Dual

Dual-Band

VHF/UHF

J-Pole Antenna Theory

The J-Pole is a vertically polarized antenna with two elements: the radiator and the matching stub. Although the antennas radiator and stub are ¾ wavelength and ¼ wavelength, respectively, it operates as an end-fed half-wave antenna. Here is how you determine the length of the J-poles two elements.

L ¾ = 8856xV

L ¼ = 2952xV

Where:

L ¾ = the length of the ¾ wavelength radiator in inches

L ¼ = the length of the ¼ wavelength stub in inches

V = the velocity factor of the ladder line

F = the design frequency in MHz.

These equations are more straightforward than they look. Just plug in the numbers and go. This design assumes that 146 MHz. is the center frequency on the 2-meter band. You may of course substitute a center frequency of your choice. Even though the antenna is designed using a 2-meter center frequency, it also works well on 70- cm.

Don’t let the velocity factor throw you. The concept is easy to understand. Put simply, the time required for a signal to travel down a length of wire is longer than the time required for the same signal to travel the same distance in free space. This delay-- the velocity factor-- is expressed in terms of the speed of light, either as a percentage or a decimal fraction. Knowing the velocity factor is important when you’re building antennas and working with transmission lines. Because of the delay, 360 degrees of a given signal wave exists in a physically shorter distance on a wire than in free space. This shorter distance is the electrical length, and that’s the length we need to be concerned about.

Copper wire has a velocity factor of about 0.93, whereas 300-ohm ladder line has a velocity factor of 0.81 to 0.85 depending on who made it. If you’re unsure about the ladder line you’re using, just use 0.85 as its velocity factor. It’s okay if it turns out to be too high. You’ll be able to compensate by trimming the antenna.( It’s better for the antenna to be too long than too short ! )

Construction

Because of the few materials needed to construct this antenna, you’ll find it surprisingly easy to build. Start with approximately two feet of 300-ohm ladder line, three and a half feet of 12ga. Speaker wire and if you are going to use this in a twenty foot Crappie pole, you’ll need twenty-five feet of coax with a suitable connector.

Start by stripping off ½ inch of insulation at one end of the ladder line. Solder the two exposed wires together. This is the bottom of the antenna. Next, measure up 1 ¼ " inches from the soldered wires and remove the insulation from the ladder line to expose 1/8 to ¼ inch of wire on both sides. Be careful not to nick or break these wires. They are your connection points for the coaxial feed line. Solder the center conductor of your coax to one side of your ladder line where you stripped the insulation up 1-1/4 inches and the shield from the coax to the other side. Cut a piece of 12 ga. Speaker wire about 52 inches long. (You will trim it later for the best match). Solder one end of the speaker wire to the top of the ladder line (on the same side as the center conductor from your coax).

Apply a generous amount of weatherproof silicon sealant to the exposed coax to prevent moisture from seeping into the line. Now tape the coax to the ladder-line to relieve strain on the soldered connection points. Heat shrink tubing also works well for this application.

Tuning

Tuning the J-pole is easy. Using a high accuracy VHF/UHF SWR meter (borrow one if necessary), simply trim the length of the elements until you read a 1:1 SWR-or as close as you can get. Trim in very small increments; don’t chop off an inch at a time. Remember to trim in a 3:1 ratio to maintain the ¾ - to ¼ - wavelength proportions.

Summary

A J-pole antenna will never replace a beam or a full-sized vertical mounted at 30 feet, but it offers relatively good performance for a minimum of materials, time and effort.