A portable 2-element VHF yagi

Article by K3MT

 

Here’s a simple Saturday project: build a portable VHF yagi antenna for 2 meters. All you need is two rabbit ear antennas from Radio Shack, two CATV baluns, four feet of 3/4″ CPVC pipe with one tee, and a bit of time.

This sketch shows a semi-exploded view of the antenna.

Prepare the antenna’s boom from two pieces of 3/4″ CPVC plastic plumbing pipe. PVC can also be used. Each half of the boom should be about 10 3/4″ long, with a mounting hole drilled and reamed through a diameter, one inch from one end.

I recommend you drill and ream the small hole first: if you oversize it, you can easily make the bigger hole, and start another hole as the small one. The two holes should be diametrically opposite one another. This is an end view of one end of the boom:

Drill the smaller hole. Ream it carefully until the antenna’s mounting post can just snap into the hole. Then drill another hole diametrically opposite the first. Ream it until the mounting post can pass through the boom and enter the small hole. Ream the larger hole for a snug fit. Don’t assemble the rabbit ears to the boom yet.

Two Radio Shack 15-235A rabbit ear TV antennas are used. One is the driven element. The other is used as a reflector. These antennas come with a few feet of 300 ohm twin lead attached. Cut off all but 1″ of ribbon on one rabbit ears. Expose the two conductors and twist them together, to make a short circuit. This rabbit ears is the reflector. It doesn’t connect to anything else.

For the driven element, cut off all but 5 1/2″ of twin lead from the other rabbit ears.Strip it so 1/2″ of bare wire is available for connections. This will be the driven element

Now attach the two baluns to the driven element. The baluns I use are tubular, with an “F” coax fitting on one side, and about 3″ of twin lead on the other. Cut the twin lead of both baluns to a length of 1 1/2″ or so, and splice them together, 300 ohm to 300 ohm sides. Examine this figure as you prepare the baluns:

 

Take the twin lead from the driven element and solder a few pieces of solid wire about 3″ long to each conductor. Trim one wire to make a pin that enters the center hole of one balun’s F connector. Wind the other wire around the outside of the F connector (the grounded part) and fasten it securely. You probably will want to use a good deal of electrical tape to keep this jury-rig connection from coming undone.

Connect regular CATV type RG-59 coax to the other balun’s F connector. Thread the coax through the boom, down the center of the Tee, and down the mast. Its far end will attach to a receiver or low-power transmitter/transceiver. Pull the baluns into the boom far enough that the driven element’s mounting post won’t hit the balun.

Assemble the yagi. You will need to displace the driven element’s twin lead to insert its mounting post in the boom. Pull the rabbit ears out to these dimensions:

• reflector – 21″, tip to center of the insulator assembly
  driven element – 18 1/2″, tip to center of the insulator assembly

This should resonate in the 146 – 148 MHz band. Check the antenna against a known signal source. Adjust the reflector for best front/back ratio or best gain. Then adjust the driven element for best VSWR.

This antenna can be used on other frequencies as well. The rabbit ears pull out to about 41″, and collapse to about 10 1/4″ which gives a tuning range of approximately 76 to 260 MHz. Of course, to use the antenna over this wide a range, a longer boom with additional through-holes for the mounting posts will be needed. The dimensions shown here are those that are approximately correct for 2 meter use. I have found this antenna to be useful with my low power mobile rig, which delivers about 5 to 10 watts to the antenna. I have not examined what the CATV baluns do to the signal – whether they saturate, clip, or anything like that – but only note that I can successfully use this antenna with low power ransceivers.

Originally available at users.erols.com/k3mt/yagi2m/yagi.htm