A wise man once said, “Waste not, want not,” and while the saying has no doubt been used to collect some pretty silly things at times, there is little doubt that it can prove fruitful for the ham operator. One example played out this week as I built two antennas for use at the shack.


    ‘Wasting not’ came in the form of collecting an old damaged television antenna from someone who was otherwise just going to throw it away. While the bent, broken, and in some cases, missing elements made it nearly worthless for receiving television signals, the square tube aluminum boom almost shouted out to the homebuilder to give it a new life as a yagi antenna and rescue it from it’s destiny in the landfill.

    The television antenna actually consisted of two booms - a longer two-part boom that was about 15 feet long, and a lower brace boom that was about seven feet long.
With dreams of a high-gain yagi sniffing out faint signals running through my head, I lashed the antenna on the roof-rack of my Jeep and headed off to see our local antenna guru - Jim Searcy (WA5WRE). His eyes brightened as he took a look at the find, and we started planning the construction.

    After some consideration of such concerns as materials, size, weight, etc. - I decided to scale back from the 21-element yagi I had imagined - and we decided on a six-element version of a traditional design. The lower brace tubing would serve as the boom, and some one-half-inch aluminum tubing would be trimmed for elements. A old gamma-match would serve to move signal to air, and with some labor and luck - my new antenna would become a reality.
    
    Building an antenna with Jim is a learning experience - as those who have shared his shop can attest. An old-time-ham, he believes in not only good design and construction, but also in understanding the WHY of how an antenna is designed and built. The experience can be challenging, entertaining, and is always informative. First step is always research. Specifics such as boom length, element length, spacing and number have to be determined, as well as type of match and intended application.
    
    Once these decisions were made, it was time to start fabricating. The lower brace tube was easily unfastened from the upper boom, and a quick bump with a drill press was all it took to remove the old rivets in the boom that were in the way.

     “It can’t just work good - it has to look good too,” commented Jim - so we spent some time running the boom and the elements through a wire-wheel mounted on a bench grinder. The resulting “brushed aluminum” look is pleasing and in fact more efficient because the oxidized metal of the boom and elements have been removed.
    
    Once they were prepared, it was time to start to make measurements and cuts. A “Sharpie” is a good way to make the marks on the boom and the elements to indicate lengths, placements for holes, etc. See the chart below for the measurements we used.

Reflector     Driven Element       Director #1       Director #2           Director #3        Director #4
40 5/16"            38"                     37 1/8"             36 1/8"                35 1/4"            35 3/16"

    The boom conveniently had a hole already drilled about one-inch in from one end, so we used that as a starting point. Based on a design from a widely available yagi, we measured from the center of the first hole and made marks for the holes of the rest of the holes for element mounting.

The holes were placed as follows:

Reflector            Driven to            Director #1 to        Director #2 to        Director #3 to      
to Driven            Director #1            Director #2           Director #3           Director #4
13 1/2"                 9"                         14 1/2"                    14"                     16"

    The trusty drill press again worked its magic and 3/16" holes were drilled through the boom at the marked locations. Once we knew where the last element would mount, we measured another inch beyond that point and cut off the boom for a final length of 5'4" (64") long. We put the drill press back into action to drill 3/16" holes at the exact center of each element, then mounted each element to the boom in place with small (10-24) hardware.

    With all the elements mounted and checked for alignment, it was time to take the yagi outside for a brief test. It was cold outside - so I know we must have looked a sight - me holding
the beam antenna overhead and Jim with the trusty MFJ 269 Antenna Analyzer checking SWR. After a few minor adjustments, we achieved an almost perfect match - 1.1-1 SWR at 146.100 Mhz.

    Time then to check performance. We didn’t want to mount it on a pole, so we lifted the yagi overhead and twisted back and forth in what we guessed was the correct direction for several local repeaters. One thing we found is that the pattern is very narrow - if you get more than about 10 degrees off pointing directly at the signal source, the signal drops from full
scale to about a one. All-in-all, a definite success, and I am looking forward to getting my hands on a rotor to mount it at home for regular use.

    And the sections of boom that Jim kept? “You’ll have to wait and see what I build with that,” was his cryptic comment.


    The yagi described above was not the only creation of the week. I also had the chance to put together an 80-Meter dipole out of some old wire, an SO-239 connector and a small piece of lexan as the center section.
    
    The first step was to cut the legs for the antenna. Using the old formula that should be familiar to every person holding even a technician class license, (468 divided by frequency in Megahertz, I quickly determined that the antenna would be 133.71 feet long. Dividing by two, we see that each leg would be 66.86 feet long.

    Because I plan to hang the antenna in an “Inverted ‘V’”, however, I know to subtract 1.5% of length from each leg. Doing the math quickly, that left each leg at 65.85 feet. I measured out the wire, (14 gauge stranded - insulated) and cut each leg at 66 feet - leaving about two inches per leg for trimming and attachments.

    I then turned my attention to the center section. The first step was to drill out the space for the SO-239 connector to sit in the middle of the lexan. Using a Forstner bit, I made a hollowed-out area for the SO-239, then switched to a smaller bit for the center hole for the connector. The center punch hole from the Forster bit made centering the smaller hole easy - and a clean hole straight through the lexan was quickly achieved. Two smaller holes were drilled out for the mounting hardware for the SO-239, as well as smaller holes about one-inch from the outside edge of the lexan for the leg mountings.

    The SO-239 was bolted into place, and a short wire jumper was soldered to the center pin of the SO-239, and it terminated at one of the leg mounting bolts. Another wire jumper runs from one of the SO-239 mounting bolts to the other leg mounting point.The legs were then attached using ring connectors to the leg mounting points, and after a generous covering of clear silicone was applied to seal the back of the SO-239, the dipole is ready to be mounted and used.

    A quick raid on the old junkbox turned up two old cable reliefs that were pressed into service as ends for the dipole legs. They provide a place to tie off the legs without taking the chance that the legs will break due to bending strain. The only addition planned from the picture, right, is the addition of strain relief where the legs leave the edge of the center piece of lexan. These will be made from large eye-connectors.