St. Louis POTA 20 Antenna

Revised 6 October 2025

• Added endcap to attach the antenna to the top of a fiberglass driveway marker.  This is easy to deploy:  just push the driveway marker into the ground and slide the antenna on top!  In the photos, this is the orange cap.  Named "polecap.stl"
• Added endcap with thread 1/8 NPT threaded rod.  This type rod is used in the US to make the major structure of electric lamps.  (All the lamp parts are assembled on this type of rod.)  In the photos this is the green cap.  Named “lampcap.stl”

Print Settings

• Supports:

No.  Orients as oriented in the files can be printed without any support

• Resolution:

Recommend 0.15 to avoid problems with the printed threads

• Notes:

This is a shortened vertical antenna designed to use a 10-segment 118cm collapsible whip antenna tuned to the 20m amateur radio band when used with a 13.5 ft (4m) counterpoise.

Bill of Materials (for US builders)

• BNC female bulkhead connector (such as https://tinyurl.com/bdfa553u)
• ~ 8ft of 18 gauge magnet wire for 24 turns (16 gauge may work also)
• 2x M4x25mm screw
• 3x M4 nut
• M4 lock washer
• M4 thumbscrew
• BNC adapter
• Hillman 1/4-in x 20 Brass Wood Insert Nut - found at Lowes, Item #137388 | Model #880549 - this is an easy & sturdy tripod mount
• 3x 1/8" steel rod (also found at Lowes cut to ~ the same length as the collapsed whip
• 115cm (actually 118cm) 10-segment whip antenna (such as https://tinyurl.com/yc7rjh8a)
• 14ft (4m) stranded wire. Any gauge, but I use 18
• 2x wire spade connectors such as (https://tinyurl.com/3xpvz26v)

Short instructions:

1. Print the plastic parts. All parts should be oriented as in the supplied files. You will need 3 feet for the table mount.
2. Assemble the Brass tripod mount in the base.
3. Assemble the antenna section by placing the M4 nut in the hex hole and place the M4 screw through the "pointy end" of the bushing to engage the nut. As you tighten the screw, you will pull the nut into the bushing and will feel significant tension when the nut reaches its base.
4. Solder a short (2") length of wire to the BNC ground ring and attach a crimp-on spade connector to the other end.. Use a spare piece of stranded wire from your counterpoise.
5. Place a M4 nut on one of the M4x25 screws. Then attach the wire to the screw and tighten. Insert the screw and wire into the hole FROM THE INSIDE pointing out. Attach another nut to the screw from the outside of the base. Use the BNC connector hole to access the head of the screw while tightening this nut. Leave the end of the wire with the BNC ground ring hanging out the bottom hole of the main body.  DO NOT OVER-TIGHTEN THE NUT - IT WILL DAMAGE THE PLASTIC.
6. Attach the BNC cable mount, making sure that the ground ring goes against the inside plastic before the lock washer and nut. This is the most fiddly step to accomplish because the stiffness of the ground wire tries to keep the ground ring from sitting on the BNC threads properly. Be patient and use good lighting. Tighten the nut with needle nose pliers.
7. Start winding the coil by first passing the wire though the top hole of the coil body and out through the center hole where the antenna will sit. Remove insulation from the end of the wire and attach to the head of the M4 screw that is installed in the bushing. Then pull all slack out of the wire and seat the bushing onto the coil body. Now is a good time to screw in the upper retaining ring.
8. Wind 18 turns of wire around the coil base - WITH NO SLACK IN THE WIRE. Feed any remaining wire into the small hole at the bottom of the coil and then out the bottom of the body. Pull tightly. Folding the wire against the inside of the coil will keep enough tension on the wire that the coil will not slip.
9. Cut the wire as short as you feel comfortable. I leave about 3/4" out of the base. Strip the end and solder to the center of the BNC adapter.
10. Screw in the base.
11. Attach safety feet to the 3 tripod legs if . The feet aren't really very functional but keep you from poking your eyes out! I roughed the ends of the 1/8" wire and used epoxy to secure them. The legs fit at a slight tilt in the 3 holes you see in the perimeter of the base.