Ground Loops in TRX-SDR Switch

Application notes

On Fig. 1 are shown two possible parasitic ground loops, which might influence the performance of the switch. We had a case where the resistors R1 and R2 in the PTT input of the Audio Switch are damaged due to the high RF currents flowing in the ground loop 1.

Fig.1 Parasitic ground loops. For GND Loop1 even if we use a separate power supply (PS) for the Audio switch there will be a ground loop since the TRX ground and PS ground must be connected.

These currents are due to excessive common mode currents in the antenna feeder. The user should take measures to eliminate these currents – how to do that is explained in numerous ham radio publications e.g.

In any case the problem with the Audio switch might be cured by inserting a common mode balun in the PTT cable. Insert a balun in the PTT cable. Wind as much as possible turns on toroidal ferrite core. The minimal inductance must be above 100 uH preferable above 400 uH. Use ferrite materials N30 (Epcos) , 77 (Amidon) or 31 (Fair-Rite). I have used a small N30 10x6x4 mm core with 15 turns bifilar thin twisted pair. For those who use ready made PTT cable, an E-core might be used thus avoiding the need to cut the cable .

Read: Using ferrite E cores for baluns


If your station is wired and grounded properly the balun in the PTT line is not needed. How to interrupt Ground loop 2 is described in the Manual. Ground loop 2 might increase the band noise in the SDR.