Heavy Noise Across 15 Meter Band, Any Idea What It Could Be?

Heavy Noise Across 15 Meter Band, Any Idea What It Could Be?

About five days back, I noted this extensive noise across the 15 meter amateur radio band. It persisted for quite some time and then subsided. I was using my modified 40 m inverted V antenna on 21 MHz, presuming that it will resonate as third harmonic. Though the VSWR was higher on 15 m than on 40 m, it was well within the tunable limit of my radio. I am yet to hear any station on 15 m. Not sure whether it is because of the high band noise, which is not so bad at all times. Or is it because my 40 m modified inverted V is not good enough for 21 MHz?

On usual days I have a thick band of noise around 21.3 MHz. It is not like the usual digital signals which I see as a narrow band on the waterfall, often on 21.074 MHz.

I do not have a balun at the feedpoint as the purchased one is still to reach me. Feedline is 20 m of HLF 200 cable. Actually it is a pair of two 10 m cables joined together with an SO 239 barrel connector. I have used a few turns of cable as a ‘common mode choke’ tied to the antenna mast. The antenna has two masts, one a conducting mast of 6 m GI pipe, about 1 m from the feedpoint and another non conducting 3m CPVC pipe near the feed point. Second support was given to prevent sagging of the feedpoint. Ends are tied to metallic compound fence frame using nylon threads of short length, separating insulated antenna wire from the frame. This is of course a compromise configuration to fit in my small space.