> "vintageaudio2004" <[EMAIL PROTECTED]> wrote: > Hi, > Thank you very much for you very detailed explanations. > They are very helpful. Bellow please find some responses > to your message.
Believe it or not, some of us live for this stuff. > > "nj902" <[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. Not nearly enough > > 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 had DCI make a number of repeater window filters for me. It took a bit of time on the phone with Sean to find out what they could do vs what trade offs I would allow in the specs. The end result UHF 10 pole filters they made to my specs work just killer. VHF is another animal... > 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? Excellente' you will luck out by having all your receivers within a same window. Imagine doing receive preselection when the FCC gives you "pop-corn picked receiver input frequencies". My VHF rx pre-selection system is a very special network made for use with non-sequencial (mixed spaced) receiver inputs. Oh the joy it was to make... > > 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. The root of the problem is the engineering... Larger companies don't want to keep higher paid Engineers on staff and smaller radio shops can't afford them. It would be really nice to have access to the mentioned 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. Sinclair supplies some of their equipment with Angle Linear preamps. Good stuff, but it's probably also bi-polar at VHF. might want to check that... > 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. You don't mention if the system will be mostly mobile or combo mobile and portable operation. If you don't have any portables, bypass the preamp right away... you don't really even need it at this point in the game. > > 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. Square one along with desense and effective receiver sens. There's a really killer Dubus Magazine Phempt Preamp article available for free on my www.radiowrench.com/sonic web page, which well describes some of the problems with preamps and example noise floor levels of a location. Well worth a look if you have time... > 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. Some of us only dream of a low noise floor... but then there's that city of 1.2 million just down the mountain and that [EMAIL PROTECTED] military radar and eplers system nearby. > 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. Sinclair is probably right, but they should also have told you the bandpass filter is not nearly enough protection if you ran a system description by them before you bought the pre-selector assembly. > Still waiting for Telewave's response (the combiner vendor). > Thanks again for all your help. > Alex > Lunch was good... cheers, skipp www.radiowrench.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/
