A low-cost vector-network analyzer (VNA) from AAI makes a handy tool for field measurements of frequency response, impedance, and VSWR.
The ability to perform network analysis measurements is probably not well-known among product designers or even some EMC engineers. Such measurements are more common among RF test engineers who characterize RF components, but you can use vector-network analysis for EMC work as well. I recently purchased one such low-cost network analyzer for my toolkit. Because I travel a lot, I needed a small, handheld instrument. My colleague, Robert Witte, Chief Technologist, Signal Blue LLC, assisted with some commentary.
Recently, Accuracy Agility Instrument (AAI), introduced an affordable and decent quality RF vector network analyzer (VNA), model N2061SA (Figure 1). The product is available through Amazon for approximately $267 (see references).
There are two types of network-analysis measurements: scalar and vector. Scalar measurements measure only the magnitude of the input versus output voltage and may be used to measure things like shielding effectiveness, resonant circuits, or general filter response. You can easily perform this measurement with a spectrum analyzer and built-in tracking generator. A tracking generator is merely an RF source whose frequency sweeps between the same limits and speed as the spectrum display. I’ve written a number of articles on this technique (see the related articles at the end of this post).
A vector measurement makes both magnitude and phase of the same input/output measurement, capturing both the real and imaginary information. This is very important information when characterizing antennas and matching circuits, because it includes the resistance and capacitive/inductive reactances.
It will measure S11, S21, VSWR, R, |Z|, X (inductive or capacitive), and equivalent series capacitance (ESC) or inductance. The VNA can measure S21 from 1.1 to 600 MHz and S11 from 1.1 MHz to 1300 MHz. Both the frequency and vertical scales are user-adjustable. There is a single marker control for reading specific frequency and vertical scale peaks/dips in the displayed measurement, indicated by a yellow triangle.
The product comes with an English manual and wall charger. The Li-ion battery is advertised to last for four hours and the percentage of charge is displayed in the display’s upper right. The 2.4-inch TFT color display is clear and nicely backlit and uses 280 horizontal pixel resolution for the scan data in a 320 x 240-pixel design. The input and output connectors are both female SMAs. There is a mini-USB connector on the bottom for charging only, it’s not designed as a data port. The unit comes calibrated or you can recalibrate it at any time using a set of “open”, “short”, or “50-Ω” calibration standards, ordered separately.
The user interface is a bit sparse, with a single digital-encoded rotatable and push control on top and power/set, mode, and cursor buttons along the bottom of the display. It takes some mental training to operate, but after a bit, it starts to come naturally. To turn on the unit requires pressing the CTRL and Power buttons simultaneously. Pressing the Power button (long press) or leaving the unit idle powers down.
The top knob and two cursor buttons let you select the field to change and set the values. The Mode (M) button sets the type of measurement.
Here are some sample measurements I made.
150 MHz low pass filter
In Figure 2, I measured a 150 MHz low-pass filter. I set the instrument mode to S21 and frequency from 1.1 to 600 MHz.
Figure 3 shows the filter’s response.
S11 measurement of a 50 Ω termination
Next, I measured several 50 Ω terminations. Most commercial models started to degrade above a couple hundred megahertz (Figure 4) shows the setup while Figure 5 shows the measurement.
VSWR, R, and |Z| measurement of an antenna
Figure 6 (setup) and Figure 7 show measurements of a short monopole antenna, which displays a resonance at 450 MHz. Note that the characteristic impedance measures at 490 MHz.
Robert Witte adds: Figure 7 shows the minimum VSWR of ~1.5 at 450 MHz. We would usually say that’s where the antenna is resonant. Really, it’s just the best VSWR. Ideally, at resonance the antenna would have a real impedance of 50 Ω (with no reactance). The antenna is a 1/4 wave monopole without a ground plane under it, so it won’t be perfect. But it’s not too bad.
Witte: Figure 8 shows the max R (probably displaying the real part of the impedance) of about 68 ohms at 490 MHz. But we don’t look for max R at resonance, we want the R to be 50 ohms, corresponding to a VSWR of 1. Note that the R plot crosses 50 ohms just to the left of the peak. It’s hard to read the horizontal scale but this is probably close to the 450 MHz point where the VSWR = 1.5. Interestingly, there is a 50 Ω point to the right of the peak, too. There is probably a bunch of reactance at that frequency causing the VSWR to be higher (and not another VSWR minimum).
Witte: Figure 9 shows the Z, R, X, VSWR at 444 MHz (not quite 450 MHz, but close). Note that |Z| is pretty close to 50 ohms (48.5 Ω), so you would expect a pretty good VSWR. But there is some reactance at this frequency (-23 Ω), so it is not quite a perfect match. I suspect that if you bump the frequency to 450 MHz you would see a bit less reactance and a better VSWR reading (matching Fig. 7).
The VNA seems to work well once you get your mind around how to set up the measurements. The display updates quickly and the unit compared favorably to another similar affordable VNA from MFJ (model MFJ-226). It’s really nice to have the ability to make these measurements using a portable battery-operated unit for field work. Recommended.
You can use the VNA for the following applications listed here.