The TeleLCO "Slim Jim" Antenna

Several people have asked what's inside the PVC radome of the "Slim Jim" style antennas we deliver with the TeleLCO component of our TeleLaunch wireless launch control system. The kind of antenna we provide is called a "Slim Jim" in the amateur radio world, it's a variation on a class of antennas often referred to as "J-Pole" antennas.

Because there are a number of issues involved in antenna design that come into play with this antenna, including a significant de-tuning effect from putting antenna elements inside a radome made from a material with the dielectric coefficient seen in small-diameter PVC pile, we decided to explain exactly what we build in case our customers have reason to want to just make another antenna for use with a TeleFireEight or to replace the one shipped with their TeleLCO.

If the materials and dimensions we use are copied exactly, the antenna should "just work". Note, however, that the real world being what it is, each antenna built is likely to have slightly different performance characteristics, and so if you or a friend have access to suitable test equipment, feel free to go completely nuts optimizing an antenna using your specific materials and techniques based on this design!

Even if you don't have access to a fully-equipped RF test bench, each of our TeleLaunch products has built-in RF debug tools that you can use to verify the performance of your system, compare the performance of different antennas, etc.

Like many documents we provide, this work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Basic Design

Bdale started with this calculator, adjusting dimensions to achieve a reasonable match and good operating efficiency at 434.5 Mhz in a PVC radome. The result is a relatively inexpensive, reliable antenna that is robust enough to survive typical launch site handling.

Required Materials

For the PVC pipe, we use 1/2" thin-wall pipe rated for "315psi" found on the shelf at our local Lowes home improvement store, not the thicker-wall schedule 40 pipe. This is because the ferrite chokes fit better in this pipe (you don't have to remove the plastic holder around the ferrites). If you change this material, all dimensions may need to be adjusted due to a difference in dielectric coefficient.

The 300-ohm twin-lead we use was once super-common for use with outdoor TV antennas. Check with local ham radio folks or other friends to see if you can find someone with some still on hand.

15" length 1/2" PVC
1" length 1/2" PVC
1/2 PVC tee
2 pieces 1/2 PVC cap
10'+ RG-58 coax with BNC, cut 25' BNC cable assembly in half
16-1/2" 300 ohm TV twin lead
ferrite choke assembly
electrical tape
nylon zip tie
PVC cement

Prepare PVC Sub-Assemblies

drill 7/32" hole in the middle of one end cap
glue this cap to one end of 1" PVC pipe
glue the other cap to one end of 15" PVC pipe
glue one side of tee to the other end of 15" pipe

Prepare Twin-Lead Sub-Assembly

cut 420mm of twin lead
cut approximately 6mm insulation away from one end
bend conductors and solder together
repeat at the other end, such that end to end, the loop is 407mm
mark 14.5mm from one end for coax attachment point, cut away enough insulation to be able to solder to conductors
mark 133mm from the same end, and 263 mm from the other end, leaving two marks approximately 11mm apart
cut one wire only at these two marks, trim insulation to get the 11'ish mm piece of wire to go away

Final Assembly

slide drilled end cap + 1" PVC sub-assembly over wire
strip and trim coax end for attachment to twin lead
solder center conductor to long wire on twin lead, shield to the short wire, at the marked feed point 14.5mm up from the bottom
put ferrite around coax close to feed point
put a zip tie around coax to act as a strain relief against drilled PVC cap
insert twin lead assembly into radome via side port of tee
dry fit PVC assemblies
quick performance check, tweak if absolutely necessary
glue PVC assemblies together