Summer 2008 Issue

Up in the Air

Field Day Balloons

By Bill Brown, WB8ELK

Photo 1. Argent Data Systemís ADS-SR1 simplex repeater. (Photo courtesy of Argent Data)

Field Day is a great time to demonstrate the amazing capabilities of high-altitude balloon payloads. Each year I try to fly something different that we can show to the public at the Huntsville Amateur Radio Clubís Field Day site. The site is uniquely positioned in a field next to Space Camp, which always attracts quite a number of curious Space Campers and their families. Iíve found that balloon flights into near space definitely get their attention.

Since Field Dayís main goal (in addition to the goal of eating tons of great food!) is to demonstrate emergency communications, this year I flew a 2-meter FM simplex voice repeater. From its vantage point over 100,000 feet above the Earth, this repeater would cover a great majority of the Southeast and Midwest. What better way to demonstrate wide-area emergency communications using low-power ground stations?

Simplex Repeater

A simplex repeater is a fairly easy thing to put together, consisting of just two things: a handheld radio and a voice record/playback unit. A few years ago, RadioShack sold a great module that made this a plug-n-play solution. Sadly, it discontinued the unit, so Iíve been relying on the increasingly rare eBay find for these. Fortunately, Scott Miller, N1VG, of Argent Data Systems ( stepped in to fill the gap and introduced the ADS-SR1 simplex repeater module at the Dayton Hamventionģ this year (see photo 1). Scott added quite a few bells and whistles to his version. Through touch-tone commands, you can set up voice and CW ID announcements, voice-mailboxes, and a host of other features, including up to 218 seconds of record time. However, due to the amount of traffic expected through the repeater, I chose to limit the record and playback time to 24-second intervals. Just hook up the interconnect cable from the ADS-SR1 to your handheld radio (I recommend using Evereadyģ lithium AA batteries), adjust the audio levels for best audio quality, stuff it all into a StyrofoamTM box with lots of duct tape, and youíre ready to fly.

Although you can use the simplex repeater module with any HT (Argent makes a variety of interface cables for various HT models), Iíve had great results with several of the Alinco family. The dual-band Alinco DJ-C7T is very easy to interface and is very lightweight and rugged. Since it has an SMA connector, I find that the Comet SMA-24 whip antenna works well with it. However, I do recommend powering the DJ-C7T from an external 6-volt lithium battery pack for maximum operating time and to also increase the output power to 500 mW. Another Alinco radio Iíve had great success with for balloon flights is the very inexpensive and lightweight DJ-S11T. You wonít need an external battery pack for this radio, since itís already designed to use internal AA batteries. I once lost a payload carrying two of these radios. Seven months after my flight, a hunter found it lying in a mud puddle covered in fire ants. I powered up the radios and they worked just fine after months of abuse.

However, I do have one recommended modification if you plan to use a DJ-S11T. The tiny whip antenna that comes permanently installed in the radio is terribly inefficient. I usually remove it and just solder two 19-inch wires to the antenna pads inside the radio and then tape the wires to a small wood or carbon-fiber dowel rod taped to the side of the radio to create a half-wave vertical dipole. You could also just install an SMA connector instead to use an antenna of your choice. In either case, youíll be amazed at the difference. Photo 2 shows the DJ-S11T with the vertical dipole modification.

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