Tuning a Monopole Antenna Using a Network Analyzer

Transcription

1 11/21/11 Tuning a Monopole Antenna Using a Network Analyzer Chris Leonard Executive Summary: When designing a monopole antenna it is important to know at which frequency the antenna will be operating at. The operation frequency is what you will be tuning your monopole to. In order to tune an antenna you will need a network analyzer, an anechoic chamber, an SMA cable, and your monopole antenna. This application note will go over how to calibrate your network analyzer in order to properly use it, the significance of an anechoic chamber, and how to properly tune your monopole. Key Words: Network Analyzer, Monopole, Anechoic Chamber

2 Table of Context: Introduction...3 Objective..4 Getting Started. 4 Anechoic Chamber....5 Designing and Tuning.. 6 Application References 8 Page 2

3 Introduction: A monopole antenna is an antenna that is commonly used for its directive gain and its radiation pattern which covers a large area. The monopole is generally made out of a wire or a rod-shaped conductor. The monopole is mounted perpendicular to a ground plane for maximum radiation and directive gain. A monopole is similar to a dipole except that the other half of the wire or rod is replaced with the ground plane which in turn increases the directive gain of the antenna by 3dB. There are many applications for a monopole antenna such as an antenna to broadcast for radio stations. The donut shape radiation pattern allows for broadcasting in all directions and they are able to utilize the earth itself as an infinitely large ground plane. Another application for the antenna is communication in the GHz range with cell phones. Many cars and airplanes have a monopole as an antenna for their wireless communication where it can use the body of the automobile as the ground plane. The radiation pattern of a monopole antenna can be seen below. Page 3

4 Objective: The objective of this document is to walk you through how to tune a monopole antenna. In order to begin the project you must first pick a frequency to design around because the dimensions of the monopole are directly related to the frequency of operation. The user must have access to a network analyzer and it would be very helpful to have access to an anechoic chamber for more precise measurements. Getting Started: First you will need access to a vector network analyzer (VNA). Using a network analyzer allows you to measure things over a sweep of frequencies and is able to plot your gain over the sweep. Before the network analyzer can be used, it must be calibrated. The calibration is a very important step in the process of using the VNA and must be done every time you use it. In order to calibrate the network analyzer you will need a very precise calibration kit with an open, short, and a 50Ω terminated load. These three together will be individually attached to the S-parameter cables on the VNA during the calibration process because they are a known load and will be used as a reference. Here in our lab we use the Agilent 85032B Type N calibration kit. These connectors are highly precise and are always used with special care. If you are using an Agilent or a similar device, the calibration process will be the same. Follow the following steps to calibrate your system before you can begin taking measurements on your antenna. 1. Power the Agilent network analyzer on. 2. Press the Preset button to return the VNA to its base state. 3. To select the frequencies that you would like to sweep through press SWEEP > LIN FREQ > START and then enter the frequency that you would like to begin the sweep at and then indicate the units of the frequency by selecting it on the keypad. Next press END and then enter the ending frequency of the sweep and again select the units using the keypad. 4. Select SWEEP and enter the number of points to plot between your linear frequency sweep. Page 4

5 5. Select CAL > N 50Ω for your calibration kit selection. 6. Press CAL > CALIBRATE MENU > S11 > 1_PORT to select that you will be calibrating the S11 port and only using that one port to test. 7. From the calibration kit, attach the Open to the end of the S11 port cable that you will be using. 8. Press OPENS > OPEN [M] this completes the open test part of the calibration; The M indicated the connector gender. 9. Repeat Step 8 for the Short and the impedance matched Load. 10. When these steps are complete, the network analyzer will be calibrated to the end of the test cable and is now ready for you to test your antenna with precision. Anechoic Chamber Now that you have your network analyzer calibrated you can begin testing your antenna. When testing and tuning an antenna it is very helpful to use an anechoic chamber. Using an anechoic chamber allows you to reduce the amount of noise and RF interferences there are in the testing environment. The chamber itself is designed to eliminate RF noise by the shape of the walls and the material that it is made of. An anechoic chamber used for antenna tuning looks much like that of an acoustic chamber with the small difference of the material that it is made out of. Anechoic chambers are most commonly used for radiation pattern measurements and electromagnetic compatibility but will be very useful when tuning an antenna. Page 5

6 Designing and Tuning When designing your monopole, the length of your antenna must be close to onequarter of the wavelength for resonant frequency. This is why you much know at which frequency you will be operating at. When testing your antenna, it is good to test it over a frequency sweep around your resonant frequency because loading effects and electromagnetic interferences will shift the resonant frequency of the antenna. A shift in resonant frequency just means that your antennas maximum gain is at a frequency higher or lower than what you have designed around. This can be fixed by extending or shortening the length of the monopole. The easiest way to test your monopole antenna is to start with a longer length than you have calculated with the one-quarter wavelength and then run multiple tests and shortening the antenna each time. This helps to narrow down your search for the resonant frequency and shift it yourself to where you would like it without having to start over each time because the antenna is too short. Below is a simple rod-monopole designed at one-quarter wavelength sitting perpendicular to a large ground plane. Below is the radiation pattern that you would see from this monopole above. Page 6

7 Magnitude (db) Application As a design team, our project at hand includes a transmitting and receiving monopole antenna. The antenna we made uses a small piece of wire for the monopole and a simple household coffee can for a waveguide. Adding a waveguide to the equation requires more calculations in order to get the correct length of the monopole, however the concept of tuning it is the same. The waveguide has some electromagnetic advantages for giving us more of a directive gain out of the system, but it adds some interferences such as loading the antenna. The main idea in designing our antenna was still to have a monopole about one-quarter of a wavelength at the frequency that we were designing around. The frequency that we are operating around is 2.4GHz so after we had calibrated our network analyzer we did a frequency sweep between 2.26GHz and 2.59GHz. We tuned our transmitting and receiving antennas individually so that we did not get interference between the two while we tuned them. We ran a frequency sweep many times while we tuned the monopole antennas so that we could take very small bits of the antenna off at a time. This allowed us to get a very precise antenna without overshooting our resonant frequency. Below is a graph of our data that we had collected from our network analyzer while tuning our two antennas. As you can see, we have our resonant frequency just above 2.4GHz. Data Collected from Antenna Tuning Frequency (khz) Page 7

8 Conclusion Monopole antennas are a simple and easy antenna to design and tune. The generally need to be perpendicular to a ground plane. When designing the monopole you must know at which frequency you are going to be operating at because the dimensions of the antenna are frequency dependent. It is very helpful to have an anechoic chamber on hand because it helps to eliminate noise and some interference that could affect the resonant frequency of your antenna. If you happen to be using a waveguide in your antenna as we did you will run into some extra calculations as well as some added electromagnetic interferences. These interferences are easy to design around if you just make the monopole a bit longer than what you have calculated and trim to size while monitoring the resonant frequency you are designing for. References Fenn, Alan J. "Antenna Design for the Laptop Radar Project." MIT Open Courseware. 14 Jan Sep < Bevelacqua, Joseph. "The Monopole Antenna." Antenna-Theory. 17 Nov < Page 8