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 --- On Wed, 8/19/09, tahrens301 <[email protected]> wrote: From: tahrens301 <[email protected]> Subject: [Repeater-Builder] DB-224 Matching To: [email protected] Date: Wednesday, August 19, 2009, 8:40 AM In my quest to get rid of desense with the Quantar, someone mentioned that having the 'wrong' antenna could make the desense worse. I've got a DB-224 - not the 'ham' version, but the 150-160 MHz version, and there is a bit of a mismatch. Has anyone ever had any desense that they could attribute to less than perfect matching? (I'm not talking about a gross problem, like one of the elements broken, etc) Has anyone built a 'tuner' for that version of the 224, so that the transmitter/ duplexer pair would see a better SWR? If this thing ever gets up, it's gonna be perfect! :-) Thanks, Tim W5FN
