It's not the 600 KHz that is the problem. The problem is that the paging transmitters are spaced the same as the TX/RX spacing on 2M, thus creating the possiblity of a third-order mix.
Here is what is happening, mathematically: 146.94 + 152.24 - 152.84 = 146.34 When all the transmitters are on, all three signals are in the air at high levels around the site. Anything that can mix could be creating the intermod problem, from one of the amplifiers themselves, to a preamp or even a piece of baling wire tied to a fence post. The mixer doesn't have to be especially efficient, since it is so close to the affected RX, a few microwatts of re-rediated power may be sufficient. That is why this can occur even when everyone has BP filters and isolators and the transmitters look clean on a spectrum analyzer. So a 600 KHz filter would be of no use. Joe --- DCFluX <[EMAIL PROTECTED]> wrote: > > I didn't specify it had to be a cavity. > > Try shorted stub type, just steal a 1000 ft roll of > standard issue > cable guy RG-6 and go to town. > > Or use a L/C filter. > > It would be an intresting experiment anyway. > > > On Wed, 22 Dec 2004 00:51:36 -0500, Thomas Oliver > <[EMAIL PROTECTED]> wrote: > > > > Do you have any idea how big a 600 khz notch > cavity would be? > > > > About 400 ft > > > > We had problems here in the Flint area with two > paging transmitters that > > were 600 khz apart also 152.240 and 152.840 one or > both are off the air now. > > > > tom n8ies > > > > > [Original Message] > > > From: DCFluX <[EMAIL PROTECTED]> > > > To: <[email protected]> > > > Date: 12/21/2004 11:09:12 PM > > > Subject: Re: [Repeater-Builder] Digest Number > 2865 > > > > > > > > > What about building a notch circuit tuned to 600 > kHz? And then put > > > one each on both TX and RX? > > > > > > > > > On Tue, 21 Dec 2004 20:06:10 -0800, Neil McKie > <[EMAIL PROTECTED]> > > wrote: > > > > > > > > > > > > A new VHF paging system just was installed > near here in the > > > > last few weeks. > > > > > > > > Rich wrote: > > > > > > > > > > 155.820 > > > > > BEND, CITY OF WNNU934 > > > > > 100 watts Overturf Butte (Bend) > > > > > 100 watts Awbrey Butte (Bend) > > > > > 300 watts Wampus Butte (La Pine) > > > > > 300 watts Gray Butte (Madras) > > > > > > > > VHF paging is apparently here to stay - > whether we like it or not. > > > > > > > > Neil - WA6KLA > > > > > > > > > > > > Dan Hancock wrote: > > > > > > > > > > We had exactly the same problem in Ann > Arbor. The > > > > > pagers were exactly 600 kz apart and up in > the 158 Mhz > > > > > segment. We installed notch filters and > sharp > > > > > band-pass filters on the repeater with some > success. > > > > > Nothing kept it our entirely. I was about to > try a > > > > > crystal filter on the front end when the > interference > > > > > just ended. One of the transmitters was > taken off the > > > > > air. Note though in our situation, the > pagers were > > > > > there first. > > > > > > > > > > I can't imagine in this day and age, with > the death of > > > > > VHF paging being on the near horizon, why > anyone would > > > > > put up a NEW VHF paging transmitter. > However,if one of > > > > > these pager transmitters is indeed a new > installation, > > > > > it may be possible to force them off the > air. I can't > > > > > quote the section, but the FCC told me one > time in a > > > > > different interference situation that a new > or changed > > > > > transmitter operation it totally responsible > for > > > > > solving interference related to their > transmitter > > > > > within 5 miles of their transmitter, even if > their > > > > > transmitter meets specs. This rule might > possibly just > > > > > apply in this situation. They have installed > a new > > > > > operation that produces an uncurable mix > that wipes > > > > > out your operation. That mix could be > occurring in > > > > > your transmitter, your receiver, one of the > paging > > > > > transmitters, someone else's transmitter, > etc, etc, > > > > > etc. I would suggest that you immediately > contact your > > > > > nearest FCC field office and discuss this > with them. I > > > > > wish I could give you the section, but the > engineer > > > > > who told me about it never actually quoted > the > > > > > section. > > > > > > > > > > Good luck. > > > > > > > > > > Dan Hancock N8DJP > > > > > President, RADAR Inc. > > > > > www.qsl.net/wr8dar > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > [EMAIL PROTECTED] > > > > > > __________________________________ Do you Yahoo!? The all-new My Yahoo! - Get yours free! http://my.yahoo.com Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
