This is a few more photos of the new repeater antenna build, and some observations.While tearing down a 4-bay antenna on 154mhz for parts & materials, I measured things with my MFJ Antenna Analyzer, and took note of some dimensions...
First, the larger dipoles are from the 154mhz antenna. I made a little pigtail connector for the MFJ, and took some readings. Resonance and impedance of these types of antennas is impacted by proximity to the mounting mast and the width (these are 3") of the loop. They also require at least a 1/4 wl of mast above and below for proper operation. The larger units were resonant at about 154mhz, where I expected. These dipoles are 17" per side, from the mount to the end of the loop, total 35" tip to tip (counting the 1" dia mount in the middle). The loop is 3" wide, edge to edge, and they are 3" from the mounting mast to the edge of the dipole loop. These are the dipoles I will modify for 2 meters.
The smaller dipoles are 15" per side, from the mount to the end of the loop, total 31" tip to tip (counting the 1" dia mount in the middle). They were resonant at about 168mhz. I altered the formula to calculate the proper dipole size, and it clearly doesn't track as I alter the dimensions, so I'll be doing a bit empirical testing (read trial and error, hahaha) to work the bugs out of the 220 and 440 antennas.
Another interesting observation...using DB Products RG-11 cable, the matching sections were pretty much the same length as the tip-to-tip dimension of the folded dipole elements. We'll see how this works out when I get these tuned and installed on a test mast...
I plan to use the single mast section on the right (with the rain bonnet) to install a pair of folded dipoles for 2m, 220, and 440. My plans are to keep this a compact antenna. I realize it'll be a compromise antenna, but I need to keep size down for a DC grounded tri-bander.
RG-11 in the original installation...
154 mhz vs 168 mhz...
This pair will be modded for the 147.39 repeater.
Dave WB4IUY
www.WB4IUY.net
