Build 1: 7 Element Yagi Commercial FM Band Antenna___________________________


or how to turn a Channel Master CM 3671B into a useful FM antenna.

Measurements and comparisons at the bottom of the page.


Taking the plunge to make my own, there were some steps to consider. Right-click/save any picture to see a full-size version.

Design: I used Martin E. Meserve's excellent site K7MEM.com, since it allowed me more elements for the length of boom I had (vs John Drew's Yagi Calculator).

Materials: A Channel Master CM3671 supplied the boom and elements. It's a current model, but it favors the upper VHF and UHF bands (since most TV stations have actually moved their signal to UHF). The lower VHF band is largely deserted now, and the Channel Master is designed to that (as are more and more current model antennas). Consequently, signal strength on the lower band was half that of the upper two, and my old Philips antenna beats it badly on FM. So, apart it came.

Construction: The advantage of cannibalizing the Channel Master (purchased for less than $100 off Craig's List) was primarily the element mounting hardware. The elements themselves appear much easier to purchase.

The black insulator is for mounting elements on a log periodic antenna. This style wasn't used on this build. The brackets left and right of it are for the yagi, which often ties the elements directly to the mast.

The boom split in three pieces. Here, all the original elements have been removed, and most of the holes covered with HVAC metal tape.

Here's the original dipole, for hooking up the transformer and coax. The elements don't touch here, which will be different on the new antenna.

Since element material was being reused, it was necessary to splice pieces. 3/8" copper plumbing nipple connectors fit the old elements nicely. They were crimped using a coax connector crimper. They aren't long enough to hold the elements pieces steady. Except for T-connectors, it was impossible to find any longer pieces of copper with a 3/8' inside diameter.

This shows two elements crimped together. One was opened up along its split and the other was slid inside for about a 2" overlap and then crimped. Looks gnarly, seems to be holding up for now. Future antennas will use one-piece elements.

I used stainless screws to mount everything. Fastenal offers wholesale pricing, but you have to buy a decent quantity. I spent $17 for 50 2.5" 10-24 Phillips head screws. That's about 35% off Lowe's price.

As mentioned, the dipole was rebuilt. Here is one end, with a 40 mm spacer and #10 bolts.

On the lower side, there is a 15 mm gap between the ends of the elements. On top, the element is screwed to the plastic for stability, but isolated from the mast (as is the whole dipole).

Future plans are for a honking big 12 element unit. Stay tuned.

Results

Here are reception figures, which are readings on an Onkyo T-4711.
Stations are a mix of local and not. For instance, 88.5 and 90.9 are about 100 miles, etc.

Frequency Magnavox MANT-901, roof mounted  DIY 7 Element,
 roof-mounted
CM-3671* DIY 7 Element*
88.1 45-47 dbu 45-47 18-22 36-46
88.3 14-17 19-20  --  19-20
88.5 18-20 21-22 --  21-25
89.3 17-21 25-26  --  16-18
89.5 45-46 40-45  20-24 30-38
89.7 41-45 46-47  40-43 43-46
89.9 16-19 25-29  11-13 14-17
90.1 48-49 48-49  17-19 40-43
90.5 47-48 49  24-31 19-24
90.9 42-46 47-48  11-14 22-26
91.1 32-41 48-49  17-20 45-46
91.7 37-41 --  20-24 36-42
92.1 85-91 83-84  76-77 80-82
92.5 49 78-80°  45-47 46-48
93.1 45-46 46-47  17-22 40-43
93.3 42-46 46-47  22-26 33-40
94.5 73-76 82-85  49-50 69-70
94.9 36-41 81-82°  40-42 26-31

*- for these measurements, the antennas were mounted on a tripod
     on the drive, three stories below.
°- these stations may have increased power between measurements

 

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