Hi Rick, Thank you for your comments on Howard's and my posts.
Of course, we prefer using SSB on VHF, because the range is longer. First tests indicate that DominoEX with SSB has at least a 3 dB advantage over using FM with DominoEx. We are arranging more tests to be sure. However, the fact that today, maybe half of the U.S. amateurs hold only a Technician license, and do not have access to full HF priviledges, together with the fact that many hams only have inexpensive FM-only transceivers (but only a relative few may have VHF or multimode 2m transceivers with SSB capability), we have decide to explore ways that more hams can participate in emcomm activities, which means finding out how to use FM-only transceivers without repeater assistance. Although you have previously pointed out that many hams already have vertical antennas, the fact remains that a vertical antenna close to the ground (2 wavelengths), has about 6 dB less gain than the same antenna horizontally polarized. At VHF, a 6 dB disadvantage is an enormous disadvantage, plus many of the directive antennas used for FM are fixed on a particular repeater, and cannot currently be rotated anyway. Just model a vertically-polarized antenna over real ground at 2 wavelengths and compare the gain to the same antenna rotated 90 degrees to horizontal polarization to see the difference. In order to confirm Cebik's assertion about the gain difference, I did the modeling myself and found that he is absolutely correct. No difference in free space, but a huge difference over real ground. So, putting it all together, we can get significantly more range by simply investing in a horizontally-polarized antenna, using the same FM transceiver that people already have, and, better yet, in an inexpensive TV antenna rotator so we can communicate in any direction. The optimized two-element quad that we used for the FM/DominoEx tests (7.5 dBi in free space) can be built for less than $15 in an hour with all parts from Lowes, plus a SO-239 connector, and turned with a $60 Philips TV antenna rotator from Walmart, because its wind loading and boom length (13") is so small. A picture of the little quad is here: http://home.comcast.net/~hteller/OptimizedQuad.jpg. It is only 20" x 20" x 13", so it will fit in the trunk of a car without having to be dismanteled. Construction uses schedule 40 PVC, fiberglass "driveway markers" for spreaders, and #14 insulated house wire, so it is very rugged. I wish that all existing equipment could be used intead, but without a gain antenna and horizontal polarization, range without repeater assistance appears to be just too limited. It would be useful to know how much range you can get in your hilly rural area by using FM, DominoEx, and horizontal antennas on 2m. 73, Skip KH6TY NBEMS Development Team ----- Original Message ----- From: "Rick W" <[EMAIL PROTECTED]> To: <digitalradio@yahoogroups.com> Sent: Wednesday, November 26, 2008 3:38 PM Subject: Re: [digitalradio] Re: How Can We Push HF Emcomm Messages to the Field? > Hi Howard, > > If you respond to someone's response to a question, with asking > questions of your own, then it might be reasonable for some to respond > as Skip did. It seems reasonable to me considering you asked "Is the > volunteer out of VHF range?" You also asked about setting up something > in the bed of a truck and asked about setting up something on HF after > arrival at the destination. All good questions. > > While your particular job situation does not seem relevant to this > discussion, the use of VHF, especially SSB VHF does seem particularly > relevant since it is the only other way to get increased distance of > communication between a mobile and fixed/portable/mobile station if HF > is not workable. > > The most expensive HF equipment may of of no value when you are trying > to communicate between two points that do not have NVIS propagation. It > can be frustrating, especially during high QRN as well as the skywave > signal just going through the ionosphere and not reflecting back down. > For those experienced with Section level nets that only use 75/80 > meters, you know what I mean. > > Going higher in HF frequency doesn't work any better (actually shorter > ground/direct wave), and that is why STANAG systems won't work for > "local" communication. > > VHF simplex with FM and with minimal antennas are not going to go all > that far as you point out. In fact, in our area, it is difficult enough > for mobiles to repeaters. Sometimes 15 to 20 miles is the best you can > do in shaded areas. With 2 meter SSB, we seem to be able to still get > through when FM can not get through although signals can be very weak. > That is using half wave base to quarter wave mobile antennas. With > improved antennas, depending upon terrain, the distance is going to > extend out to as much as 50 to 100 miles. > > This is important because you reduce QRN problems from lightning static > and other noise (admittedly less likely though during a time when power > has failed), and you rarely would need that much distance for Incident > Command to the dispatched mobile. > > Bottom line is that HF may not be able to do it 24/7, but 2 meter SSB > may be the best choice. With today's relatively low cost > multimode/multiband rigs, the cost is around $700 or so for 50 watts on > 2 meter SSB. As you point out, these rigs are more expensive than 2 > meter FM, but tremendously more flexible and a very good value since you > also get an HF rig too. > > 73, > > Rick, KV9U > Moderator, HFDEC yahoogroup
