Product Review: The Array Solutions VNA2180 Vector Network Analyzer
Phil Salas – AD5X
Introduction
In August 2007 QST reviewed the Array Solutions AIM4170 antenna analyzer. In the
ARRL review it was pointed out that “… the accuracy is exceptional …” and “… the
AIM4170 is really more of a laboratory instrument…” I couldn’t agree more as I’ve
found the AIM4170 to be indispensible for my home lab work – so much so that I also
wrote a review of the AIM4170 (elsewhere on this website). And while the AIM4170
has been outstanding in all that it does, I’ve wanted a 2-port version so I could design and
analyze 2-port devices. Enter the VNA2180 Vector Network Analyzer.
Photo A: Array Solutions VNA2180 Vector Network Analyzer & Supplied Accessories
VNA2180 Description
The VNA2180 is a 2-port instrument where Port A is an enhanced version of the
AIM4170 featuring increased frequency range, higher output signal with a programmable
level, and the ability to handle a higher interfering signal level while making antenna
measurements). The VNA2180 also has a built-in optically-isolated USB interface which
reduces the effects of ground loops. As this review focuses on the Port A-to-Port B
transmission path capability of the VNA2180, refer to the above referenced AIM4170
reviews for the Port A features. The complete VNA2180 specifications are show below:
Parameter Specification
Frequency Control Digital Synthesizer
Frequency Range 5KHz-180 MHz
Stability +/- 25 ppm
Frequency Step Size 1 Hz minimum
Calibration Software controlled
ADC resolution 12 bits
SWR measuring range 1 to 20
Impedance Measurement Range 1 ohm to 5K ohms
Accuracy +/- 5% to 60 MHz, +/- 10% to 180 MHz
Phase Angle +/-180 degrees (true phase)
RF Output +7 milliwatts max, nominal output Z = 50 ohms.
Spurious output –30dBc or better
Max stray RF input while measuring 2V peak (+16dBm)
Max safe RF input 5V peak (+24dBm)
S21 nominal dynamic range 100dB up to 50MHz, 80dB to 160MHz
Port B nominal input impedance 50 ohms - Return Loss greater than 30dB.
RF Interfaces (Ports A & B) Type N connector
PC Interface Optically isolated USB, FTDI chipset
Display Graphics output on PC. Mouse controlled cursor for
digital parametric readout. User-specified
frequency markers. SWR audible tone & speech
output. Displayed parameters: SWR, S11, Return
loss, |Z|, Phase angle of |Z|, S21, RS, XS, RP, XP, and
uH or pF. Two Smith Charts with zoom, phase
offset and markers. Data can be referenced to the
antenna terminals.
Power Requirements 11-15 VDC, 500mA max (power supply included)
Dimensions (approx) 7” x 5.3” x 1.5” (17.8 x 13.5 x 3.8 cm)
Preparing to use the VNA2180
The VNA2180 software has been tested with Windows 2K/XP/Vista/Win7 32/64 bit. No
software installation is required - it can run directly from a flash drive or CD if desired.
The VNA2180 includes three standard N-connector calibration loads (open, short, and
50Ω), a 120VAC power supply (or optional DX power supply with Australian, European,
US and UK adapters), a 2.1mm DC cable, two N-male terminated cables, and a USB
interface cable. You must download the VNA2180 manual and software from
w5big.com. Incidentally, you can run the software in demo mode to get a feel for the
product prior to purchasing.
Calibration requires no tools or adjustments, and you’ll have everything up and running
in minutes. However since the VNA2180 Ports A&B uses N-connectors, you must
purchase BNC/F-to-N/M and UHF/F-to-N/M adapters for BNC and UHF measurements
(I purchased mine from www.therfc.com). And while not absolutely necessary for many
lower frequency measurements, for maximum accuracy you’ll want BNC and UHF
calibration loads. However these are easy to build (see the last section of this review).
