Portable APRS Station
I have been wanting a complete portable APRS station for casual and emergency use for some time now. This would allow me to have a complete station to bring with for events like Field Day, Hamfest, Emergency Drills, you name it and its ready to go for it. My main goal though was for it to be as self contained and portable as possible to eliminate having too many items that could get lost or misplaced in transport.
When considering what equipment to use I started to make a list.
1. VHF Radio - I went with a Motorola Maxtrac as
that's what I had handy and
readily available.
2. Tracker - I decided to purchase a
Opentracker + from
Argent Data, mainly due to cost for a unit that can be a tracker as well a KISS
interface.
3. Power - I wanted both battery and a small power supply that can
go with. The unit itself has a 7ah gel cell battery which is charged off
the power input, making it capable of being run and charged at the same time
from any 12-14v DC source. Power outlet in a vehicle, AC outlet and power
supply, solar panel, whatever is made available.
4. Antenna - An attached
antenna to make it truly portable, but also the ability to easily connect an
external antenna when needed to increase coverage and range. I went with a
simple 1/4w antenna cut to length for 144.390 as well as a NMO to UHF adapter
that lets me quickly change from attached to external antenna.
5.
GPS - Of
course you need a GPS unit, so in this case I purchased the
ADS-GM1 from Argent
Data, an excellent SiRF III chipset 20 channel based GPS that gives excellent
performance for its cost. Gets a 3D lock holding 5-6 satellites in my
basement shack, impressive!
6. Enclosure - Waterproof or at least weather
resistant to protect the unit when being used standalone as a portable unit.
I wanted it to literally be able to sit out in the rain if the worst happened.
I had an old 50 caliber surplus ammo can that seemed to fit the bill nicely!
So now I had my list and it was time to start building. I didn't take
detailed photos along the way for a good reason, this article is merely to
illustrate what can be done, the enclosure is up to you. There is many
choices here, from simple to elaborate. So be creative! Here you can
see what I ended up with. There is a NMO mount through the top for the
antenna connection, the GPS is simply stuck on the can for the photos as its a
magnetic mount. The gps can be positioned wherever its convenient.
The back has all the connections for power, and serial. The serial port allows connection to the GPS receiver or a computer via a null modem cable. The binding posts allow DC input to be connected from any sourced, fused internally. Many would say "what no powerpoles", NO.. First off I hate them with a passion. Second I want to be able to make any connection that is necessary, from a power supply to a chopped wall adapter to a hank of wire to a car battery, I want VERSATILITY.
Inside the unit... You can see the radio on the right, the tracker in the middle, and the blue foam to the left covers up the gel cell battery. When not in use the USB to Serial adapter and all the cables go on top of the blue foam covering the battery. There is also a NMO to UHF adapter stored in the foam near the front. In the back there is a bridge being used as an isolation diode for the battery and external power input, and a large power resistor which acts as a current limiter to float charge the battery. This is a very simple setup, if you wanted to though you could easily build in a smart battery charger. I didn't feel it was necessary at this time. The Opentracker is configured to not allow transmit below a certain voltage, all done in the configuration program, this allows it to run on battery without fear of pulling the battery too low on transmit. Once the battery reaches that point it will no longer transmit until an external source is connected. For me the battery is merely a way to keep everything running in-between power sources, transferring from vehicle to stationary, etc. Like having a UPS to keep everything running.
Complete unit... Ready to travel, just stow away the cords inside and connect the gps up to the back and its good to go.
Power is being supplied by a small 3 amp power supply which is float charging the battery and running the unit in this picture. It fits in the laptop's bag and doesn't add a lot of weight.
The computer doesn't have a serial port, therefore I had to supply one. I use an FTDI chipset based unit that gives you two serial ports and a 5v output. These units can be found on eBay for $15-$20 and are well worth the investment. I have been using one for years in the shack for interfacing to radio keying interfaces for PSK and many other uses. The FTDI is the best chipset out there and behaves like a hardware serial port unlike some adapters that will do nothing but provide data. I made a quick pigtail that has a DB9 male to a DB9 female to separate out the power leads to feed the GPS receiver. Argent Data also sells a similar cable that gives you the same ability, in my case I felt it was easier and cheaper to just build one myself. The other important factor here is this allows me to connect the laptop to both the GPS and Opentracker at the same time to use with UI-View or any other mapping application. It also has RX and TX led's showing status while in use.
The laptop running UI-View with the Opentracker using KISS firmware, this allows me to use it with the computer to send and receive, acting as a digipeater, igate with an internet connection, weather station, and the ability to send and receive messages. I can also disconnect the computer and connect the gps receiver to the back of the unit and its instantly a portable tracker, using the settings programmed into the Opentracker.
Also since I have a GPS providing data to the computer any program that is GPS enabled can now make use of it. Storm tracking is another application. With GRLevel3 running and the GPS plotting my position on the radar image I can use UI-View to place objects on my current position. For example.. Lets say you had important weather data to relay and you also wanted to post an object on the map for other APRS users, you can have UI-View running, click on that, click place object, use current location, and put a warning or notice on the map when you spot something important. To be able to do this you need to run Franson GPS Gate software and create a virtual serial port for each application, this splits the data and sends it to each program. Works great!
This site was last updated 07/19/10