Larry, That's an excellent question! Since the Micor circulators were designed and intended primarily for protection against PA destruction due to load mismatches, they are a "special case." When protection against intermodulation was needed, an additional outboard isolator was specified- and this additional isolator has narrow tuning. The built-in circulator has a wider frequency range when compared to an outboard device, since it was designed to perform its rather limited function over the same range as the PA it was installed in. However, it is fairly common knowledge that most Micor PAs are unhappy when tuned below their specified ranges. Some discussion on this topic is available here: <www.repeater-builder.com/rbtip/micoruhffilters.html>
I should point out that current Motorola MTR2000 stations in the 100 watt models are equipped with built-in circulators for PA protection- and these are not adjustable. However, the VHF high-power stations have two different PAs- one to cover the 132-154 MHz band, and one to cover the 150-174 MHz band. I have four VHF MTR2000 station in service, two of them in the low split, and ! am confident that the internal circulators are working just fine. If a UHF Micor circulator responds to the tweaking described in Kevin's article, I don't think it's necessary to take any more action. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, June 12, 2009 9:50 AM To: [email protected] Subject: RE: [Repeater-Builder] Sinclair Dual Circulator Help What about factory-installed circulators, such as those in a UHF Motorola MICOR station/repeater? Who can those be sent to for proper retuning? -----Original Message----- >From: Eric Lemmon <[email protected] <mailto:wb6fly%40verizon.net> > >Sent: Jun 12, 2009 8:48 AM >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: RE: [Repeater-Builder] Sinclair Dual Circulator Help > >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:Repeater-Builder%40yahoogroups.com> >[mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Wullie >Sent: Friday, June 12, 2009 12:22 AM >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >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
