This antenna will be the transmit/receive antenna for the Digi m10 satellite data modem. Please see the main page for dates needed.
Suggest a J pole ant. This type of antenna does not require a ground plane. It can be made from light gauge Teflon wire. The transmission cable should be RG174 Teflon jacketed, with the proper connect (need to research) Teflon is good to -70C. A standard Amateur Radio 2meter J pole antenna will work. The 2meter band is from 144 - 148. Just below and above the sat comm TX and RX freqs. The antenna should be the below the ballast assembly. This will keep the RF as far from the electronics as possible.
A Dipole antenna will be built. The first design is two 1/4” wooden dowels for the vertical elements, a 3/8” dowel for the attachment to the payload. The connecting block is two pieces of 1/2 inch balsa wood glued together. The assembly forms a T. This can be mounted vertically or horizontally. The length of each element is determined by the formula of 234 / design frequency = each dipole side length in feet, 234/149 = 1.57 or a total length of 3.14 feet. For the first prototype 30ga magnet wire was used. This was connected to RG174. A SMA connector was used to match the satellite antenna connector. If the evaulation board is not used the connector will be MMCX male connector.
Future experiments will be with brass tubing and the center conductor and shield of the RG174 will be used for radials. Other materials for the center block will be experimented with. A laminated piece of ½ in balsa wood and 1/16in birch plywood and Lexan are two materials that will be tried. Most material is available in a 3ft length. Making an antenna that is 3.14 ft with 3ft material means either having to use two 3ft pieces cut to about 19in and having an unusable piece of 17in. The solution is to use the 3ft piece and add some higher gauge wire to the end of the radial. A 16ga to 20ga wire will be stiff enough to not need support and can be trimmed to obtain a good SWR. If the brass rods are used, the wire can be soldered to the open end of the tube.
Two items to note. RG174/U that meets MIL-C-17F specs has a temperature range of -40C to +80C. An example of this type of wire is Beldon Type 8216. A 100ft spool of this cable is under $50. The other item is a glue was found. Loctite Stick’N Seal Extreme Conditions has a temperature range of -50C to +120C. This is a multi surface glue. It was used to glue the two ½ in block of balsa wood together for the prototype antenna. The first antenna was checked at 149 MHz for a SWR of 1.2. At 148 MHz and 150 MHz the SWR was 1.3. The length of each radial was 1.6 ft. When building antenna’s the formulas give you a starting point. It is best to make the radials longer then needed and trim them back until an acceptable SWR is achieved.
Speedball 1 version. An T connector was designed and printed on the Maker Bot. The radials were made from .062 piano wire. The end was bent 90 deg. The piano wire must be sanded to allow the coax to be soldered to it. The braid was solered direclty to the piano wire. To solder the center conductor, a wire was soldered to it first, then center conductor was soldered to that. The end of the piano wire had part of a crimp on terminal put on the end. Be carefull not to change the lenght of the radial. After the connector is put on several layes of red heatsrink was put on. the was done for two reasons. One for eye protection and two to protect the balloon envolope. A 3/8” wood dowel was used as the support to connect it to the payload box. The dowel should be painted to seal it. RG174 was used to keep the weight down. A MMCX male connector is needed to connect to the modem.
This should be made as lightweight as possible. It can be oriented vertically or horizontally, attached to the exterior of the payload.
Keep in mind that there will be a vertical run of wires, going up from the payload box into the balloon, and possible a single wire dangling below the payload for an HF antenna.
The wiring going up and down from the payload may make a groundplane vertical hard to implement. A dipole may be sufficient. Low weight is key - use the smallest gauge wire possible, and shortest feedline possible. Avoid heavy PL-259 interconnects.
|Transmit Power||5W (37 +/-1 dBm|
|TX Freq||148MHz to 150MHz|
|RX Freq||137MHz to 138MHz|
|Sensitivity||>= -119 dBm|
|Dynamic Range||40 dB|
|Antenna Gain||>=0 dBi, in at least one direction|
|Antenna Impedance||50 ohms nominal|
For more details, see page 17 of the Digi m10 Satellite Modem Hardware Reference