Learn how to use LiteVNA to accurately measure FPV antenna SWR, a critical aspect of antenna performance that refers to power reflection. This guide covers everything from initial setup and calibration to performing precise SWR measurements to ensure optimal antenna performance for your FPV drone.
Check out our recommended tools for building an FPV drone: https://oscarliang.com/fpv-tools/
What is LiteVNA?
LiteVNA stands for Lite Vector Network Analyzer.
A vector network analyzer is an instrument that can measure various properties of an antenna, such as the antenna's tuned frequency and how well it is tuned to this frequency.
Traditionally, lab-level vector network analyzers have been very expensive, costing thousands of dollars. However, LiteVNA breaks this price barrier as a small, inexpensive vector network analyzer that retails for about $130. The latest LiteVNA64 model can measure up to 6.3GHz and covers the 5.8Ghz band commonly used for FPV drones. For example, you can display Smith and SWR charts from 5.6 GHz to 6.0 GHz, and lower frequencies such as 433 MHz (radio link), 868/915 MHz (radio link), 1.3 GHz (fpv link), and 2.4 GHz (FPV link). It also works. and standard RC link) etc.
LiteVNA measures antenna SWR and can be used to tune antennas (if you build one), evaluate filters, and identify defects in coaxial cables. Although not as accurate as laboratory-grade products, they are accurate enough for hobbyist use. There are cheaper SWR meters available for FPV, such as the OwlRC SWR meter, but they lack the reliability and extensive features of LiteVNA.
Purchase LiteVNA from the following vendors (Important: Make sure it supports up to 6.3 GHz!):
What is SWR?
SWR or VSWR stands for (voltage) standing wave ratio and is an important factor in antenna performance.
This metric represents the amount of power reflected back to the source. Ideally, all of the power from the transmitter travels to the antenna and is emitted as radio waves. But in reality, each antenna has a specific tuning, and some of the energy is reflected back and converted into heat, which reduces the maximum range. VSWR values vary depending on the frequency being used and antenna tuning.
When transmitting using an antenna, a high SWR may cause damage to the wireless device. Conversely, if the antenna is for reception, high SWR will not harm the receiver but may degrade signal reception.
Learn more about antenna SWR in my antenna guide: https://oscarliang.com/best-fpv-antenna/
Some antenna manufacturers list SWR values on their product pages, but these numbers are not always accurate and may not specify the frequency at which they are measured. Over time, damage to the antenna may cause the SWR value to change. Having a tool to measure this can help you evaluate the condition and performance of your antenna and determine the best channel to use it on.
For example, take a look at the SWR diagram of the RushFPV Cherry antenna below. The lowest SWR between 5600 MHz (marker 2) and 6000 MHz (marker 3) is exactly 1.113 at 5.8 GHz, which is excellent. Even at 5600MHz and 6000MHz, the SWR is only about 1.4 and 1.6 respectively, making this antenna ideal for FPV use.
How much signal loss and range reduction does SWR 1.1 have? Let’s check it out in this table I created. With about 0.2% signal loss and 0.1% range reduction, that's basically nothing. For SWR 1.4 and 1.6, the range reduction is 1.4% and 2.7%, respectively.
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How to use LiteVNA
Trace display
By default LiteVNA displays all four traces, which can be quite messy. Since we are primarily interested in SWR, we can remove the remaining three traces.
- Go to “Show” and uncheck all traces except Trace 0.
- Change “Format S11” to SWR.
- In “Scale/DIV” set it to 0.5 and enable “Show scale values”.
sweep range
Here you set the frequency range you want to analyze.
- Go to “Stimulus,” select “Start,” then enter the lowest frequency you want to observe.
- Select “Stop” and enter the highest frequency you wish to observe.
- Typically, for FPV, it is set in the 5.6GHz to 6.0GHz range.
It is recommended that you recalibrate your VNA each time you adjust the sweep range.
calibration
It is important to calibrate the LiteVNA before performing measurements. Using an uncalibrated VNA will result in unreliable results, essentially rendering them useless and misleading.
LiteVNA comes with a set of calibration tools (open, short and load connectors) essential for the calibration process.
Go to the calibration menu and connect the “open connector” (without the middle pin) to the first port. Select “Open” from the menu.
Repeat this process for the “Short” and “Load” connectors. Both of these connectors have a center pin. However, the “short connector” is shorter while the “load connector” is taller. Make sure the screws are tight to ensure a good connection.
If you are testing a filter, you will also need to calibrate “Thru” by connecting port 1 to port 2 using the cable provided. However, if you are only testing antenna SWR as described here, you can skip it.
When finished, click Done. You have the option to save this calibration to memory, which will be automatically loaded the next time you start the device, including all settings and frequency sweep ranges. This feature is very convenient as it allows you to save multiple calibrations with different settings and switch between these profiles in the “Recall” menu.
You can also check calibration using a Smith chart. The short point should be at the center left of the circle, the load point should be directly at the center, and the open point should be at the center right of the circle. It is recommended that you recalibrate your VNA each time you adjust the sweep range.
Use adapter or cable
If you plan to use adapters or cables during testing, it is important to recalibrate any connected adapters. This step helps eliminate the potential influence of test cables or adapters that may skew the results.
If you lack a calibration set with the appropriate connectors, you can use “Electrical Delay” to make adjustments. First calibrate the VNA without the adapter, then connect the adapter. You can see that the open point has moved on the Smith chart. You can fix this by going to “Scale”, “Electrical Delay (E. Delay)”. Enter values until the open points are realigned to the correct position. This range is typically 50 to 150 picoseconds for SMA adapters (or longer delays for cables).
How to measure antenna SWR
To test an antenna with LiteVNA, first connect the antenna to the first port. To ensure a solid connection, make sure it is securely fastened. Place the antenna away from metal or conductive materials. The VNA then displays a graph showing the VSWR within the specified frequency range.
important:
- Make sure the surrounding area is clean and free of obstructions during testing. In particular, install it away from metal or magnetic materials.
- Also inspect the SMA connector to make sure it is clean.
Most antennas will have a dip in the graph to indicate the frequency they are best tuned to. Some antennas may not exhibit a significant dip, but it is generally acceptable to have relatively low SWR values within the desired frequency range.
With a VSWR of 2, only about 10% of the power is reflected, which is considered very good. If VSWR is less than 2, the difference in real-world performance will be barely noticeable, but of course lower is better.
Determine the best FPV channel to use by identifying the frequencies with the lowest VSWR. For optimal performance, it is recommended that you select an antenna for your receiver that matches the tuned frequency.
conclusion
LiteVNA gives you detailed insight into your antenna's SWR, helping you select the best frequency for best performance. You can also use it to check if your antenna is suitable for FPV. LiteVNA stands out as an affordable and practical tool for hobbyists, offering reliability and extensive features to efficiently fine-tune and evaluate antennas.
Purchase LiteVNA from the following vendors (Important: Make sure it supports up to 6 GHz!):