Re: [Repeater-Builder] Re: Multiple receivers one antenna ???
At 12:29 PM 12/19/2005, Coy Hilton wrote: It may sound interesting (and cheap) but the reason that no one else has suggested it is because the impedance miss matches it causes. That is why you need something like a multicoupler whis is first a pre amp to keep the loss to a minimum then sends the pre amp to a splitter that maintains the 50 ohm match required by the receivers on each output port. You can handle the impedance matching by using 1/4 wave sections of 75 ohm coax between the receiver input and the T. The 1/4 wave 75 ohm section steps the 50 ohm receiver input impedance up to 100 at the other end, two of those in parallel at the T gets you back to 50 to match the feedline. Any number of receivers other than 'powers of 2' is more complicated. This does nothing for the loss of course. Roger Grady K9OPO 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/
RE: [Repeater-Builder] Re: Multiple receivers one antenna ???
It's called a Wilkinson splitter. Here is a link to some of the theory. I don't think it's fair to call it a Wilkinson without a resistor across the output ports. A real Wilkinson provides port-to-port isolation due to the addition of the resistor. A tee and 75 ohm cables doesn't provide any appreciable isolation; it's just two transmission line transformers teed together to yield a proper match assuming that the ends of the cables are terminated into 50 ohm loads. In the real world, receivers don't have 50 ohm Z across a wide range due to front end filters and other factors, so you may end up with additional loss (above and beyond the theoretical 3 dB power-dividing loss) due to the lack of isolation between receivers. --- Jeff 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/
RE: [Repeater-Builder] Re: Multiple receivers one antenna ???
You can handle the impedance matching by using 1/4 wave sections of 75 ohm coax between the receiver input and the T. The 1/4 wave 75 ohm section steps the 50 ohm receiver input impedance up to 100 at the other end, two of those in parallel at the T gets you back to 50 to match the feedline. Any number of receivers other than 'powers of 2' is more complicated. This does nothing for the loss of course. Roger Grady K9OPO The math works in the case of the tee being at the antenna connection, but the ASCII drawing that accompanied the previous email put the tees on the backs of the receivers, sort of like old-style thin Ethernet 10base2, except instead of having terminations at the ends of the backbone, one end is connected to the last receiver, and the other end is connected to the antenna. Matching goes out the window with this design. The problem with even doing the tees with odd quarterwave 75 ohm transformers is that you have virtually no isolation between receivers. If your receivers have tight front ends, unless all of the receivers are very close in frequency, you may end up with more than 3 dB of theoretical loss due to phase cancellation back at the tee. If you're really pinching pennies, Kevin's recommendation of using 75 ohm CATV/MATV splitters is better since they provide port-to-port isolation, and the losses due to the impedance mismatch (50 versus 75) are insignificant (theoretically approx. 0.4 dB + normal dividing loss). --- Jeff 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/
