It proved to be a nuisance while receiving weak signals using either of the shack antennas - a 1/4λ Ground Plane at a height of 15m and a Slim Jim at a height of 10m, located 5m apart.
The noise was presumed to be of commercial origin as it was not present at nights and on weekends.
Then, on an impulse, both the antennas were connected to the rig through a BNC-T connector. Surprisingly, the QRM disappeared and the audio level went up considerably.
The antenna feeder lengths were then changed one at a time. The QRM promptly returned, confirming that it was indeed phase cancellation at work.
SWR was 3:1 with both the antennas connected. Inspite of that, it was now possible to comfortably work through a distant repeater.
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