Chuck Kelsey wrote: > First, don't expect to land exactly where you expect simply by adjusting the > lengths to a specific dimension. You really need to place the antenna on a > tower to adjust properly and use an antenna analyzer.
<grin> Reminds me of my first experience with a Cushcraft AFM-4DA (4 bay exposed dipole array for 2M) about 30 years ago. I put it on a nice galvanized mast and mounted it to a tall building free and clear of anything. While the antenna worked - it really didn't perform any better than a 5/8 wave ground plane. I realized that optimum gain was not being achieved because the dipoles were spaced around the galvanized support and weren't really "in phase" to help each other out. OK, no problem, I'll put them in a line all above one another. WOW, the gain in the direction where the dipoles were facing was fantastic - so, I fixed the "gain" problem - BUT I created another problem. The null in the opposite direction was so deep that the system didn't work where I needed it to (I really needed omni coverage - I couldn't suffer with a null anywhere). OK, no problem, I'll put the elements on a FIBERGLASS pole. I bought a hefty fiberglass pipe and carefully mounted the Cushcraft array to it - mounting all of the dipoles above one another will retain the gain, and the fiberglass pipe won't create a null - problem solved right? NOPE - a rubber duck would have performed better! What happened? This type of array REQUIRES the element to be in the presence of a metallic mast - otherwise the impedance of the element isn't close to 50 ohms. I didn't have the ability then to measure return loss or actual impedance of the individual dipoles, but I can guarantee you it wasn't close to 50 ohms. The antenna didn't work and the reflected power was awful. I used a Bird and hand-held to feed one dipole - while moving the distance between the dipole and the metal mast pipe - sure enough, when you got to the distance the mount was designed to give, the reflected was at a minimum. SO.... The distance between the antenna and the support varies the port impedance. Bought a Sinclair SRL-229 (super stationmaster type) and life went on.... Certainly on low band the concern of the elements exactly above on another is of a lesser concern because the difference in phase angle is likely less - so obtaining good low angle gain in an omni directional pattern is doable. Kevin Custer
