By Ron Hege, K3PF

Element Trimming
For this job, a fine-toothed saw works well. (Caution: During the following steps, be sure that you cut half the total amount from each half [ie, each side] of an element.) For operation at the low end of the band (144 MHz), cut the 66-inch element to a total length of 41 inches (see Figure 1B). This element becomes the reflector. Cut the next element in line (the driven element) to a length of 38 and seven-eighths inches. Cut the next three elements (directors D1, D2 and D3) to lengths of 38 and one-eighths, 37, and 36 and five-eighths inches, respectively. If you want to trim the elements for use at higher frequencies, cut one-fourth inch off of each element for each 1-MHz frequency increase. For instance, cutting a total of one-half inch from each element tunes and maximizes the antenna for 146 MHz. (Again, cut half the total amount from each half of an element section. For 146 MHz, the preceding example, that’s one-fourth inch from each half-element section.)

SO-239 Connector and Mount
Refer to Figures 4 and 5. Attach the SO-239 connector to the bottom of the boom beneath the driven element using an L-shaped piece of aluminum. Fabricate the bracket from a 3 and three-eighths inch length of aluminum cut from the 9×1 inch strip. Bend it at a right angle so that one side is about 1 and one-quarter inches long. Make the necessary holes to mount an SO-239 connector on the 1 and one-quarter inch long section and secure the connector to it. Fasten the bracket to the boom bottom using bolts and nuts, positioning the bracket so that the tip of the SO-239 center pin faces the reflector. Position the tip of the pin about three-sixteenths to one-fourth inch in front of the center of the driven element toward the director side.

Making The Gamma Match
Remove the outer insulation and braid from an 11-inch piece of RG-8 coax, leaving the center conductor and its insulation. Strip off one-half inch of the insulation and solder the center conductor to the SO-239 pin. At the pin, bend the wire at a right angle so that the wire is parallel to and about 2 and three-sixteenths inches away from the driven element along its length (see Figure 5). This lead forms the inner plate of the gamma capacitor. Next, select a piece of the scrap three-eighths inch tubing you cut from one of the antenna elements and cut it to a length of 11 inches. Slip this tubing over the RG-8 inner-conductor insulation to form the outer plate of the gamma capacitor. Position the tube seam so it faces the ground when the antenna is at its operating position; this allows moisture an easy way out. To complete the capacitor construction, wrap the remainder of the 1-inch aluminum strip around the driven element on one side and around the 11-inch tube on the other. Construct the strap so that the centers of the tubing sections are approximately 2 and three-sixteenths inches apart. Leave a tang on each side of the strap to accept a locking screw. Trim away any access material.

Tuning The Gamma Match
Before applying RF to the antenna, connect an SWR meter to the SO-239 connector at the antenna, not at the transmitter end of your transmission line. This insures that you are tuning just the antenna. For a quick adjustment of the matching network, you can try positioning the antenna straight up toward the sky, with the reflector setting on the ground. Using this approach, however, I found that when I raised the antenna to a height of 10 feet on a metal mast, the gamma capacitor needed readjustment. If you’re a perfectionist, it might ultimately be less work to tune the antenna while it’s mounted in the clear a few wavelengths above ground or setting at its intended operating position. If you’re going to use a metal support mast, attach it to the antenna prior to tuning. Use a non-metallic mast if you’re going to mount the Yagi vertically (so that the elements are in line with the mast); otherwise, antenna performance will suffer a bit. It’s okay to use a metal mast when using horizontal polarization.

Reduce your transmitter’s output power to about 1 or 2 W for safety use or, an antenna analyzer. Don’t use more than a few watts – you don’t need it. Set the transmitter frequency to that for which you cut the antenna. First, adjust the gamma strap (sliding it back and forth) on the driven element for the lowest SWR. Then slide the gamma tube (capacitor) back and forth within the strap for the lowest SWR reading. You should be able to get a match by alternately making adjustments to the strap and gamma tube. I was able to tune my antenna to a 1:1 match. (An SWR of 1.5:1 or less is acceptable.) Recheck the SWR after finally tightening the strap to be certain that everything is still okay. Check by eye to ensure the gamma-capacitor tube is parallel with the driven element from one end to the other. It doesn’t matter if the gamma-capacitor tube is slightly in front of or behind the driven element, but it should be parallel to it.

[A Five-Element, 2-Meter Yagi for $20 – July 1999 QST] To read the article in its entirety and see all the diagrams click here. Go back to Part 1 here.