How To read a VNA Sweep.

A Vector Network Analyzer (VNA) is one of the most useful pieces of equipment in both my shack, and on my test bench. I use a VNA for everything from tuning antennas to testing the design of my Amp circuits. Although I have several VNAs, when I want to take a good screenshot I tend to use my Nano-VNA-F and the NanoVNA Server software. The software allows me to adjust all of the settings on my Windows computer and monitor rather than on the 4" Nano-VNA display. It also makes taking screenshots far easier than posting a photograph of my small green H.P. VNA display.  

Which are the important numbers?

When it comes to your SDR Switch, reading a VNA sweep will tell you all the most important numbers you need to know when choosing which SDR Switch is best for your application. Below I will highlight the areas to look and why.

VSWR over Frequency Range.

This overview shows you the VSWR of your SDR Switch over its entire design range. In this case you can see the VSWR remains well under 1.20:1 from 50 KHz all the way to 70 MHz. Think of this as checking your VSWR into a very good Dummy Load attached to the ANT connector of your SDR Switch. This is a very important reading as it shows you if your SDR Switch is even usable over your desired frequency range. This does not take into account the VSWR of your antenna. Likewise your TX or RX will not see a VSWR better than this number no matter how resonant your antenna is. If the VSWR of your antenna is 1.50:1 but the VSWR reading of your SDR Switch is 3.00:1 then the VSWR your TX and RX will see is 3.00:1.

Markers.

As you can see on the VSWR over Frequency range picture there are 3 coloured inverted triangles along the sweepline.

These indicate the specific frequencies chosen for the main table measurements. These are far easier to read when using Nano-VNA Server than on my HP screen. When choosing a SDR Switch the following numbers from the main table are of the most interest. 

VSWR.

Although we have already looked at the VSWR over the entire desired frequency range we also want to look at specific frequencies. The picture above is looking at the 6M band (50.414 MHz) and showing that the SDR Switch has a VSWR of 1.08:1. Again think of this as your TX or RX looking into a very good Dummy Load. The same factors mentioned above still apply. 

Insertion Loss.

As previously mentioned many times insertion loss is one of the most important numbers to look at when choosing a SDR Switch. It contributes to both the loss of the signal prior to your receiver even seeing the signal, as covered on the Insertion Loss page, and equally it is one of the factors that sets the very important Noise Figure of your entire receive chain. As with coax loss prior to your LNA, this loss, and increase in your Noise Figure cannot be compensated for by installing a Pre-Amp after the switch.

The Insertion Loss is shown as the S21 Gain on the same reading as your VSWR. It is a negative gain so will always be shown as a negative number measured in dBs. In this case on 6M the insertion loss (Gain) is -0.057 dB. The closer to 0.0 dB the better your SDR Switch will perform. Insertion Loss is frequency dependent and will vary from band to band, also affecting your RX N.F. as you change bands. 

Isolation.

As mentioned on the on the Home page this is a dB measurement used to indicate how much signal is seeping past your relays when you TX. It represents how much signal your RX System sees from your own transmitter when you TX. 

Again the Isolation is shown as a S21 Gain and will be a negative number measured in dBs. The further away from 0dB the better the isolation. For example -45 dB of Isolation provides more protection than -10 dB of Isolation.

As both the Insertion Loss and Isolation are shown on a VNA Sweep as a S21 Gain we need an easy way to know which reading is which. This can be done by looking at the VSWR reading. If the VSWR reading is within what you would consider an acceptable range, say 2.00:1, then you are most likely looking at the Insertion Loss.  If the VSWR number is extremely excessive, effectively infinite, say 35700:1, then you are looking at the Isolation figures. 

VNA Sweep measurements matter.  

When choosing your SDR Switch you should pay particular attention to these VNA measurements: Insertion Loss, VSWR and Isolation. They are the most important sets of numbers that will affect the performance of your RX system.