR due to high fluctuations. This measured data is confirmed by a S21 vs angle NanoVNA measurement, where the 12-gauge helical antenna measured 7.4 dBi gain at its peak and the 8-gauge helical antenna measured 8.7 dBi gain at its peak.