RF Systems Engineering, of sorts
My big work project right now involves setting up localized differential GPS base stations. I work for a John Deere dealer and we are trying to get some coverage for our area of responsibility on what is called "RTK", Real Time Kinematic correction. Basically, the idea is to put a localized differential GPS station up (like WAAS if you have a hand held GPS) - the units have a 1" accuracy at the GPS receiver out to a 12 mile radius (theoretically, based on propagation). You can read more of the systems and learn a lot more than I can put in a post here on John Deere's site.
My project has been piecing together the systems. The main components are coming from John Deere, so those are taken care of. However, power supply, back up power supply, RF routing, power levels, and ultimately building the the systems will be mostly my task.
This past week I have spent a lot of my time thinking about the systems and what should go in to them to get them on the air. We got the information filled out for the FCC license (coordinates, elevation, etc) so once we get out frequency and power allocation I can finish the EIRP calculations. Just for the mean time I plugged in some known component values - antenna gain and coax loss (the BRAND new 7/8" AVA5-50A from Andrew/Comescope/Heliax [replaced the LDF series, even lower loss] - the GOOD stuff!!). The rest of it we should be able to control from our radios. The power level will be adjustable between .2 and 2 watts with an amplifier of a max PEP of 50 watts.
Some challenges left to tackle - enclosing the components to where they are protected (in most cases from the weather - outside mounting) while allowing adequate cooling of the power supply. The amplifiers are not quite out yet, but they are supposed to be weatherproof for direct outside mounting negating the need for cooling (the whole case is a heat sink). The power supply is another ball game. I ordered some Astron RM-50M's for easy rack mounting. They will be the most versatile in our application since they can either be rack mounted or desk mounted. If we put it in a case it will go in a rack, however one location is inside a building on a desk more or less.
For stand-by power I am using two ~110Ah batteries in parallel and a PWRGate from West Mountain Radio. The PWRGate allows constant battery system maintenance with instant switching of the load from the power supply to the backup batteries. It actually is not powered itself, it takes power off of the power supply and passes that on to the batteries - however much it needs (you can set it for up to 10 amps of charge current). Once the voltage of the batteries comes up the charging current drops off until it gets to 13.8v. At that point the PWRGate switches to a float charge mode.
One aspect of this project that adds to the challenge is the fact that once these are on the air they will run 100% of the time at 100% load, except if the power goes out. We will have around 7 to 8 hours of reserve back up power. This is why the ventilation/cooling of the power supply is critical. I ordered one size larger power supply than I really needed (based on constant amperage) so they are running at a lower percentage load. This should help out on the reliability. However, if they get too hot, which can happen even at 60% or so load, they are going to fail.
All in all I have my hands full right now! This is a really neat project, though. I am very glad I can apply some of my ham radio know-how to getting these systems up and running. I don't necessarily know all the answers but I have a lot of resources to tap and know which ones to use - people, companies, books, websites, you name it. On top of this RTK project I still have to juggle piecing together the GPS systems that our customers run on their farm equipment (tractors, combines, sprayers, etc)...
My project has been piecing together the systems. The main components are coming from John Deere, so those are taken care of. However, power supply, back up power supply, RF routing, power levels, and ultimately building the the systems will be mostly my task.
This past week I have spent a lot of my time thinking about the systems and what should go in to them to get them on the air. We got the information filled out for the FCC license (coordinates, elevation, etc) so once we get out frequency and power allocation I can finish the EIRP calculations. Just for the mean time I plugged in some known component values - antenna gain and coax loss (the BRAND new 7/8" AVA5-50A from Andrew/Comescope/Heliax [replaced the LDF series, even lower loss] - the GOOD stuff!!). The rest of it we should be able to control from our radios. The power level will be adjustable between .2 and 2 watts with an amplifier of a max PEP of 50 watts.
Some challenges left to tackle - enclosing the components to where they are protected (in most cases from the weather - outside mounting) while allowing adequate cooling of the power supply. The amplifiers are not quite out yet, but they are supposed to be weatherproof for direct outside mounting negating the need for cooling (the whole case is a heat sink). The power supply is another ball game. I ordered some Astron RM-50M's for easy rack mounting. They will be the most versatile in our application since they can either be rack mounted or desk mounted. If we put it in a case it will go in a rack, however one location is inside a building on a desk more or less.
For stand-by power I am using two ~110Ah batteries in parallel and a PWRGate from West Mountain Radio. The PWRGate allows constant battery system maintenance with instant switching of the load from the power supply to the backup batteries. It actually is not powered itself, it takes power off of the power supply and passes that on to the batteries - however much it needs (you can set it for up to 10 amps of charge current). Once the voltage of the batteries comes up the charging current drops off until it gets to 13.8v. At that point the PWRGate switches to a float charge mode.
One aspect of this project that adds to the challenge is the fact that once these are on the air they will run 100% of the time at 100% load, except if the power goes out. We will have around 7 to 8 hours of reserve back up power. This is why the ventilation/cooling of the power supply is critical. I ordered one size larger power supply than I really needed (based on constant amperage) so they are running at a lower percentage load. This should help out on the reliability. However, if they get too hot, which can happen even at 60% or so load, they are going to fail.
All in all I have my hands full right now! This is a really neat project, though. I am very glad I can apply some of my ham radio know-how to getting these systems up and running. I don't necessarily know all the answers but I have a lot of resources to tap and know which ones to use - people, companies, books, websites, you name it. On top of this RTK project I still have to juggle piecing together the GPS systems that our customers run on their farm equipment (tractors, combines, sprayers, etc)...
posted by Steve at
7:06 PM
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