> > I have seen some multiipath, especially when I have tested PSK31 on VHF, > but much of that was from aircraft. I am not sure how I can discern > multipath when on HF. Is there any clue in the waterfall or do you go by > the sound? > > 73, > > Rick, KV9U
You will see three kinds of multipath on VHF, which you can see on the waterfall. One is reflections from airplanes, which tends to look like a ghost signal accelerating across the main signal. When it coincides with the main signal, all copy will be momentarily lost, no matter how strong the signal. The second correlates with wind conditions, and the ghost signal moves slightly in and out of the main signal during wind gusts, especially when a weather front is moving through. The third is reflections from fixed objects, and the ghost signal tends to stay a fixed distance away from the main signal. PSK63 is less affected by multipath reflections than PSK31 is on VHF, and PSK125 even less so. When cancellation does occur, if you are using ARQ, that frame is just resent and the transfer is delayed by that much. Of course, only ARQ is going to guarantee error-free copy. FEC only helps, but does not insure no errors. QRN seems to be the biggest problem on HF and QSB second. During a period of thunderstorm activity, as we often have in South Carolina, and more especially in Florida, PSK125 is greatly disturbed and PSK250 so much that it is unusable, but PSK63 not nearly as much. All the decoders seem to have this problem, and there may be a way to improve that cascaded loss of sync in the faster modes, due to QRN, but we have not yet tackled this problem. Fortunately, for our 100 mile emcomm uses, QRN and QSB are not problems on VHF, and ARQ takes care of the multipath reflection problem. 73, Skip KH6TY Announce your digital presence via our Interactive Sked Page at http://www.obriensweb.com/sked Check our other Yahoo Groups.... http://groups.yahoo.com/group/dxlist/ http://groups.yahoo.com/group/contesting http://groups.yahoo.com/group/themixwgroup Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
