And further, this thought should be considered as VHF FM, or VHF SSB? In a base/mobile or mobile/mobile environment, SSB on VHF works over much greater distances.
With voice communications, VHF SSB benefits from having flutter resulting in the desired signal amplitude going up and down, while the background noise level is held pretty constant, by the AGC in the receiver. FM is opposite in that regard - when the signal gets weak, the background noise level comes up, at least until the squelch closes. In my perception, I seem to be better able to fill in the gaps in syllables when the signal drops out, than when it's filled with noise. VHF SSB also has the benefit of probably not requiring the mobile station to have to take time to set up an antenna. If the mobile station is parked in a null, chances are that moving the vehicle a few inches will change a multipath situation enough to provide good copy. If there's benefit to be had by setting up a portable (v.s. mobile) antenna, putting a VHF omnidirectional stick up 10-20' is a pretty trivial task. While there can be benefit to be had by using horizontal antennas, unless you're into serious weak-signal work, it's not necessary to realize large gains in coverage, even using omni antennas on both ends, using SSB. Digital modes that are designed to work well in weak signal circumstances on HF SSB rigs will similarly work well on weal signal VHF SSB rigs, because the same "linear-mode" technology is involved. Probably the biggest caveat to that will be frequency accuracy and stability. Radios on a net will need to be well warmed up, or have high stability oscillators, if they are operating unattended, and expected to be able to be received by the sender. I've encouraged those that are working on upgrading our regional hospital network to use the IC-706's that they already have set up for HF pactor, to try VHF pactor using the SSB mode, as a way to gain from the more readily available spectrum, so they don't have to compete for access to the very few frequencies available on HF for digital operations. It'll be interesting to see how they do. 73, Bob, KD7NM -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Howard Z. Sent: Tuesday, November 25, 2008 3:59 PM To: digitalradio@yahoogroups.com 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 --- In digitalradio@yahoogroups.com, "expeditionradio" <[EMAIL PROTECTED]> wrote: > > The following questions are asked to the amateur radio Emcomm > community... how can we work together on this? > > THE TYPICAL SCENARIO > It is a dark and stormy night... > You are an amateur radio operator, volunteering with a relief > organization, for communication to set up shelters in a hurricane > disaster. > > There has been no power in the area for 24 hours. > There is no mobile phone service, and all the VHF/UHF repeaters and > digipeaters in the area are out of range or out of service. > > It is 3AM. You are driving in your vehicle, half-way to your first > shelter destination, making your way on back roads. The main highway > is flooded. You use your chain saw to pass a downed tree. The road > ahead looks worse. > > THE CALL > The relief organization wants to call you now. > They have new information since you left on your mission, and they now > want to change your destination, to divert you to another shelter > location not far from your route. They want you to give the workers at > the other shelter a list of supplies that are on the way. They want > you to check the shelter's status. They want to know where you are, > and if you can possibly divert to the other shelter, so they won't > need to send out yet another expedition to the other shelter. > > THE QUESTIONS > How will the relief organization call you? > How will they get the actual message to you? > How will they know where to route the message to be sure it gets to > you? > How will they get urgent feedback from you? > > THE BACKGROUND > In the past, Ham radio has generally been very good at a "One Way > Traffic" situation. > > We can initiate messages. > We can pull messages into the field using automatic email systems. > > It is easy to send messages initiated from the field. But, not as easy > to call someone in the field, unless the operator in the field decides > to actually initiate some sort of 2-way contact. > > CAN WE PUSH MESSAGES? > > What about pushing calls and messages to the field? > > What are the types of ham radio methods presently in place to call > hams in the field when the ham in the field doesn't initiate the > contact? > > What are the existing techniques, and how can these be improved? > > How is the ham in the field alerted to a call? > > Can we devise standard method(s) for routing Emcomm "push messages" to > the field? > > Is ham radio HF viable for pushing messages? > Can we make the call day or night, without prior notice? > > Bonnie VR2/KQ6XA > > P.S. In case you are wondering, the scenario above was taken from the > Katrina Hurricane Disaster. > > . > ------------------------------------ Announce your digital presence via our Interactive Sked Page at http://www.obriensweb.com/sked Yahoo! Groups Links
