Hi, Thank you very much for you very detailed explanations. They are very helpful. Bellow please find some responses to your message.
--- In Repeater-Builder@yahoogroups.com, "nj902" <[EMAIL PROTECTED]> wrote: > --- In Repeater-Builder@yahoogroups.com, "vintageaudio2004" > <[EMAIL PROTECTED]> wrote: As Skipp indicated - your Sinclair RX preselector is less than > optimum. If you look at the filter's curve on the Sinclair web site > you will see that your transmitter frequencies are only being > attenuated by about 25 dB. There are many 'window' filters available > with steeper skirts, for example, look at this DCI filter: > > http://www.dci.ca/html_commercial/graph_165d92-1-6_wo143.htm I've taken a look at the DCI site, and also contacted them. They promptly responded and seemed very eager to help. Have you had any previous experience with their products and so forth? > If we allow the system IM point to be degraded - then strong receive > signals from our own close-in mobiles and control stations become a > very real problem. Motorola has several white papers that warn > System Engineers of this issue. Thanks for the excellent explanation. It would be nice to be able to have a look at some of those /\/\ white papers. > It is not clear from your system description if you replaced the > Sinclair amp with the Anglelinear amp - or - if both are in line. No, the Anglelinear came with the multicoupler. As it is now, the multicoupler is still completely stock, no modifications or adjustments have been done to it. Even the 2dB pad came originally with the unit. > To get the receive design right - you should start by measuring the > site noise floor. Directly measuring site noise is a complex > subject but for starters why not just test one MTR station directly > on the recevie antenna. Will do that when we return to the site to redo the present grounding system, and hopefully resolve the IM issue. Although I need to point out, this area being very remote, and scarcely populated, I expect that the noise floor, even in VHF will be much lower than average. Terrain is mostly flat, with lot's of vegetation towards certain areas (tall trees, a few smaller elevations, etc). Still we will measure the noise floor as described just to be sure, and have the numbers at hand. BTW, yesterday I received a response from Sinclair, the multicoupler vendor. They ran (another) IM study, and also indicated that they believe that we have an "External IM Problem". The data we sent them was exactly the same as we posted here on the group (system description, tests, etc). It strikes me that no one else has so far arrived at the same conclusion. For the benefit of the group, bellow I've taken the liberty to copy the message text as it was received. Still waiting for Telewave's response (the combiner vendor). Thanks again for all your help. Alex ======================== I have attached copies of some IM study results. The system frequencies produce direct IM product hits starting at 9th order as shown in study titled AlexR. 11th is the highest the study can show, but it is likely they continue with higher odd multiples as well. A single antenna solution is not recommended due to the presence of these products. Check and see if these are the subtle products that you are hearing. Under normal circumstances with a tower in good condition and decent T-R antenna space isolation, I would not expect that you would have a problem with these higher order IM products. Adding any portable Tx frequency to the study will cause multiple direct A + B - C product hits due to the 5 MHz offset, as shown by AlexR+portable. I believe that you have an external IM problem. The mobile Rx antenna connected to the power divider with no amplification still detected the interference, which indicates an external mix that is coming in on the Rx frequencies. Also strong enough to cause a problem despite the splitter loss that is experienced by the signal without benefit of the LNA. A participating Tx carrier would encounter this split loss and the resulting IM level would be substantially reduced if the IM was occurring in the receiver itself. Diverting Tx power to a dummy load, prevents the RF currents from being induced in the tower or antenna network where the mix is being produced and then radiated to the Rx antenna in turn passing through (on frequency) the multicoupler to the receivers. ======================== 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/
