Tuesday, October 4, 2016

Amateur Radio - Building a 3 element yagi for 6 meters

I've been looking for a suitable replacement for the simple 6m rigid dipole.  Some of the designs that I've seen look flimsy and perhaps not well suited for high winds or snow.  I started ruling out all the designs that used dipole driven elements.  In looking for the perfect stout yagi I came across a gamma match driven design by ON6MU.  It boasted a good 2 Mhz bandwidth, with all the elements bonded directly to the boom without the need for isolation.  This appeared to be the ideal solution.

I bought a 1" square tube from the local hardware store cut to 2 meters in length, and obtained some surplus 1/2" and 3/8" aluminum tubing from my local club.  The tubing was quite old and corroded, so I had to polish the outside of the tubes.  I followed the dimensions precisely as given in the original design:

Reflector = 2940mm at 10mm spacing from the end
Driver = 2870mm at 985 mm spacing from the end
Director = 2660 mm at 1990 mm spacing from the end

I chose #10 stainless screws and nylock nuts to fasten the structure together.  I made spacers for the inner tubing as not to crush it out of solid 3/8" aluminum rod.  The holes are spaced at 0.700 inches so that the screws are directly against the walls of the square aluminum boom for maximum strength when tightening down.  I drilled the boom on the drill press prior to taking all the parts outside to insure all the holes were drilled perpendicular to the length.  Everything was assembled with Silicone Grease to prevent corrosion between the metal joints.
One screw was dropped in while the spacer was pushed down the length of the tube.  A dental pick helped align the pre-drilled holes.
Here is the reflector about to be fastened down.
The whole thing went together right well.  I used some strips of aluminum gutter for the shorting strap.  These were formed first, then drilled and sheet metal screwed used to fasten them together.  The first Gamma Tube I created was out of 1/4" copper with a RG-8 center conductor and dielectric.  This is where things started to go wrong later.
When I erected the antenna on a pole for tuning I couldn't get a good match.  Of course I wasn't using ON6MU's recommended dimensions for the Gamma Tube since I didn't have the right material.  I referenced a few other designs for Gamma Tubes on 6m Yagi antennas and finally created a new Gamma Tube made from 3/8" Aluminum at 12.75" long and a center conductor and dielectric from RG-213 coax at 14" long.  This proved to give a good reactive match.

Further difficulty presented itself when the antenna exhibited a 200 ohm resistance at 50.3 Mhz.  This meant that my driven element was too long and the antenna was resonating below what my analyzer could read.  I kept cutting each end till the resistance came down to 50 ohms, which was about 2.25 inches.  This could be the material I was using, or the way I attached the elements which may have differed from the original design.  So the final measurements ended up being:

Reflector = 2940mm at 10mm spacing from the end
Driver = 2870mm 2756mm at 985 mm spacing from the end (final tune by antenna analyzer)
Director = 2660 mm at 1990 mm spacing from the end
The Gamma Tube was easy to tune as it was almost completely pushed into the coax as far as it would go with 0.25" space at the end, and then the shorting strap was attached at about 1.5"" from the end.  Shrink tube was applied on both ends of the tube and around the coax section.  The SO-238 connector was sealed with first a layer of hot glue, and then coated with liquid electrical tape for a final seal.
A quick check SWR with the rig was done and I contacted my friend a few miles away.  Another friend (in the opposite direction) also chimed in saying that my signal was weak.  A quick twist of the "Armstrong Rotator" put the other station at S7 with minimal power, proving that the beam-width of the antenna was what it was designed to be.  I didn't worry too much about the director being proportionally longer than the driven element after being trimmed.  Some tests with the NL7XM/B beacon proved that the beam-width was satisfactory.  It won't be hard to lop off some of the director if I think the beamwidth is a problem later.  I can always insert some 3/8" tube into the elements if I need to lengthen any element.  I think the original design intended this, but I wanted to avoid points of corrosion by adding different pieces.  The aluminum tubing I am using was very old and internally quite corroded.
Hauling the antenna up to the roof was easy since it is so light.  It clamped right onto the pole along with my 2 meter slim JIM.  My antique Alliance Tenna Rotor model T-20 has no problem spinning it around.  While it doesn't have the claimed bandwidth of an M2, it tunes just fine with my external tuner.  The bottom end of the band is where the SSB weak signal work occurs anyway.  I can still point it at the local club 6m repeater and bring it up, so that makes me happy too.

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