This has happened three times now on three different nights and three different contacts. Here's how one instance went: I was calling CQ on 40m PSK31 and Fred, K5QBX was trying to reply. His signal looked sort of okay but just wasn't enough to read. It took two attempts before I even got his callsign. It was enough for a contact but not enough for a decent conversation. It was late and the band was empty so he switched in the amp and dialed it up to 500 W PEP. His signal was still perfectly clean on the waterfall and IMD was -21 dB, and now I had him at about 98 percent. So then I suggested, just for fun, switch to PSK63 and drop back down to barefoot. He did, and guess what? I had him at 95 to 98 percent. He had never tried PSK63 before and was very pleased. We had a long chat.
I've observed this same phenomenon two other times where on a weak contact on a noisy and weak 40m band at night, PSK63 provides better readability than PSK31. Of course, this makes no sense. PSK63 should be 3 dB poorer than PSK31 and yet I observe the opposite and then some.
I am at a loss to explain this effect. Perhaps the noise or path characteristics on 40m are such that PSK63 modulation is harmed less than PSK31? I'd be interested to hear any theories about this and maybe someone can duplicate my results.
73 de KW2P
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