Wullie, If the isolator was made for a frequency that is more than 4 MHz away from your 2m transmit frequency, it cannot be tuned to work properly that far away from its design frequency. No matter who makes the unit, each isolator (which is called a circulator if dummy loads are not provided) is made-to-order for a specific frequency. The catalog descriptions of such devices are very misleading, since the description often says something like, "136-174 MHz," which means only that the company can manufacture an isolator to work within that range; it does NOT mean that an isolator can be field-tuned to any frequency within that range. A very informative article on this topic is found here: <www.repeater-builder.com/antenna/ant-sys-index.html#iso-cir>
Several years ago, I acquired more than a half-dozen Sinclair dual isolators that were part of a military VHF radio system. Some of these isolators were made to work around 138 MHz, while others were made to work around 163 MHz. I quickly discovered that none of them would perform properly at 2m. I contacted Sinclair and learned that I could have any one of them re-manufactured to a specific 2m frequency for $250- about 1/3 the cost of a new unit. The only prerequisites were that the two dummy loads be in good condition, the isolator works properly at the frequency stamped on the unit, and that the case had never been opened since it left the factory. The radio club shipped the used isolator to Sinclair along with a check, and we received a perfect isolator, with a guarantee, in about two weeks. It is still in service. Every time this topic comes up, a few posters claim that they have re-tuned a commercial-frequency isolator to 2m, and further claim that it works perfectly. Baloney! Proper tuning of a circulator or isolator takes a network analyzer, or at least a good spectrum analyzer with tracking generator and a return-loss bridge. There's no way that an isolator can be tuned properly by simply adjusting the caps for maximum forward power. It takes some precision grinding, machining, and lapping to very close tolerances to manufacture a circulator, or to modify it for a new frequency. The magnetic properties of the pole pieces must be carefully adjusted to match those of the ferrites. This is a very exacting process, and it results in a unit that will work properly only over a narrow band of frequencies- usually less than +/- 2%. Nevertheless, there are misinformed people "out there" who will lead you astray. If you want your repeater to function properly, please have your isolator properly tuned to your frequency. An incorrectly-tuned isolator may SEEM to be working properly, but can be very unstable when the load impedance shifts, and can cause severe damage to the PA or to itself. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Wullie Sent: Friday, June 12, 2009 12:22 AM To: [email protected] Subject: [Repeater-Builder] Sinclair Dual Circulator Help Hi All I recently purchased a Sinclir I2210A Dual Isolator for our local 2m repeater. I was wondering if there is anyone out there who has information on how to align this device ? Kindest regards Wullie GM0MZB Keeper GB3FE and GB3OH
