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australia:vnas

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vector network analyzers (VNAs)

see also:

  • I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed them, they are listed here to give you perspective

Introduction

  • VNAs are devices that assess the impedance and resonance characteristics of radio antenna systems
  • they work in somewhat similar ways to frequency response analyzers but have different use cases and different outputs
  • a professional VNA will have a noise floor of ~100dB and cost $20,000 with a calibration kit costing $2000
  • a nanoVNA will suffice for most purposes and will have a noise floor of ~75dB, upper frequency limit of ~3GHz, and the calibration kit and the nanoVNA will cost under $200
    • you may need VNA-QT software for firmware upgrades or other analysis
    • nanoVNA-Saver software is more sophisticated and with a steeper learning curve
  • AVOID damaging your VNA:
    • DON'T connect a transmitter to the VNA - it won't be able to handle the wattage
    • Coax cable has a capacitance which can retain large static voltages
      • 50 foot of RG8/U cable with capacitance of 25pF/foot has 1.25nF capacitance which could hold over transient 1000V discharge
      • Bleed off any residual static charge in your antenna cable before connecting it - ground the male end of the cable

the basics

  • standing wave ratio (SWR) is a measure of the return loss of a device under test (DUT) such as an antenna for a given signal frequency describing the ratio of standing wave amplitude maxima to minima which can be expressed in terms of voltage, current, or even power
  • VSWR (Voltage Standing Wave Ratio) or return loss is a specific way of expressing SWR that relates to the ratio of the maximum and minimum voltages along a transmission line

using a Vector Network Analyzer (VNA) and measuring the standing wave ratio (SWR)

  • one method to do this is to use a VNA antenna analyzer and ascertain the SWR value (see above) which needs to be as low as possible
  • VNAs will come with three calibration connectors - Open, Load, Short
  • if you are just doing a reflection SWR measurement of your antenna, you will just use the SW11 port so only this port needs to be calibrated for Open, Load and Short for the frequency range you wish to measure for.
  • once calibrated, you then attach the antenna, ensure the Trace is set to display SWR, and then run the sweep
  • if you choose a wide range of frequencies you will probably see many dips in the SWR which signify the frequencies the antenna is resonant for
  • if the SWR dip frequency is lower than what you want, you will need to shorten the length of the antenna
  • if the SWR dip frequency is higher than what you want, you will need to extend the length of the antenna
  • you MUST calibrate for the frequency range you are going to test BEFORE running any tests
    • adding connection adapters or cables will require re-calibration
      • if you add a male out connector and you only have male out calibration kit, then you can calibrate without the male connector but add a Port Extension Electrical Delay on a nanoVNA eg 165psec which will put the open calibration back where it should be on the Smith chart
    • when calibrating you must choose your frequency range first and this should not be too wide

Using the Smith chart

  • when calibrated the points should lie on the horizontal line with the open will be at far right, the load at 50 Ohm mark and the closed at far left
  • the area at the top half indicates excessive inductance whereas the area in the bottom half indicates excessive capacitance
  • https://www.youtube.com/watch?v=_pjcEKQY_Tk&t=19s

Using software to use a nanoVNA via USB

Cheap nanoVNAs may not be as accurate or reliable for RF work

  • for most RF enthusiasts a cheap nanoVNA will be adequate for their needs for non-critical tasks
  • a major issue is the lack of a sine wave generator
    • most if not all nanoVNAs only output a square wave signal due to the on-board clock chip (e.g., Si5351)
    • square waves contain the fundamental frequency plus odd harmonics (3rd, 5th, etc.), which means the VNA injects energy at multiple frequencies at once rather than a single, pure tone. This contaminates S-parameter and frequency response measurements, especially when characterizing narrowband or resonance circuits where harmonics may overlap with other system frequencies.
    • many RF devices, filters, and components are designed to respond only to a clean, single-frequency (sine) excitation. Square waves defeat this by exciting unwanted resonances, giving misleading results for insertion loss, reflection, and filter performance.
    • VNAs rely on precise calibration at known frequencies. Harmonics from a square wave interfere with measurement by creating multiple frequency responses simultaneously, making accurate calibration and measurements impractical for demanding applications.

Example VNAs

what else can you use a VNA for?

  • see https://www.youtube.com/watch?v=ZK7-M7U-_1I
  • measure how long your coax cable is or where a breakage is
    • connecting a coax to port S11 then sending a impulse out and measuring how long it takes to get reflected back allows the device to calculate the point it got reflected back - the end or the breakage point
    • this is the TDR mode
  • measure the impedance of coax cable
  • trap resonance
    • determine the frequency a trap resonates at
  • measure component impedance
  • you can use the S21 port to:
    • look for coax losses due to a damaged coax
    • insertion loss of devices
    • filter characteristics such as band width / stop band
    • attenuator characteristics - are they correct amounts and what frequency do they operate over
    • amp gain

Using a VNA for a 75 Ohm TV antenna

  • the professional way is to use a $20,000 VNA and a $2000 cal standard kit with 50 to 75 ohm minimum loss matching pad as well as 75 Ohm calibration kit
  • you do need to use two 5.7db loss 50 to 75 Ohm minimum loss pad connectors (~$AU45 each) to avoid the impedance mismatch reflections
    • SMA male and 75 Ohm female TV coax
  • but a cheaper way is https://www.youtube.com/watch?v=X2a31PExqM0:
    • use two 5.7db loss 50 to 75 ohm minimum loss connectors plus 4x 75 Ohm terminators F male (4th is a 2nd port load for 21 calibrations) and modify two of them (to make open and short circuit terminators) to create a 75 Ohm calibration kit
    • to avoid the weight on the VNA terminals connect each 50 to 75 Ohm minimum loss connector via an intermediary short SMA cable and SMA-SMA adapter and use a torque wench or a standard 8mm wrench to slightly tighten more than hand tighten
  • 75 Ohm F male connector terminators: https://www.aliexpress.com/item/1005008567398808.html
  • 50 Ohm SMA male to 75 Ohm F female minimum loss pad connectors https://www.aliexpress.com/item/1005009153002838.html
  • and for Australian PAL TV antennas, add:
    • F male to PAL adapters
  • and for your BNC port oscilloscope you may as well add:
australia/vnas.txt · Last modified: 2025/09/09 01:09 by gary1
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