At 07:45 PM 09/29/08, you wrote: >I apologize if this has been addressed previously, or even close! Do >not have a substantial amount of time to complete this research! > >Situation: An event in a remote area, one hill top is well enough >that coverage at around 10 watts VHF for the repeater could cover most >of our Aid Stations, 2 with HT's, the other 3 with Portables at 25 >watts or so, could get into the repeater! > >In the past, we have used crossband UHF in, VHF out, and it worked ok!
Can you continue to use crossband, or do you have to go inband this time? >Except for issues with a couple of HT's not being able to cut the >input out during transmit! ??? I thought you wanted to talk on the input? >My goal, is to design a lite weight, low >power consumption (i.e. fewest batteries possible, as the hill top is >only assessable by hiking or horseback! Fewer batteries means lower power out, or lower battery life. You've not said how long this operations is going to last? . At some point the weight of an additional battery is more than a 600 watt generator and a few gallons of gas (about 8 pounds per gallon). The genny can be run for an hour now and then to recharge the batteries. If you are going to pack the repeater into a site on horseback for a multi-day event it might make sense to lead a second horse that is carrying a small genny and a jerry can of gas. See <http://cgi.ebay.com/600-800-WATTS-GENERATOR-GONI-GN-1996_W0QQitemZ250167777951QQcmdZViewItem?_trksid=p3286.m20.l1116> >What I have on hand: > >Single band 2m HT for receive >Single band Yaesu 2M FT2800R for transmit @ 12.5 watts! >Pair of homebrewed 2 m aluminum j-poles > >Need to acquire: > >simple controller - NHRC-2 looks workable! If there is going to be a ham on site at the repeater then you can get by with a carrier delay timer and a timeout timer and no remote control shutdown ability. >batteries - based on estimated power consumption of final configuration! Double what you think you need, or take additional gasoline. The receiver and controller can have as much or even greater impact on the battery power requirements as the transmitter. Look at the DC current requirements of a modern synthesized radio compared to a crystal controlled radio and you will see why 90% of the solar powered repeaters are crystal based. I have a crystal based 2m receiver that draws less than 20 milliamps at 12v. The old Motorola HT200 was about half that. One thing you can do to maximize the battery life is to think of ways to minimize the key down time. Tell the users to pause, think, plan the entire transmission in their mind, then push the PTT button. No pressing, then thinking, then saying Ahhh, errr, ummm, etc. as that wastes repeater batteries. And remember you can't drain the batteries down to zero - some battery chemistries are such that if you go below 25% charge you damage the battery. >Basically the question is: at a 2 mhz seperation (odd split) on VHF >can one get away without using duplexers (cans), utilizing separate >rx/tx antenna's, spaced about 20 ft apart! Without desense or other >issues! See <http://www.repeater-builder.com/antenna/thoughts-on-isolation.html> and <http://www.repeater-builder.com/antenna/separation.html> Read both articles twice. 20 horizontal feet of separation and no vertical separation will not be NOT enough for an in-band repeater. The distance needed depends on how much isolation you can get. Keeping the transmit antenna radiated energy out of the receiver antenna pattern helps a LOT, zero leakage coax and and a quarter wave stub can work wonders. See <http://www.repeater-builder.com/ge/datafile-bulletin/df-10002-01.pdf> (but use better cable than RG-8). At 2m you will find that 40 or 50 feet of vertical separation is worth a lot more than 500 feet of horizontal. Do what you can to maximize the vertical antenna separation. Twenty years ago I saw a portable military 60 foot guyed mast with the receive antenna AND a Motrac receiver at the top, and a multiconductor cable carrying power, audio and COR strapped to one of the guys. The receiver was up there to keep it out of the transmitter antenna radiation pattern. The transmit antenna was 10 horizontal feet away at the top of a 10 foot pole. The transmitter had a single pass cavity on the TX frequency. >Being such a remote area, and the nearest 2 meter repeater is well >over 50 miles away, and nowhere near these frequencies - 145.500 >rx/147.500 out using a tone of 179.9, I do not see any interference >issues there! Why run tone if yours is the only activity? It's one more layer of complexity you don't need. Let the repeater play in carrier squelch - that way if you have desense or any other problem you will hear it. A tone decoder does not improve receiver sensitivity or selectivity, it just hides the interference - the interference that you need to eliminate to make the repeater work. If your local transmitter is the only one around then you will find that your local RF cabling, your receiver front end (or lack of one), and your transmitter noise are the limiting factors. Why not run more repeater offset ? The further away you are the more isolation you have - and it's free. Here in SoCal we have a special portable 2m repeater pair coordinated - it's 144.93 out, and 147.585 in. That's over 2.5 mhz of offset, and you can get just over 3 mhz if you use 147.99 as your input. From what I'm reading in your posting this is a temporary / special event system and your use of a 144 channel for an output and 147.99 as an input shouldn't be a problem. Just avoid 144.39 and the weak signal and satellite allocations. With a 2.5MHz or more offset you can fit an entire GE MVP repeater, a 12v 18ah gell cell AND a 2mhz duplexer into a backpack. And I've seen it run on a single antenna with no desense. The secret is the GE MVP mobile radio - the receiver has a real helical front end, and can be tuned to be very narrow. The transmitter is very clean. Both the receiver and the transmitter are crystal controlled, so there is no synthesizer noise. See <http://www.repeater-builder.com/ge/mvp/no6bmvpconversion.html> You state above that you have an HT and an amateur grade mobile to work with. Most modern HTs are super wide and have ZERO rejection from strong in-band signals, and adding a pass cavity in front does nothing because the signal goes in through the unshielded plastic case. Modern mobiles that hear equally well from 136-174 MHz are no better. You need to find a better set of radios or set up a split site with a much greater distance between receiver and transmitter. >Observations, suggestions, and your crazy are appreciated! > >Thank You, > >K1STX If your hilltop geography is such that you can separate the receiver and transmitter by a good distance that will help. I once set up a split-site portable UHF repeater with four borrowed 100-foot extension cords set up as two 200 footers. One cord carried audio, the other carried PTT. A balanced-to-unbalanced audio transformer at each end of the audio cord got rid of the hum picked up by the unshielded cord. It took 12v at one end of the PTT link to reliably key a 6v relay at the other end due to the DC voltage drop in the cord (the controller was at the receiver end, and its PTT output keyed a relay that put 12v on the PTT extension cord, which were connected to the coil of a relay at the other end. Normally open contacts on a 6v relay at the other end keyed the transmitter PTT circuit. The geography was a ridgeline overlooking a valley. The receiver and transmitter antenna footprints were different, but the required service area was encompassed by the overlap, so it didn't matter. Don't forget the downtilt. And don't try to duplex on a pushup mast. Mike WA6ILQ
