How to choose an SDR Switch.

There are 3 specifications to consider when choosing your SDR Switch. Insertion Loss (S21), VSWR, Isolation (S21).

For a full description of how to understand Insertion loss, VSWR and Isolation, and how they affect your receive capabilities, please see the section on Understanding VNA Sweeps. Fully understanding these three variables will allow you to choose the best SDR Switch for your requirements.

The S21 gain, expressed in a negative number, can be used to measure both the Insertion Loss of the SDR Switch and the Isolation between the TX connector to your transmitting rig and RX connector to your SDR RX.

Insertion Loss

This is the loss, in dBs, of inserting an SDR Switch in your station. The lower the number the better. A -0.5dB insertion loss is far better than a -3.5dB insertion loss.  The greater the insertion loss the more detrimental it will be to your RX N.F. and how well your station will hear. Think of this in FT8 terms. If you are receiving a station at -27 with a 3 dB insertion loss they become a -30. Can you still RX them? On EME and VHF (6M and up) every dB matters. Just a single dB can be the difference between working a station or not. On EME most of us are working stations in the -27 to -30 range. Add a 3dB insertion loss and we would never even hear the other station.

VSWR

This is the VSWR that your transmitter (and receiver)  sees via the SDR Switch. Pay attention to this number so as not to blow your radio up when you transmit. Attention should be paid if the SDR Switch has a VSWR of over 2.0:1 on TX as this may cause damage to your radio. 

Isolation

Think of this as how much signal your SDR RX sees via the RX connector when you TX on your radio. How many mW sneak by the relays when you transmit. Your SDR RX will still see your TX signal, think of it as you would a MONI option, but you do not want to blow your SDR RX up. 

K7IU, Marv, WA2FZW, John, and the ARRL have taken independent Insertion Loss and VSWR measurements. K7IU did a very nice comparison between MFJ and the SDRSwitch.com. We have added this at the bottom as it is well worth taking the time to read.

WA2FZW Amazon SDR Switch v SDRSwitch.com (N2EME) Comparison

ARRL Lab measurements for the MFJ.

K7IU MFJ v SDRSwitch.com Comparison.

 

 

MFJ 1708B Insertion Loss

 

SDRSwitch.com Insertion Loss

 MFJ 1708B 1-30 MHz

 

SDRSwitch.com 1-30 MHz

When comparing the MFJ-1708SDR and MFJ-1708B the ARRL had the following comment regarding the minimum insertion loss of 3.5dB on HF. Please remember that any insertion loss increases the Noise Figure of your entire receiver. 

Whilst the above information compares the main three SDR Switches on the market now that you know how to read a VNA Sweep you can easily evaluate the performance of any SDR Switch.