A VHF 1/2-Wave Center-Fed Dipole Antenna
for HAB Back-Up Tracker Use

January - 2015







Continuing on from A 1/2-Wave Center-Fed Dipole Antenna for HAB Use . . . .

We've had too many close calls with only using one tracker, having it fail for whatever reason and not being able to locate and recover a payload and now always use 2, especially if one is located with other electronic items in  the payload box, like a camera, that often generate RFI which can interfere with reception of the GPS signal that the tracker requires.

Our backup trackers are enclosed in their own insulated container and are simply tied on several feet below whatever else is being flown on a particular flight which not only ensures that other equipment won't affect the tracker, but that the trackers transmissions will not affect other equipment as well.

Right - BEAR-9 payload with a back-up tracker suspended below.

We've been using RTrak-HAB trackers and following is how I've been using them to make back-up trackers, but the same could be done with most any tracker hardware.

A 1/2-Wave Center-Fed Dipole Antenna is used, much like the inline element ones shown on the 1/2-Wave Center-Fed Dipole Antenna page, but in this case the VHF antenna SMA connector on the tracker is removed and the antenna elements are simply soldered directly to the tracker PCB where the VHF antenna connector had been as shown below. Several layers of heat shrink tubing are also used over the elements where they cross over the board to ensure they'll never be able to short to the board.

The SMA GPS Antenna connector is also removed from the top side of the tracker PCB, turned 180° to face in the opposite direction and then re-installed on the bottom side of the board.

In use the antenna elements are positioned vertically with the active element below the ground element. Eye protection (as described on the previous page) is provided at the tip of the lower active element, but at the tip of the upper ground element a loop is formed that's used to tie the lift line from our payload box to. When making the upper ground element you need to start with a length of music wire several inch's longer than for the active element and make the loop far enough from the end to leave a straight section ≈ 1-½" long after the loop so the loop can be soldered closed as shown. Solder alone isn't very strong plus one needs a way to keep the loop tightly closed while it's being soldered so it's tightly wrapped with a length of bare wire-wrap wire first for a very strong easy to solder joint.

A hot wire foam cutter was used to slice some 6" wide x 5" thick foam blocks of the needed length into 2 halves of the proper thickness to end up with the antenna elements centered. Strips of card stock taped to the sides of the blocks were used to guide the hot wire cutter along at the proper depth.

I was making a number of trackers so card stock was also used to make a template for where the foam needed to be cut out for the various items to save having to layout the pattern more than once.

The template was taped to the thicker foam half and the cut outs were made using a soldering gun with a foam cutting tip made from #12 solid copper wire. The tip gets much too hot if powered for more than a few sec's so the temperature must be controlled by repeatedly pulsing the trigger on and off.

The elements can't simply be pressed down into a slit cut in the foam where they exit the enclosure as the elements will create a large cone shaped hole in the foam as they swing about.

To solve this problem, several pieces of heat shrink tubing of increasing size are used to fit a 3" piece of 1/4" plastic tubing onto the elements where they exit the enclosure. The tubing is then pressed down into a 3/8" wide slot cut in the foam for a nice tight fit followed by a 1/4" wide piece of foam to fill in the remainder of the hole.

Below is our first
back-up tracker with
a MicroTrak that had
cone shaped holes
cut out of the foam
at each end by the
elements where they
exited the enclosure.

Below is the cone
shaped hole in one
end of the tracker
shown above.

After placing the 1/4" tubing sections on the elements, the 90° bend in each element where it will be soldered to the PC board can be made at the point that will make the length, from where the active element will be soldered to each tip, 19-5/8". Once bent, the excess length after leaving what's needed for soldering each element to the PCB, can be cut off. Below is a tracker ready to be installed in it's foam enclosure and, below that, is the completed tracker ready to have the upper half of it's enclosure placed on top, the two halves taped together and the tracker used for a flight.

I wouldn't suggest also making and using a loop at the end of the bottom active element to tie on additional items as the soldered PC board connection would likely fail and you would lose what was tied on as well as the tracker signal. We have sometimes used 2 of our back-up trackers on a flight with one as the primary tracker between the parachute and payload and one as the back-up tracker tied on below the payload, but we use one continuous length of lift line from the parachute to the payload with the primary tracker simply connected to the line by tying the line through and to the trackers upper element loop and adding a few wraps of tape around the line and lower antenna element to simply keep everything in-line.

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