Using the VNA2180
My first project was a low-pass filter needed to clean up the square-wave output of a 50
MHz TTL clock oscillator for use as a standard signal source for calibrating attenuators.
The schematic of the filter is shown in Figure 1.
150nHy 150nHy
120pf 200pf 120pf
Figure 1: 50 MHz Low Pass Filter
The VNA2180-measured filter response, shown in Photo B, highlights the ability to set
vertical markers (up to 20), and simultaneously display S11 and S22. I set markers at 50-
and 100-MHz. The vertical cursor is placed at 150 MHz, where I wanted maximum
rejection. Incidentally, Port B provides a 50Ω termination >30dB return loss.
Photo B: Low Pass Filter Swept Response. Markers at 50-, 100-, and 150 MHz
My next project was a resistive 50dB tap covering 1.8-148 MHz to permit looking at a
transmitter RF output on a spectrum analyzer. My original circuit is shown in Figure 2,
and the final circuit is shown in Figure 3. The two paralleled 16K 2-watt metal film
resistors provide the necessary power dissipation for up to a 600 watt amplifier. The
VNA2180 software permits setting an offset attenuation value while simultaneously
permitting a fine attenuation range. In this way you can look at fine variations in
attenuation, even at the 50dB tap point. As you can see in Photo C, I set an offset of 45
dB with a measuring resolution of 0.5 dB.
16K 2W 16K 2W 16K 2W 16K 2W
51Ω 51Ω 62pf
1/2W 50dB 1/2W 50dB
down down
Figure 2: Original Resistive Tap Figure 3: Final Resistive Tap
Photo C: Original Sweep of the 50dB resistive tap
As you can see, the isolation degrades at higher frequencies due to stray capacitance
across the paralleled resistors (even a few picofarads impacts isolation at higher
frequencies in a high impedance circuit). The 62pf capacitor across the 50 ohm resistor
(Figure 3) compensates for this. Photo E shows the final swept response. The three
horizontal markers are on 10-, 6-, and 2-meters. Horizontal rulers at -49dB and -51B
show that the tap circuit provides 50dB +/- 1dB attenuation in all ham bands through 2-
meters.
Photo D: Compensated 50dB tap. Vertical rulers on 10-, 6-, and 2-meters
Next I wanted to characterize a home-brew 4-element crystal filter. The schematic is
shown in Figure 4 below.
10MHz 10MHz 10MHz 10MHz
230pf 270pf 230pf
Figure 4: 10 MHz Crystal Filter
I ran two scans. The first (Photo E) shows the ultimate rejection of this crystal filter
which is in excess of 100dB just 5 kHz to either side of the 9.998MHz center frequency.
Photo E: Crystal Filter ultimate rejection
The second scan has a narrowed pass-band and increased vertical resolution to better see
the pass-band response. The VNA2180 software calculates the shape factor of the filter
for you when you insert horizontal rulers at the desired measuring points. Because the
filter loss is approximately 4.5dB, horizontal rulers were set at -10.5dB and -64.5dB so as
to calculate the standard 6/60dB shape factor. The results are shown in Photo F below.
Photo F: Determination of Crystal Filter Shape Factor
Finally, I looked at a commercially available 40 meter band-pass filter used in contest
environments to protect a receiver from high-power adjacent band transmitters and
nearby antennas. A broadband scan is shown in Photo G. This filter has rejection
specifications of 42dB (horizontal red ruler) on 80 meters, and 70 dB (horizontal blue
ruler) on 20 meters. Note the simultaneous display of both S11 (dark blue scan) and S22
(dark green scan). Vertical markers are set at 4 MHz and 14 MHz. The vertical curser is
placed within the 40 meter band to show the in-band insertion loss (0.430dB).
