On Mar 7, 2009, at 6:59 PM, Kevin King wrote: > Bob Ok so much for the software. > > One other user today mentioned a possible issue with another repeater. > 147.300. All I had been looking at with the software were the > transmitters > at his site. > > Ok so I plug that into the mix of xmiters in the software and bingo. > > A+B-C 146.70 + 146.70 - 147.30 = 146.100. > > The repeater 146.70 does have a circulator.
Doesn't matter if it has a circulator or not. That 146.70 signal doesn't have to mix in THEIR PA, or even in a PA at all, to be a mess. All you need is something outside on the tower -- rusty bolts, bad guy wire connection, tin on the roof touching at seams, colinear antenna coming apart internally, cracks in aluminum dipole antennas, corrosion of just about anything -- to act as a diode at RF. When you put more than one repeater at a site with the same OFFSET, you're set up for that kind of mixing. Multiple Amateur machines at a site means, it's ready to go. (Check the math sometime... three 600 KHz offset VHF machines, three 5 MHz offset UHF machines... doesn't matter what frequency. Once you have two on the same offset, you're set up for mixes external to both systems to hit whoever's input is unlucky enough to get hit.) To find out which system is really mixing, you may have to either a) temporarily lower the power output of systems one at a time to see when the mix diminishes, or b) the more "active" way... be there when it happens and slap the power switch/TX disable switch and see if it instantly stops when you apply "the kill switch" to any particular transmitter. (This of course, doesn't make whoever was talking on it real impressed with your "technical skill", since they have no idea why you just bounced their repeater. And you need buy-in from the folks who own the things... to do stuff like that.) Lowering power on systems involved in the mix can give great clues to where the mix is physically occuring. For example, if you drop to exciter power and the mix is still happening... and you can reproduce it. Either the antennas are high-gain and RIGHT on top of each other on the tower... or the mix is being amplified somewhere. Start looking at PA's and receive pre-amps... If the mix disappears from the receiver being hit, with a very slight reduction in power of one of the mixing partners, the problem is not likely happening in an "active" component. But those are just hints, there's too many variables to know 100% at that point. It's just some of the "homework" you have to do. The fact that it seems to happen only in cold weather would tend to indicate to me, that it's a metal to metal joint that pulls apart when the metal gets cold and shrinks. Fixing that type of problem can rapidly turn into a hunt for a needle in a haystack. Been there done that. It sucks. Looking at all the transmitters for noise with a spectrum analyzer is also a key homework component. Make sure something simple just isn't "broken"... if a dirty TX is at the site, all bets are off until it's fixed. And of course, always think about "what changed right when this started"? Did someone up their power? Put up a new antenna? New feedline? Is there a new tenant on the site? Did anything about the tower or antenna structure get changed? These and similar questions can all be clues to the ever-important question -- "When did this start?" The software will spit out so many possibilities it'll make your head spin -- you have to add those to the other things you know about the site, and it's just tough... no matter how you slice it. Nate WY0X
