Introduction: What is an antenna?
An antenna is a device which is used to transmit and/or receive electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned (matched) to the same frequency band as the radio system to which it is connected, otherwise reception and/or transmission will be impaired.
There are several reasons as to why we need or why we use antennas, but an important reason as to why we use antennas is that they provide a simple way to transfer signals (or data) where other methods are impossible. For example, take the case of an airplane. The pilot needs to frequently communicate with the ATC personnel. It would not make any sense if we tie up a cable (of dynamically variable length) to the tail of the plane and connect it to the ATC. Wireless communication is the only feasible option and antennas are the gateway for that.
How does an antenna work?
The antenna at the transmitter generates the radio wave. A voltage at the desired frequency is applied to the antenna. The voltage across the antenna elements and the current through them create the electric and magnetic waves, respectively. At the receiver, the electromagnetic wave passing over the antenna induces a small voltage. Thus, the antenna becomes the signal source for the receiver input.
Types of filter design
There are various types of antennas and their sub-classifications. Some of the common types of antennas are: Wire Antenna, Log Periodic Antenna, Aperture Antenna, Microstrip Antenna, Reflector Antenna, Lens Antenna, Travelling wave Antenna and Array Antenna.
Wi-Fi Antenna
Wi-Fi antenna is an omnidirectional antenna. There are five different bands used for Wi-Fi transmissions: 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, and 5.9 GHz. How the bands are used varies from one country to another. The most widely used is the 2.4 GHz band which you will be using in this lab.
General Best Practices
Before starting this lab, there are few practices one should keep in mind. They are as follows:
- It is always a good practice to warm up the VNA before using it to get accurate measurements.
- Second important thing is to check if the cables are in a good condition. A quick way to do this is to connect the cable end to Short, Open and Load and see the corresponding S11 graph in Log Magnitude.
- Both the open and short connections must yield a flat line very close to 0dB. The load connection must give you a very low return loss (around -40dB).
- Always make sure to use a torque wrench while making the connections.
- Also make sure that the VNA is in an ESD (Electrostatic Discharge) safe environment and the operator is wearing an ESD wrist wrap.
- With these prechecks performed, you can now go ahead and make the measurements.
Pre-Lab
- What are the different antenna parameters that are accounted for while characterizing an antenna?
- What is directivity and gain of an antenna? How are they related to each other?
- What are the three main types of antennas?
- In your own words explain what a radiation pattern is?
- What is meant by radiation pattern of an antenna?
- Explain the concept of isotropic antenna.
Objective
To learn how to characterize antennas and the transmission between them by using a VNA.
Lab Procedure
Click here to watch the video
- Turn the VNA on and launch the S2VNA software
- Connect a cable to Port 1 of the VNA and secure the connection using a torque wrench.
- Let us ensure that we have our stimulus settings ready before we go ahead with the calibration procedure.
- On the sidebar menu, click on Stimulus. Set the start and stop frequency to 1 MHz and 8 GHz respectively. Let the points be 201 and the IF bandwidth be 10 KHz.
- Since the antenna that you are using is a 1-port device, perform a one port calibration using the steps given below.
- To perform a one port calibration, click on Calibration > Calibrate > Full 1-Port Cal.
- If you are using port 1 to make the measurements, keep Select Port as 1 (S11) and if you are using port 2 then change the Select Port to 2 (S22) as shown in Figure 4.1
Note: The Kit number is indicated on the N-Type cross kit (N911/912)
- Now go ahead and connect an open standard to your port (either port 1 or port 2). Use torque wrench to secure the connection and click on the open option on the sidebar menu.
- Now repeat the steps for short and load standard.
- Once you are done with short, open and load, click apply.
- Now remove the load and connect Wi-Fi antenna (2.4 GHz) to port 1 cable (use a torque wrench to secure the connection).
- Make sure that the format is in log magnitude and the parameter you are measuring is S11.
- You must now be able to see a graph that looks like Figure 2 as shown below. Please note that this response may look a little different based on the position the antenna.
Figure 4.2: S11 measurement of Wi-Fi antenna
- You are now looking at the S11 or return loss of the antenna.
- The device is always expected to give low return loss. The frequency you notice at this dip in the screenshot is the resonant frequency of the antenna. Resonant frequency is the frequency at which the antenna radiates maximum.
- You can also see the Standing Wave Ratio (SWR) measurements. To do this, change the format of S11 from Log Magnitude to SWR. The plot must look something like the one given in Figure 3. SWR must have a value closer to one.
Figure 4.3: SWR measurement
- You can also see the corresponding smith chart graph. To do this, change the format of S11 from SWR to Smith (R+jX) and the plot should resemble the one given in Figure 4. The point for a matched load in a Smith Chart is at the very center.
Figure 4.4: Smith Chart Plot
- Now to take a screenshot of your graphs/traces, click on System > Print > Print Windows. This will give you the screenshot of the graph and you can go ahead and save the file on your computer.
- To save this data onto your computer, click on Save/Recall > Save Data to Touchstone File > Type 2-port (s2p) > 1-port (s1p) > Save Fileā¦
- You can use this screenshot to submit your lab report.
- Now repeat the calibration for a 2-port VNA. Place another antenna next to the first one and measure S12 and S21 parameters.
- Measure the two antennas in parallel and in perpendicular position to one another. What do you conclude?
Lab Report
- What are the different frequency ranges of a Wi-Fi antenna?
- Submit a screenshot of various antenna parameter measurements that you made.
- State the value of your resonant frequency.
- Explain what resonant frequency is and why does the antenna radiate maximum power at this frequency?
- What category of antennas do Wi-Fi antennas fall under? Directional or Omni-directional?
- Simulate the far-field of a dipole type Wi-Fi antenna using HFSS, Comsol, or AWR. Attempt to simulate the transmission from two such antennas. Some software have tutorials or demos that you may follow.
- Experimentally show and simulate the temporal behavior of a Wi-Fi antennas