Photo G: 40 Meter Band-Pass Filter Response
Conclusion
The VNA2180 is a lab-grade analyzer that you will find to be an indispensable item for
both home and industrial lab environments. And with software and firmware updates
continually available for download at no charge, you don’t have to worry about product
obsolescence. I’ve only touched on some of the transmission capabilities of this
instrument. As an example, group delay measurements permit you to determine the
length of cables and potential distortion in filters. There is also an external port with both
digital I/O and analog input capability for possible control and monitoring of other
accessories. Finally, there is a wealth of measuring and analysis capabilities associated
with Port A. Detailed information is readily available for further investigation at
w5big.com. The complete manual is included with the program zip file.
The VNA2180 is available from Array Solutions (www.arraysolutions.com).
Price - US Version: VNA-2180 @ $1495. With DX supply: VNA-2180-1 @ $1524.
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Addendum: Building BNC and UHF Calibration Loads for the VNA2180
Phil Salas – AD5X
For maximum accuracy, BNC and UHF calibration loads should be used when making
measurements on equipment and components with these connectors.
BNC calibration loads are easily built into RG-58 BNC crimp-on connectors. The parts
list is shown in Table 1. Mouser (www.mouser.com) part numbers are shown.
Table 1: BNC Calibration-Loads Parts List
QTY Description Part Number
3 BNC crimp-on connectors 601-27-9208
1 49.9Ω 1% ¼-watt resistor 71-CMF5549R900FHEK
1 Buss wire or resistor lead
1 Toothpick
1 Dab of epoxy
For the BNC 50 ohm load, clip one 49.9 ohm resistor lead just long enough to go into the
BNC center pin and solder the resistor lead to the pin. Slide the center pin/resistor into
the BNC plug, fold the resistor lead over the BNC collar, and solder the lead to the collar.
The short circuit is assembled the same way, using just a resistor lead or buss wire.
For the open circuit, insert the BNC center pin into the plug. Coat a toothpick end with
epoxy and push the toothpick into the BNC plug so it is snug against the center pin.
When the epoxy cures, snip-off the excess toothpick.
When complete, put heat-shrink tubing over the BNC collars and mark with a silver
SharpieTM pen (∞, 0, and 50). Photo A shows parts prior to assembly, and Photo B
shows a 50 ohm marked load.
Photo A: 50 Ohm Load & Open BNC parts Photo B: Final BNC 50 ohm load
The UHF calibration loads are also easy to build. The 50 ohm load consists of two 100
ohm resistors in parallel. Begin by putting a slight bend in the end of one lead of each
resistor. Slide one resistor through the UHF center pin, bent lead end first, and feed the
resistor lead through one of the shield holes in the connector. Pull the resistor through so
it clears the pin inside the connector but with minimum lead length on both ends. Do the
same for the second resistor. Solder the resistor leads in the UHF connector center pin,
and then solder the ground ends of the resistors to the UHF body. The UHF short is built
the same way, only use two pieces of bus wire or resistor leads. An empty UHF
connector is used for the open circuit. The parts list is shown in Table 2. Photos of the
before and after UHF 50 ohm load is shown in Photos C & D below. The connectors
were marked with Casio labeling tape.
Table 2: UHF Calibration-Loads Parts List
QTY Description Part Number
3 PL-259 connector 523-83-1SP-1050
2 100Ω 1% ¼-watt resistor 660-MF1/4D52R1000F
1 Bus wire or resistor lead
Photo C: PL259 and two 100 ohm resistors Photo D: Final 50 ohm load
The return loss of the 50 ohm loads was measured at 180 MHz with a spectrum
analyzer/tracking generator and MiniCircuits ZFDC-20-5 directional coupler. N adapters
were used for the BNC and UHF connectors. The results are shown in Table 3. As you
can see, the performance of the homebrew loads is quite good.
Load Return Loss/VSWR
Supplied Array Solutions N-load 36dB/ 1.03
Precision Array Solutions N-load (optional) 42dB/1.01
BNC homebrew 30dB/ 1.07
UHF homebrew 40dB/ 1.02
