Howard, We already achieved successful, error-free, VHF communication (with no repeated blocks) using NBEMS software over a 70 mile path in flat country between two 50 watt FM transceivers, one with a 7.5 dBi antenna at 10 feet off the ground and the other with a 7.5 dBi antenna 25 feet off the ground. I have also developed a DOX interface for FM transceivers which have no VOX. A schematic is here: http://home.comcast.net/~hteller/Interface%20schematic.jpg
We are now in the process of determining just how much farther we can go using FM. However, using SSB with DominoEX, we have already reached 100 miles consistently between a 9 dBi antenna and a 13 dBi antenna. We think that a 100 mile capability is sufficient to reach outside connectivity for email or phone delivery and confirmation. If so, then VHF can be used most of the time. By using 2m, if the S/N is sufficient, we can also use phone and data interchangably on the same frequency, which is not permitted on HF. When the terrain is too hilly for VHF, NBEMS also supports Hf using NVIS antennas with several modes specifically tailored to work under very high static conditions. However, it obviously easier to put up a small beam than it is to always find supports for a NVIS antenna for portable use. A picture of my 2m portable setup is here: http://home.comcast.net/~hteller/sideview.jpg. By using a two section mast, everything will fit in the trunk or in the back seat. NBEMS does not support "push" emcomm email, because there is no confirmation of delivery. Instead, there must just be an operator present at each end of the link. This also helps prevent transmitting on an already active frequency. As you correctly note, VHF FM transceivers cost only a couple of hundred dollars instead of a thousand for SSB-capable transceivers, however, it is absolutely necessary to use horizontally-polarized, gain, antennas to go farther than a repeater can go. The portable station antenna is usually going to be near the ground, and at 10 feet off the ground, there is a huge 6 dB penalty to using vertical polarization. We are now changing the emphasis of NBEMS from SSB to FM with DominoEX in order to make it possible for more people to use NBEMS and also take advantage of the low cost FM-only transceivers in the field. There appears to be a 3 dB or greater disadvantage to using FM over SSB, even with horizontally-polarized antennas, but that can be made up with increased antenna gain or power. Phone will not work on VHF over the same long distances as DominoEX or MFSK16 will work, because the noise level is often so high, the voice just cannot be understood or even heard at all. However, DominoEX and MFSK16 can still decode when the S/N is 10 or 12 dB UNDER the noise level, and that is how we get such long distance communication on 2m. 73, Skip KH6TY NBEMS Development Team ----- Original Message ----- From: "Howard Z." <[EMAIL PROTECTED]> To: <digitalradio@yahoogroups.com> Sent: Tuesday, November 25, 2008 6:58 PM Subject: [digitalradio] Re: How Can We Push HF Emcomm Messages to the Field? Is the volunteer out of VHF range? If the base station has a 100 watt VHF radio like the 746pro - you might be able to still reach the volunteer, but he may not have enough power to get back to you. Or he may be out of VHF range. HF is the way to go - but both ends of the conversation need NVIS antennas. HF antennas tend to be large, and NVIS needs to be horizontal. I'm not sure there exists an NVIS antenna for a car or truck. Maybe something horizontal can be setup in the bed of a pick up truck? In general HF antennas for vehicles do not perform very well - but they are better than nothing. There are portable NVIS HF antennas available that can be setup rather quickly. Perhaps this is something to be done when he arrives at his destination, and then call the base on HF? Also keep in mind that HF radios typically cost over a thousand dollars compared to maybe two hundred for a VHF radio. Howard N3ZH
