Being a technology geek for me doesn't stop at writing code or hacking gadgets, it extends to even communications and specifically RF (Radio Frequencies). I'm a licensed HAM radio opeartor and have been for some time now. It's true that I love electronics but dang I hate that they require support from a fragile supply of electrons from the wall. When it comes to HAM radio and it's historical use as emergency communications for our nation, I wanted to build something that would allow my small 2M HTs to communicate over wider distances and be free from mains power. I wanted something portable, tactial, that I could use anywhere anytime I wanted. While this is certaintly not the first tactical simplex repeater built by someone, I think it turned out amazingly and much less expensive than something you would buy off the shelf.
First I had to create set of criteria or goals for the build:
- Cheap(ish) cost to build (< $400)
- Used for days without intervention
- Self contained and portable
- Support 2M frequencies
- Remotely enabled/disabled
- Maximize 2M HT distance
- Rugged enough to withstand the outdoors
- Ready to use whenever I needed them
My first idea was to use some Pelican 1300 cases I had laying around, free is always better than not. I also wanted to keep the batteries maintained while in the vehicles (these were portable devices I wanted to keep in each car). Unfortunately as you'll read below the original batteries I wanted to use (always do your math first) didn't provide me with the parameters I needed for talk time so I had to get larger batteries and thus had to ditch the Pelican cases and opted for the fatty .50 caliber ammo cans (not the regular .50 cal cans). These boxes are metal, water/air tight, rugged and cheap.
Unfortunately my first idea was to purchase 12AH 6V batteries which did fit in the pelican boxes but really didn't provide me with enough talk time, I was shooting for 12.5% duty cycle which equates to about 3 hours of transmit time a day and 21 hours of receive time a day. I calculated duty cycle using DC=100*(Time On) / (Time On + Time Off). I figure 3 hours of TX time a day would be suffecient in the case of an emergency but I also wanted to build in some padding just in case. Once I had calculated my duty cycle I needed to calculate the power consumption of my system in both TX and RX states. Using my handy dandy multimeter, I measured the power at RX (idle) to be 110mA and TX to be 1.4A. That means that my average power consumption over 24 hours would be 271mA. With this number is was trival to plug in the values to figure out what sized battery I would need, I used an online calculator that used Peukert's formula for calculating battery life. Ulitmately I decided on a 35AH 12V battery which would give me approximately 37 hours of runtime at 25% discharge (higher discharges ultimately shorten the life of the battery).
Now the task was to figure out how to replinish the juice in my battery, solar is the way and I settled on a Renogy 30W solar panel. Why you ask, well the panel gives me about 1.3A of power on a sunny day, it's somewhat portable, and Renogy sells great quality solar panels for outdoors.
As with any solar setup, I had to purchase a solar charger to ensure my battery was correctly charged by the panel and ensure the battery was never overcharged. The solar charger I picked up is the Renogy 30A model, more than enough for a single 30W solar panel configuration. The solar charger also has another nice feature which is that it has load terminals. These terminals allow you to hook up your load (the radio gear in my case) and the charger will automatically disconnect the load if the battery voltage drops too low. This particular model solar charger has about 20 different modes you can select for how it manipulates the load terminal (dusk till done, dusk then xx hours, etc).
With the battery and solar system figured out, I had a few more power conversions I needed to address. Namely the radio which has a max input voltage of ~8.4v, the repeater module accepted a wide range of voltages so our 12v battery voltage was fine for it. As for the radio, I had to purchase a buck converter that would effeciently drop the 12v to the ~8.4v required. I found a cheap one on Amazon which was rated for way more power than the radio would need and was manually adjustable so I could easily dial it to the voltage I required. I hacked up a battery eliminator cord for my radio and ran it directly to my buck converter, works perfectly and never gets even warm to the touch at 8W TX on the radio.
As for the radio (the most important part actually), I decided on a Baofeng, more specifically the BF-F9 V2+ HP 8Watt Tri-Power HT. I wanted to keep costs down, but also wanted the little bit extra TX power this version gave over the cheaper 5W models available. Baofeng is a manufacturer of cheap HAM radios (but have been surprisingly reliable) but costs containment is a goal so no Yeasu HTs or the like were an option. The decision of which simplex repeater (voice recorder and playback device) was easy, there are not many available from reputable supplieers so the ADS-SR1 from Argent Systems was my choice. It had great specifications and was easily managed remotely using DTMF tones. Fit all the criteria I needed.
The last tiddly bit was the connectors for the radio and the antenna selection. A female SMA connection is more common now that these chineese radio manufactures are using them but eBay was still the only source I could find that had them with various other ends. I chose a female SMA to BNC pigtail. The reason for BNC was simply because the antenna I wanted came in a BNC type connector and I really wanted a stronger attachment point that SMA. I wanted as good gain as I could but also try to maintain my goal of portability. It took me several days to find what I was looking for and I finally decided upon the MFJ-1714 telescoping antenna from MFJ. It's got great gain and works better for this setup than any others I could find. It collapses down and can easily fit inside the can but reaches ~40" when fully extracted. I'm really loving this antenna!
The build on this project was actually lots of fun. I was able to leverage my favorite tool, the hand riveter which I used to build a cradle for the battery which prevents it from moving around and damaging the other components. Other than that, it was a pretty simple process of drilling a couple of holes, one for the antenna and the other for the 12V cigarette lighter socket which is how the solar panel plugs into the box. A box of super strong Velcro from Lowes finished off my build, I simply secured the radio and other components to the sides of the interior. I'll post some pictures of the build later this week once I get a chance to get my new shack setup.
I've tested the range of this repeater on 147Mhz with it sitting on the top of my house. I'm able to clearly transmit and receive on a Waesu 5W HT from about 8 miles away with the repeater sitting on my back patio, elevation to remove any obstructions would clearly extend the range significantly but that's for another day. Overall I'm very happy with the build and the results, once I have an opportunity to get this thing higher in the air and perform a more extensive range test, I'll update this post.