Mixing products generally don't sweep through the spectrum, as usually both TXs are stable and the mix would be as well.
Joe M. Matt Harker wrote: > > > Sounds like a bad case of mixing products (i.e. 2A-B) where A is > frequency A and B is frequency B. Several years ago, I helped cure a > similar problem on 146.820 MHz. Turned out four transmitters were > involved in that fracas and it only took two to start the repeater howling. > > 152.480 is one of the common paging frequencies. There is another > around 157 MHz that is usually a troublemaker for 2m amateur > repeaters. A lot paging transmitters are NOT properly filtered. I have > run across some that were connected directly to the feedline and antenna > with no circulator, no isolator, nothing. This is unacceptable > practice. No transmitter or receiver should be looking directly into a > feedline at any fixed site where multiple transmitters are in operation > or, where interference could likely erupt. > > I doubt the UHF paging transmitters has a whole lot to do with the > problem although there is a slim chance they might. Chances are, the > VHF transmitter is the one involved. > > The next thing I'd be curious about, given the paging company's comments > about the ability to key the transmitters separately, is this: How are > they getting the data from their offices to the transmitters? By radio > link? By wireline? or both? If by radio, therein may lie the second > transmitter in the mix. If not, keep searching. > > One thing to keep in mind with the spectrum analyzer is that it is not > likely as sensitive as the repeater's receiver. Some spec an's are > pretty sensitive, others not so sensitive. So you may want to visit the > paging site with it and see what you find. Especially around 304.960 > MHz which is the double of 152.480 MHz. > > > KC5DBH Matt > > > ------------------------------------------------------------------------ > *From:* Mike <mwbese...@cox.net> > *To:* Repeater-Builder@yahoogroups.com > *Sent:* Wed, October 28, 2009 1:28:56 PM > *Subject:* [Repeater-Builder] Pager Interference to 2-meter & VHF Public > Service Band > > A couple of weeks ago, our repeater system started to experience > interference from a paging system. The repeater is on 146.850 (-600 > KHz), with the antenna system about 120 feet up a water tower. The > repeater itself is an Advanced Communications System KRP-5000 running 35 > watts through a set of 4 WACOM cans. The feedline is 7/8" hardline > feeding a DB-224. All jumpers are RG-214 MILSPEC with MILSPEC > connectors. This system has been in service for years and has never > given us any problems, and we are the only ones at the site. > > To be clear, the interference we are experiencing is clearly audible on > the repeater input. I have the capability to monitor via telephone and > have heard it on the receiver, and I've also traveled to the site and > heard the interference on my mobile radio hooked to the repeater > antenna. The interference is also audible on the input in various > locations around town. Also, the interference can be heard on the input > regardless of whether or not the repeater transmitter is on. It also > continued to be present during several days of continuous heavy rain. > > The interference typically shows up at least once a day, although some > days (rarely) it does not show up at all and other days it will show up > several times. Lately, it's been making an appearance around 10 a.m. > and hangs around for an hour or two. As it begins to disappear, it > sounds as though it is moving off frequency. > > This interference has also been heard on at least two other repeaters in > the area. One is about 22 air-miles from the 146.85 machine and is on > 145.110 (-600 KHz). It has also been heard on or near the output > frequency of that machine, and one evening I tracked it from about > 145.120 to 145.190 as it swept through each transmission. The other > repeater it has been heard on is 147.300 (+600 KHz). I also have > reports from a neighboring county a ham/deputy sheriff there has been > hearing it on VHF public safety frequencies. As you can see, it's all > over the place. > > I've been working with the owner of a local paging company and we can > clearly tell that the data we're hearing is coming from 152.480 and > 462.775. He has two sites (about 20 miles apart) that simulcasts on > both frequencies and when those transmitters are active it's easy to > tell that the data is the same. He also tells me that he can key each > transmitter separately and the data from each transmitter will be heard > on our repeater. We also believe that there are other systems on nearby > frequencies that are being heard on our repeater, specifically 462.850 > and 462.925. > > I've run IMD numbers on everything I can think of, but can't come up > with a common thread. For it to be moving all through the 2-meter band > and for it to be mixing with several different frequencies, it seems to > me that it's got to be very ugly and unstable. What am I missing here? > > Mike > WM4B > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > mailto:repeater-builder-fullfeatu...@yahoogroups.com > <mailto:repeater-builder-fullfeatu...@yahoogroups.com> > > > > > >
