Kevin, I'm sorry to have to tell you this but I think your calculator batteries need to be changed. 0 dBM = 0.2236 volts in a 50 Ohm circuit. +20 dBM is indeed 100 mW and P=EI and inserting 100 mW into the Eq. for Volts in a 50 Ohm system, E= the sq. rt. of the quantity (.100 x 50) = sq. rt. of 5 = .707 V. or 707 mV. not the 2.24 V. you indicated. Even if the rcvr had an input Z of 50 Ohms (without any X component at the operating freq. which I doubt but let's accept that as so) , the rcvr input Z at a freq. removed by 6 MHz can hardly be assummed to be any thing close to 50 Ohms and is more likely to be highly + or -X therefor not absorbing or accepting any input signal and your analysis lacks merit.
Allan Crites WA9ZZU ________________________________ From: Kevin Custer <kug...@kuggie.com> To: Repeater-Builder@yahoogroups.com Sent: Mon, July 26, 2010 3:47:08 PM Subject: Re: [Repeater-Builder] Re: Repeater receiver testing >yes I know -55db is I think around 399 microvolts which will flatten any >receiver, > I sure hope not; there are many signals coming down my antenna that are that strong. In fact, my 440 repeater 13 miles away is at about that level at my antenna connector. Reminds me of a test I did with a GE MASTR Pro VHF receiver (stock - no preamp - ER-41C) and a GE MASTR II PLL exciter (stock - 200 mW output) and a tee connector. Receiver and exciter on each end of the tee (with short cables) and antenna/service monitor on the center. As I remember, the frequencies were about 6 MHz apart and the 'repeater' would duplex reasonably well considering no filtering was used between the units. Of course, I'm not saying there wasn't desense, but it worked - pretty well. Obviously, the 200 mW was split into two paths and who knows how much was hitting the input port of the receiver, but lets say half of it was - 100 mW. That power level is +20 dBm. I can almost guarantee you that transmitter side band noise is what created the desense - not the shear power of the transmitter. +20 dBm is 2,240,000 μV or 2.24 volts of RF. My receive antenna for the UHF remote receivers is top mounted across the tower face beside the VHF stick. The VHF repeater runs 250 watts. 4 watts of VHF power appears at the splitter (in the shelter - including feedline loss) which feeds the 4 UHF receivers. The splitter (old CATV type) gets warm - you figure out how much VHF power is getting to each UHF receiver <grin>. Kevin Custer
