An Introduction to Vector Network Analysers
Keith Rawlings gets up close and personal with Vector Network Analysers, looking at their key components and functions and introducing the Nano VNA in the process.
While at the Newark rally last year, my friend spotted a stall that was demonstrating a handheld Radio Frequency (RF) Vector Network Analyser (VNA). It was a battery-powered, two-port device with a small but clear built-in colour display. It could be used either as a standalone unit or be connected to a PC running under free software. Its frequency range was from 50kHz to 900MHz, and it had a claimed dynamic range of 40dB. It was no surprise that sometime later my friend announced he had purchased a version of the NanoVNA off of Amazon for about £40. He then very kindly lent it to me to play with.
I have covered Vector Network Analysers before, but it is worth returning to this topic, in my view.
There are two main types of network analyser, ignoring the Large Signal Network Analysers. These are
(1) An SNA or Scalar Network Analyser, and
(2) A Vector Network Analyser (VNA).
An SNA generally will only display the amplitude of a measurement whereas a VNA will display both amplitude and phase (Don’t worry about these terms at the moment).
The VNA is the more versatile of the two, as it provides more comprehensive information.
These devices are used to characterise the RF properties of a ‘network’. These networks will have ‘ports.
For example, an aerial can be regarded as a ‘network’ and will (usually) have one single port. A low pass filter or an amplifier will have two. Devices such as directional couplers, mixers and the like can have three ports, and a multi-coupler can have many more.
There are a number of relatively cheap VNAs available. Remember that aerial analysers such as the MJF269, Sark, Rig Expert etc. are in fact network analysers.
Mostly, these later units have a single connector and are termed single-port-analysers. They can measure parameters such as impedance, SWR and so on.
The Nano VNA, being a two-port VNA, can also perform two port ‘through-measurements’, in addition to single-port measurements. Therefore, with the NanoVNA, we can make single port measurements on ‘single-ended’ DUTs (Devices Under Test) such as aerials dummy loads, and so on. We may also perform two port/through measurements on devices like filters, attenuators, amplifiers and others.
Put simply, it does this by injecting RF into a given port of the DUT (Device Under Test).
If it is a single port, it measures the level of RF at that port as reflected power.
In case of a through-test, the RF level at port 2 is measured.
These measurements are represented by S Parameters (Scattering Parameters, for instance, ‘S11’, ‘S21’).
A more detailed description of these is beyond the scope of this column, but I can direct readers who wish to learn more to the links provided at the end.
The NanoVNA promises to be a very powerful tool for the aerial/RF experimenter. It is popular, cheap, ideally suited for the beginner as well as the experienced user, and it offers a huge amount of background information online. I think it is worth delving into the basic technicalities of it, before moving on to make some basic measurements next month.
The Nano VNA is a small unit, measuring 86 x 54 x 15mm. Along one edge, there is a USB port for battery charging and PC software, an on/off switch and a thumbwheel, which is used to select settings of the Nano, in combination with the touch screen. On the left-hand edge of the unit, there are two SMA female connectors, one for the Transmitter Port (CH0) and the other one for the Receiver Port (CH1). On top of the case, users can find makings for each port. These graphically illustrate the ‘direction’ of parameters S11 and S21.
Below this, and on the reverse of the case, the Transmit (TX) and Receive (RX) ports are marked, together with the ‘Reflection’ and Transmission’ direction. These markings help reduce confusion when setting up to make measurements.
Readings are displayed on a 2.8” colour touch screen display. Despite having an internal battery, the unit weighs next to nothing. On switching on, the user is presented with an introductory screen. After a short delay, the Nano displays its main working screen. The information displayed is, quite naturally, in a tiny format but is readable for those with good eyesight.
Moreover, a USBC lead and an SMA SOL calibration kit are provided with this Nano.
The calibration of any VNA is essential before use. From switch-on, a VNA may have no calibration parameters set, or it may store a Master Calibration, which should give passable measurements.
With my SDR-kits VNA, I calibrate for each individual job, in order to avoid mistakes with incorrect calibration parameters.
First, I set the frequency span I require, sweep speed, data points, and so on; subsequently, I perform a calibration based on these settings. These also include the parameters of the calibration kit in use. When not using software, it seems that (with the Nano) just a simple SOL (Short-Open-Load) calibration is undertaken.
The Nano ‘Master’ calibration procedure involves placing each element of the calibration kit on the TX port (Ch0) in turn. Reflected measurements are then possible, of values like VSWR, Return Loss, Impedance and so on, as used for making measurements on aerials, checking dummy loads and similar cases.
If you want to make through-measurements, such as to measure a filter, you will have to undertake a SOLT (T= through) calibration. This is simply the same procedure as an SOL, and you will now need to add the Load to CH1 and then connect a cable between CH0 and CH1 (Figs. 1, 5, 6).
However, this only calibrates the Nano at its source; remember that usually, you will need to make a calibration at some other point, such as at the end of a cable.
The calibration of the Nano involves you touching the screen (or thumbwheel) to bring the menu up on the right-hand side, navigate to the Cal option and select Calibrate. You then connect the Open component of the calibration kit to CH0 of the Nano and press the Open option. You do the same for Short and the (50Ω) Load.
If you are going to undertake through-measurements on the Nano, you have to also connect the Load to CH1, select Isoln and connect a cable from CH0 to CH1. Then, select Thru. The information on the Nano website seems to suggest that this Master calibration is made from 50kHz to 900MHz. The parameters can be saved to the Nano ‘Memory’ 0.
Ideally, a torque wrench is used to tighten SMA connectors, but finger-tight should suffice. When checking something like an aerial, it is usual to connect a cable to CH0 and make an SOL calibration at the end of that cable where the aerial is to be connected.
For, say, a filter, a second cable is attached to CH1 and is coupled to the first with an inline adaptor; a though calibration is then made using both cables.
As we now have a calibration point to the end of both cables, we can connect our filter in place of the adaptor and measure its parameters.
For a filter, we could be interested in S11 measurements such as VSWR and Impedance, or in S21 dB through-measurements, which will allow us to measure not only the shape of the filter but also bandwidth, cut off point, overall insertion loss and so on.
I used this type of through-checking when diagnosing the Aerial Facilities Coupler covered in AN recently.
Needless to say, the accuracy of measurements on a VNA is directly linked to the accuracy of the calibration kit used. Those used commercially are very expensive, and if you need more than one type of connector, you will need more than one kit.
However, for the majority of readers, the level of precision that these kits provide is not required. Although the Nano only comes with an SMA kit, it is more than possible to make your own calibration kit from connectors such as BNC and Type N.
If care is taken with construction, these can be used quite confidently over (at least) the HF range.
Also, please remember that you may find you need a female and a male calibration kit.