Norm, A collection of very informative information is available here: <www.repeater-builder.com/antenna/ant-sys-index.html#iso-cir>
As for what an isolator does, it ensures that your PA is working into a good 50 ohm load. If the antenna or feedline breaks, the power that would have been radiated is now reflected back to the PA- causing all sorts of problems. But an isolator shunts the reflected power into the attached 50 ohm load, thereby protecting the PA from a load mismatch. A circulator is the ferrite device that routes forward power to the antenna, but routes reverse (reflected) power to the dummy load. An isolator is the name given to a circulator that is packaged with one, two, or even three dummy loads. If you have a simple repeater that uses a single transistor PA that has no power control or high-SWR protection- such as a Hamtronics, RF Engineering, or Maggiore unit- an isolator on the output is necessary to keep the PA from burning up if the antenna or feedline faults. The other primary purpose of an isolator is to prevent mixing (intermodulation) in your PA of carriers from other nearby transmitters. Most commercial PAs have power available to them all the time, and only amplify when the exciter is providing an input. Being Class C, there is no amplification during idle periods. But, if other carriers can sneak into your PA through your duplexer, they can mix and be amplified to create new IM products. Keep in mind that most BpBr duplexers have a rather broad bandpass response, and can allow carriers far removed from your operating frequency to pass with ease. An isolator shunts such rogue carriers to the dummy load, thereby preventing the generation of spurious IM products. A ferrite isolator has a tendency to generate a second harmonic of the fundamental transmitted carrier, so it should be followed by a simple harmonic notch filter or a low-pass filter. As for the Kenwood TKR-750, I leave it to other and better-informed folks to respond to Item 2. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of NORM KNAPP Sent: Saturday, May 16, 2009 10:42 AM To: [email protected] Subject: RE: [Repeater-Builder] Re: Isolator Loss Kinda OT, but not entirely.... 1) What does the isolator do, and why would I need one? 2) I work for a Kenwood dealer. Kenwood has admitted to our company that the TKR-750 is rife with internal desense caused mainly by synthesizer birdies. Supposedly no two will have exactly the same birdies or desense on the same freqs. They have told us that there is a fix under warrantee but it has to go to them for the fix and you must provide freq info for custom fix. Has anyone else seen this problem or heard of this? 73 de N5NPO Norman knapp
