I use many surplus SMA relays in my microwave stations. Befor usage I test them for attenuation and isolation versus frequency.

Next pictures show attenuation of Narda SEM020 relays, two pieces (S/N 5782 and S/N 2037) were measured. Every picture contains a black and a red graph. The black plot is what was measured with the power head connected to the generator without the relay (all other adaptors were still in between). This plot is used as a reference. There are some ups and downs in the curves due to resonances. I did not figure out where the resonances came from, high grade adaptors were used. The red plot shows the measured attenuation thru the relay. The both straight lines are the linear fittings of the curves, the black line is the fitted reference, the red line is the fitted attenuation curve.

S/N 2037, "normal closed" (NC)-port, distance between the fitted curves is about 0.1 dB. At a certain frequency the attenuation can be much higher due to resonances. Attenuation gets smaller with rising frequency, not enough points for an accurate fit.

S/N 2037, "normal open" (NO)-port. Same as above, difference between the fit-lines 0.2 dB. One explanation for the higher attenuation is that this is the normal open port and the properties of the contact surface may be different compared to the NC port. Probably this will change if the relay is used for transmitting more often.

S/N 5782, "normal closed" (NC)-port. Difference between the fit-lines is rising with frequency as one should expect.

S/N 5782, "normal open" (NO)-port. Otherwise same as above.

SWR input to NC contact, SWR does not rise above 1:107 up to 6 GHz, there might be some resonaces above that are not within the range of the network analyser.

SWR input to NO contact. SWR rises up to 1:12 at 6 GHz. Dunno why SWR is slightly higher compared to NC port. Probably some corrosion on the NO port. Could improve after some switching cycles with power applied.

Commercial grade relay in the 300$ range. Almost new out of the box. Very low difference between reference and relay-attenuation at low frequencies. Again measure errors due to resonances at higher frequencies. Picture shows attenuation of the normaly closed (NC) port (red plot) and the reference (black plot) between 1 .. 20 GHz.

same as above, but attenuation measured thru the normaly open port. The threshold voltage (where the coil switched reliable) is 17 Volt. The current consumption is 60 mA at 17 Volt and about 100 mA at nominal 28 V.


Isolation measured from 300 kHz to 6 GHz between input port and NO port (relay switched), resonance peak at 3.3 GHz. The peak was also visible on two other relais.


Isolation measured from 300 kHz to 6 GHz between input port and NC port (relay not switched), resonance peak at 3.2 GHz. The peak was also visible on two other relais.

RLC "one out of six" relays, often found on fleamarkets. Reference (black), port 5 (red) and port 2 (blue) are plotted. Port 2 was the best from all 6 ports, port 5 was the worst. Attenuation (mean value) is less than 0.1 dB at 1 GHz, and between 0.1 to 0.2 dB at 20 GHz. In real world, it could be better or worse if you take resonances into account.


S21 (thru attenuation) measured from 300 kHz to 6 GHz between common port and port 1. The upper (flat) plot is the thru attenuation when just port 1 is switched on. What happens when port 2 to 6 are switched on in parallel can be seen in the lines below. All ports were terminated with 50 Ohm. Display is set to 3 dB per division.


S11 (SWR) measured from 300 kHz to 6 GHz between common port and 50 Ohm terminated port 1. When other ports are switched on in parallel, SWR increases, specially at lower frequencies.