Dual-Band Center-Fed Folded Dipole
Antenna for High Altitude Balloon Use
May - 2000
A lot of time was spent researching, learning about & building antennas to find the most suitable one to use for BEAR-1, but without something like the Bird antenna analyzer I was able to borrow it would have been impossible, or at least taken an impossible amount of time, to successfully build any.

This photo shows the gamma match on one of the numerous antennas of different types I built & tested. The dipole is 3/16" dia. aluminum rod, but, except for the folded half-wave dipole antennas I ended up using, this and every other type of antenna and method to match them to 50 ohm feed line all required difficult adjustment and were sensitive to nearby objects.
The decision to build & use 1/2 wave folded dipole antennas was easy after finding they were the least affected by nearby objects, required no tuning other then having to be made the correct length, one of the easiest to build, adjust, make light weight plus, being dual-band, good for cross-band repeater use.

I simply used 300 ohm TV twin-lead feed-line placed between (2) 3/16" x 3/8" balsa wood strips to keep it laying flat and then spiral wrapped the sandwich with thread to keep everything straight and in place while wrapping 2 layers of 0.6 oz fibreglass cloth around the assembly and applying epoxy resin to hold it all together, protect and hopefully provide enough strength for the antenna to survive whatever landings may bring.
A 4:1 balun from a TV mini-match worked perfect to match the 50 ohm coax feed line to the 200 ohm antenna. 5 minute epoxy glue was used to attach the balun and then more was applied, in workable amounts, until the balun and all of the fragile wiring was completely embedded in epoxy.
To further protect the balun, wiring and provide better strain relief for the feed line, several layers of fibreglass cloth and epoxy were then applied over the embedded balun and wiring.
The length of the folded dipole was calculated using the standard formula and after soldering the two twin-lead conductors together, at what will become the bottom end, one of the conductors was cut at the measured half way point for the balun connection. The upper half end of the twin-lead was cut and joined about 1/2" further from the center then calculated however, to allow tuning the antenna by slowly trimming the upper end shorter while checking for the best possible match.

The velocity factor of different twin-lead feed lines varies plus the balsa wood and amount of epoxy used affects the VF so expect to make and perhaps have to toss several antennas before finding the exact length of dipole needed for the best match in your particular case.

 

It took several attempts and building a number of antennas before finding the exact length required for a perfect 1:1 match at the VHF frequency each antenna was for and the match only increased to 2:1 with the antennas laying flat on wet grass. The antennas also work well for UHF and the cross-band repeater antenna also had a very good match at the UHF frequency it would be used for.
Left -
The BEAR-1 payload box hanging in the back yard during equipment tests.
Right - Capsule Interior with
APRS tracker and space for a
cross-band repeater, camera
and other future equipment.

The 2 antenna feed-lines for the
APRS tracker & Cross-Band Repeater are laid in grooves that allow installing
the lid and maintaining a good seal.

Lower Capsule Dowel
and
Antenna Bottom End Connection
Upper Antenna Dowel
and
Antenna Top End Connection
Upper Capsule Dowel
and connection
where it meets with the antenna.
The antennas are installed using elastic loops at each end made from rubber strips cut from a bicycle inner tube. The loop is part of the antenna at the lower end where it's slid over the lower capsule dowel and up against a fiber washer 'stop' that's been epoxied in place. Then a second fiber washer is slid on and glued with epoxy, but not so much to not be able to remove the washer and antenna if need be. The loop at the top end of the antenna is part of the dowel keeping the 2 antennas properly spaced and the loop is simply stretched over the antenna and kept in place by the snug fit. Between the ends of the antennas, the ends of the upper capsule dowel have a small phenolic board which is epoxied into a slot and has a cut-out that the antennas simply sit in.


 


 

I enjoy trying to make things as light as possible for HAB flights and this is a partially finished balun that wasn't embedded in epoxy glue and only a fibreglass cloth and epoxy resin shell was used to attach and protect the balun to see how much lighter this would be. However the shell was much harder to create than expected and it simply didn't save enough weight to be worth all the extra time and effort required. The shell also wouldn't have provided nearly as good of a strain relief for the feed line.
 
To BEAR Home Page