John, Over what distance are you getting flutter or Doppler on VHF? I only get the flutter (usually all the time!) when I try to work Charlotte, NC from Charleston, SC on 70 cm, which is 173 miles away, but I am not far enough north for Aurora. For emcomm, we only need to span up to 100 miles. I am interested to know if you also find flutter on VHF within 100 miles.
Skip KH6TY ----- Original Message ----- From: "John Bradley" <[EMAIL PROTECTED]> To: <digitalradio@yahoogroups.com> Sent: Sunday, March 02, 2008 9:30 PM Subject: RE: [digitalradio] Re: Keeping NBEMS in mind > This may be true at lower latitudes, but up here at 50 degrees north, we > get > sustained aurora flutter or Doppler on HF and VHF. Sometimes the audio has > a > distinct echo. PSK125 and 250 are worse. > > we do have days where we have strong signals but cannot decode anything. > > it would be nice to have something a little faster than regular MFSK for a > robust mode > > John > VE5MU > > > -----Original Message----- > From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] > On > Behalf Of kh6ty > Sent: Sunday, March 02, 2008 4:18 PM > To: digitalradio@yahoogroups.com > Subject: Re: [digitalradio] Re: Keeping NBEMS in mind > > >> >> 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 > > > > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.516 / Virus Database: 269.21.3/1306 - Release Date: 3/1/2008 > 5:41 PM > > -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.21.3/1306 - Release Date: 3/1/2008 5:41 PM
