I previously installed a 20-meter JYR end-fed antenna on the roof, and the four bands had acceptable VSWR values. Recently, there have been several days of light rain, and last night I tested my homemade VSWR meter while operating a radio. I found that the VSWR was very high, but when I disconnected the VSWR meter and connected the antenna directly to the radio, it remained the same. When setting the transmit power to 15W in CW mode, the radio showed a VSWR of approximately 3. This indicates that the radio's VSWR protection is limiting the transmit power to only 6W.
However, when I used the NanoVNA and the built-in standing wave scanner on my radio, both showed very low standing waves, around 1.5, which differed significantly from the actual transmission conditions.
Could you please advise me on the impact of rain on end-to-end antennas? Is it significant, and are there any ways to mitigate this effect?