Tom - KG6SEA - Northern California

2m Collinear J-Pole - okay, I'm a new ham. I got a low power 5 Watt radio, fully charged. I just joined a local radio club, and my first net is tomorrow. What's missing here? Well, I found out rather quickly that my HT's little antenna can't open up the club repeater from inside an apartment complex. Fortunately, I have been doing a bit of research on the web, and came upon this great antenna project from the KB1DIG Antenna Projects. This is a 2-section collinear J-Pole antenna. It has the advantages of both a 2-section collinear, and a J-Pole. And I have all the parts at home. It sure does a nice job of getting the radio waves out of a large apartment complex.

Fig 3 - Parts for the antenna - just a few feet of RG-58 and a short length of twin-lead

Fig 4 - Tools needed to build the antenna, including some clothes line and a popsicle stick

Fig 5 - Here are the plans for building the antenna from KB1DIG.

Description - The antenna consists of a 1/2 wave radiator at the top, another 1/2 wave of wire coiled up in the phase element, a second 1/2 wave radiator below that which is in phase with the top radiator, then a 1/4 wave J-Pole section with a tap at 50 ohms. There is a small loop at the top tied to the clothes line so that the antenna can be hung off the top of the PVC pipe. Check out Fig 1-4 for some shots of the antenna and the material and tools I used to build it. Click on Fig 5 to jump to the plans by KB1DIG.

Customizations - of course, I had to personalize the antenna from the stock design. First, the important, is SWR. I found that the measurements for the plan works well when the antenna is hung from the ceiling, but when encased in PVC, the SWR shot up. By shortening all the elements by 5.5%, I was able to bring down the SWR. Second, for a leaner look, I used 1/2" PVC pipe instead of 3/4" PVC pipe to house the antenna. Third, I decided to wind the phase element on the outside of the antenna (Fig 6). The antenna is divided into 3 smaller sections of PVC joined together with PVC fittings. Fourth, instead of using circuit board, I used clothes line to tie the antenna to a popsicle stick. Then I turn the popsicle stick to tension the wire. I cut a small 1/4" notch at the top of the PVC pipe to hold the popsicle stick, then trim off the wood sticking out the sides, and the end cap gets put on.

Fig 1 - antenna - looking up

Fig 2 - antenna base

Fig 6 - phase element

Fig 7 - The antenna base. The picture shows the antenna ground with 4 x 1/4 wave radials attached, ground wire to water pipe also attached to antenna ground, inductor made from 5-turns of coax to block RF radiation from being coupled back into the radio through the feedline shield.

Antenna base - this is the challenging part of the antenna. There are many elements coming together at the base (Fig 7). The point in the base where the 1/4 wave radials are protruding corresponds to the position of the 50 ohm tap on the J-Pole. I used some of the left over braid from the coax. to bring the antenna ground point outside the PVC pipe. The radials are then soldered to the braid. A ground wire connecting the antenna to a water pipe is also attached to the braid.

Below the ground point is where the coax. feed line enters the antenna. Note that just outside the antenna, I have coiled up the coax. feedline. I found through SWR measurement that 5 turns of 2" diameter coil works best to block RF from coupling to the feed line and going back into the radio.

Fig 8 - SWR meters

Calibration - A SWR meter is required to calibrate the antenna (Fig 8). A cross needle type meter is easier to use since there is no calibration required before every measurement.

As mentioned above, I shortened the measurements of the elements by 5.5%. The antenna is tuned to 146MHz. The length of the 1/2 wave bare wire elements is c / 146MHz * 1/2 wave * 0.95 velocity factor = 98cm or 38.4", where c is 3 * 10^8 m/s. Shortening by 5.5% brings it down to 36.3". The phase element is 12 turns. The 1/4 wave J-Pole element length is c / 146MHz * 1/4 wave * 0.82 velocity factor = 42cm or 16.6". Subtract 5.5% brings it down to 15.7". Of course, I did check out the SWR before fixing everything in the PVC pipes, which is done by taping the antenna to the outside of the PVC.

Essential to proper operation are the addition of radials for the ground plane, and adding an inductor to the feedpoint. I soldered 4 x 1/4 wave radials to the ground point. Also, I had 6 turns of coax. cable at the feedpoint to bring down the SWR. I am getting SWR of 1.3:1 in the center of the 2m band, and 1.7:1 at the edges of the 2m band. The antenna seems to have a very high Q. SWR rises quickly at the edge of the 2m band. The antenna does not work at 220MHz or 70cm bands. The impedance and SWR are way off outside of the 2m band. But, yes, it does pick up signals in all bands :-)