Log Periodic beam antenna for 2m
Focus your signal!
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So this is an antenna I have been wanting to build for some time now. I have hastily assembled Yagi's in the past, but ended up using the components for other antennas as I never really had a place to set up the Yagi.
Now that I am settled into the new QTH, it's time to try some new things. I have been wanting a build a log periodic, and figured it would be a cool addition to my selection of POTA antennas. Add to it the fact I just bought a like-new Yaesu FT-857D, and it almost becomes a must-have!
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A LP differs from a Yagi in a couple ways. A Yagi has one driven element (a dipole that is near the rear), and several parasitic elements (one reflector, and one or more directors). The Yagi design results in a very narrow bandwidth, but also produces a great amount of gain. Yagi antennas are built with just one boom, and all of the parasitic elements are usually connected directly to the boom. Depending on the type of match used, the driven elements may be connected directly to the boom, or they might be isolated.
A LP has multiple active driven elements (dipoles) who's lengths will have a lowest frequency, a highest frequency, and those in the middle. The LP is fed in the front, in the direction that antenna will point. The elements are arranged from front to back in a way that the polarity alternates opposite of each set. This alternating polarity occurs because the LP uses two booms; an upper, and a lower.
Similar to a fan dipole, the transmitting frequency that the antenna sees determines which dipole is active. Lower frequencies will be active on the longer dipole elements, and higher frequencies on the shorter ones.
The design of the LP results in a slight decrease in gain from a Yagi, but also an increase in bandwidth. For those wanting to use a beam antenna for both SSB (144.200) and FM simplex (146.520), the LP is an ideal antenna. As a bonus, the LP is also active on the 3rd harmonic, resulting in great performance on 70cm as well! Gain is estimated to be around 9 dBi on 2m, and 8 dBi an 70cm.
Like most any other antenna, I would rather build one than buy it. This was a fun project, and other than the arrow shafts, was built from material I already had. The main components are:
- Booms: I had several pieces of 1/2" wide aluminum channel already, and they are lighter than round or square aluminum tubing.
- Boom spacers: The booms are spaced apart 1/4" by way of three 1/4" thick nylon spacers, and held together with three 1/4"-20 nylon screws and nuts.
- Elements: I found a set of 12 target arrows on Amazon that already have the 8-32 threaded insert crimped into the shafts, which lowered the cost. Actual diameter measurement is .299", slightly larger than 1/4". These are the most expensive components to the antenna, at a cost about $43 for a set of 12. But, I now have 12 pieces of shafts left over that I can use to build a 70cm Yagi.
- Screws: The machine screws I used to attach the elements are 8-32 x 1-1/2". I drilled through both sides of the booms on the drill press, then tapped them all the way through. I didn't use any locking compound, just snugged them up tight.
- Feed line: I had a short piece of coax with a PL-259 already attached, so with the addition of an SO-239 barrel connector, that became the feedline. The coax center attached to the upper boom via the element screw, and the braid to the bottom boom. The exposed parts of the coax wires were coated in "liquid electrical tape".
- Mast mount: I took a cue from Elk Antennas to build the mount. All the parts are 1/2" PVC. None of these parts are glued together, but I did use a couple screws in the end elbows to keep it from falling apart. I did not glue anything as I want to be able to rotate the antenna between horizontal and vertical. The tee in the middle is 1/2" x 1/2" x 3/4", and allows me to use a piece of 3' x 3/4" PVC to slip over my Spiderbeam mast. In this 3' long piece of PVC I drilled a hole top and bottom and threaded them 1/4"-20 and used nylon screws to snug this piece to the mast. That keeps the antenna from spinning around in the wind. Alternatively, I can insert a short piece of 1/2" PVC to the back of the mount if I want to hold the antenna by hand for working satellites.
- Rubber caps: 1/4" diameter, to slip over the ends of the elements. A bag of 100 from Amazon was $6.79.
Dimensions are:
- Booms: 24" long each, 1/2" x 3/8" aluminum channel. You can also use round or square tube.
- Element lengths: There will be two of each length. From from to rear:
16-1/4"
16-1/2"
17-1/2"
18"
18-3/4"
- Element spacing: I drilled the first hole 3/16" from the front edge of the beams, then:
5-5/16"
5-5/8"
6-1/8"
6-5/8"
- PVC parts: you can make this however you want it depending on how you plan to use it, then just cut to length as you go.
Thank you to my friends Greg TI2GBB, and Carlos TI2BSH, for providing me with the plans for this antenna.
So there you have it. A fun project that did not cost an arm and a leg. It breaks down to a compact 24" x 3-1/2" x 2-1/2" and weighs just 23 ounces. It opened up a whole new ham radio experience for me, 2m SSB! On the day I tested it, I had SSB contacts from 52 to 148 miles. That was pretty exciting. As always, shoot me a message if you have any questions on building one for yourself.
The 2m Log Periodic in a horizontal orientation for 2m sideband.