--- In Repeater-Builder@yahoogroups.com, "vintageaudio2004" <[EMAIL PROTECTED]> wrote: "...SYSTEM DESCRIPTION ...Just as a refresher... ... a Sinclair RM201-112S1B RX Multicoupler fitted with a BP Preselector (with 1MHz pass-band, and factory tuned to 165.625), a "Hi Level" Bipolar preamp(Angle Linear brand / anglelinear.com), and a 12 port power divider. Between the preamp output and the power divider there is a 2dB pad fitted.
_____________________________________________________________________ You have certainly put a lot of effort into your investigation. I think you are on the right track to focus on the receive side of the system. 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 You indicated that you see the interference most often when a user radio is transmitting at the site - near the equipment or 3-400 feet away. This points us in a new direction. When an IM run is computed, normally the objective is to look for tx to tx mixes. Accordingly, the system transmit[and other co-located transmitter] frequencies are input as 'originators' and the system receive frequencies are input as 'targets'. However, if the IM is occurring in the receive side, your mobile tx [base receive] frequencies also need to be input as 'originators'. Now you get a LOT of hits. The problem is to understand the IM characteristics of your receiving system - system meaning the combination of all the elements - the base station receivers, the preselector, preamp, attenuator(s), power dividers, cable loss, etc. Your MTR2000 stations have an IM specification of 85 dB. Suppose we say your receive threshold is -116 dBm [0.35uv] What the IM spec means is that signals 85 dB stronger than -116 [-31] have the potential to mix and generate interference [assuming the mathematical mix is proper,e.g. 2A-B] But - your receiving system consists of more than just the MTR. The receiver filter, multicoupler, and preamp contribute to the equation. The preamp an particular. The primary use of the preamp is to overcome the loss of the power dividers. At UHF and 800 [where site noise is usually not a factor] we also take advantage of the preamp's low noise figure to improve the overall receiver system sensitivity. The unfortunate fact is that this improvement comes at a cost - that being the degradation of the receiving system IM rejection capability. For this reason - it is imperative that receiver preamp systems be carefully designed. 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. In the case of VHF systems - site noise - not the lower noise figure of the preamp - is often the limiting factor in determining the system's effective receiver sensitivity. Whereas an 800 MHz multicoupler may have a net gain of 8 to 10 dB from antenna port to receiver ports, your Sinclair unit is specified as having a net gain of only 1 dB. It is not clear from your system description if you replaced the Sinclair amp with the Anglelinear amp - or - if both are in line. Assuming the preselector and power divider loss to be on the order of 13 dB, the Sinclair amp probably has a gain of around 14 dB for a net gain of 1 dB. If you replaced that amp with the Anglelinear [which has a gain of 20 dB] plus you have installed a 2 dB atenuator - then your system net gain is 5 dB - that might either be OK or unnecessary based on the site noise floor. If you have both the Sinclair and the Anglelinear amps in line you have way too much gain. 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. If you have any degradation at all - that will be sufficient data to determine the site noise. The procedure is covered here: http://www.repeater-builder.com/tech-info/effectivesens.html 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/
