The oxidation problem was what I was trying to avoid with my wrap and clamp scheme. I just use galvanized hardware to clamp the extension since it is an aluminum to aluminum connection (no dissimilar metal) to the 224 element. Oxidation of the screw and nut are not in the contact path so it does not matter.
Also, the clamp connection is at the highest impedance point on the dipoles, so again, it is not a high current point, so the clamp scheme does not carry much current through the clamp. Not drilling holes in the existing antenna was also a goal, as I have modified several antennas on site with a tower climber doing the install. Keeps it simple! 73 - Jim W5ZIT --- On Thu, 8/20/09, tahrens301 <[email protected]> wrote: From: tahrens301 <[email protected]> Subject: [Repeater-Builder] Re: DB-224 Matching To: [email protected] Date: Thursday, August 20, 2009, 10:16 AM Hi Jim, That's what I was looking for! Do you have any oxidation/connectio n issues where the tubing is wrapped around & screwed together? Perhaps drilling a hole through the original element for connection might be better. Just wondering. Thanks for the tip!! Tim W5FN --- In Repeater-Builder@ yahoogroups. com, Jim Brown <w5...@...> wrote: > > I have modified several of the 155 mHz center frequency DB-224 antennas to > the ham band. I found that it takes a 2 inch extension to each end of each > dipole to move them down. My mod was done empirically by taking a single > dipole and hooking it to an analyzer and finding the original center > frequency to be ~ 155 mHz. With the 2 inch extension added to each end, the > center frequency moved down to 146 mHz. I did not have to modify the harness. > > I make the extension out of a scrapped TV antenna with rolled tubing > elements. I flatten the tubing so that it can be wrapped around the dipole > at the center point of each end and put a single screw through the wrapped > flat portion of the TV tubing to clamp it to the dipole. When all assembled, > I measure the distance to the end of the tubing from the 224 dipole and cut > the extension to 2 inches. This moves the center frequency of the completed > DB-224 mod down to 146 mHz from 155 mHz. > > 73 - Jim W5ZIT >
