GMRS J-Pole Antenna

2/27/2005 marc 61 comments

Construction method

The first step in constructing this antenna is to obtain the materials. The list of items I used to build my antenna is somewhat simple:

Materials:

1 stainless steel FM car antenna new or used
2 Brass collars (model airplane supply store)
2 – 5 meters RG-174 coax cable
1 Female SMA connector (small size for RG-174 coax)
1/8″ plexiglass scrap (10cm square-ish)
Large heat-shrink tubing
Liquid tape
Rosin core solder
Self-sealing tape
Silver solder
Stainless steel picture hanging wire
Plastic cable ties

Tools:

Micro torch
Rotary tool with fiber metal cutting wheel (Rotozip, Dremel, etc.)
Vice/hammer for bending steel
Vice grips
Soldering iron
Jig saw

The antenna is made from the material of a single FM auto antenna. There is enough material in one antenna to make both the main radiating element (the main body of the car antenna, including the mount at the bottom) and the quarter-wave matching stub (made from the top-end of the antenna).

NOTE: Measure the lengths of both B and A and add about 1 cm to their tops. This is so that you can construct the antenna without worrying if you have they lengths perfect. Then when the antenna is soldered together, use your rotary tool to cut the elements to the exact length.

Take the antenna and cut off the ball or very top end of the antenna. Next, measure from the tip, a distance equal to 1.1 * (B + D + 1.6D). That gives you about 20.5cm. The length need not be exact but needs to include a little extra for the bending radius of the antenna wire plus a little extra (maybe a centimeter or a little less) added to the length the matching stub B.

This piece is then bent into a “Z” shape forming the matching stub element B, the length D and a 2.5cm section which is silver soldered to the main antenna shaft.

Measure from the tip of the main radiating element of the antenna, measure down towards the threaded mount a distance of at least the length A + 10%. You can go longer, lowering the “J” of the antenna towards the base, but remember there must be room at the between the mount for the choke (4-5 turns of the coax – see pictures). Any excess length beyond A will be trimmed once the antenna is soldered. Using vice-grips, hold the “J” to the main shaft of the antenna while silver soldering.

WARNING: Torches and solder (especially, silver solder) are very HOT. Be very careful while soldering this antenna!

Once the antenna has cooled, clean aways any excess solder with a file and wrap the top and bottom of the newly soldered section with stainless steel picture hanging wire and then silver-solder these wraps to add strength.

At this point, measure the exact lengths of A and B from the base of the “J” and cut them to the proper lengths with a Rotozip, Dremel or similar rotary tool with a metal cutoff wheel. Polish and buff the ends to remove burrs and sharp edges.

The feed point is made by either directly silver-soldering the coax to the antenna at point C (shield to B and center conductor to A) or soldering (regular solder) the coax to brass collars (used to hold model airplane wheels on). The collars can then be adjusted up or down to find the lowest SWR. This is what I did with this antenna, however if I was making another, I would probably just silver-solder directly to the antenna since the distance C has proven to be very good and has needed no adjustment.

The choke is made by looping the RG-174 coax 5 or so loops in a tight circle, mine is about 5 centimeters in diameter. Then choke is attached to the bottom of the “J” and the antenna shaft below the “J” using cable ties and is wrapped in self-sealing rubber tape.

To prevent the antenna being bent apart by trees, twigs or other objects entering the “J”, a small piece of plexiglass with a sloped top is placed between the top of the radiating element and the quarter wave stub. Cut/shape your scrap piece to fit between the two prongs of the antenna. I held mine in place with contact cement and then placed a short length of largish heat-shrink tubing around it to help protect it.

The exposed contacts at the feed point are covered with a few coats of liquid tape and after drying and attaching the SMA (or proper end for your radio) connector to the end of the coax, the antenna is ready to mount and use.

The final antenna is shown here.