Chris, I plugged your frequency separation and power level into CommShop, and assumed a receiver sensitivity of 0.3 microvolts. The program responded that at least 77 dB of isolation is needed for zero desense- which is the obvious goal of any repeater builder. CommShop calculated that 77 dB of isolation can be achieved by 112 feet of vertical separation or 5,681 feet of horizontal separation. I will readily admit that CommShop is not perfect, since it makes many assumptions that may or may not be valid in your particular case. That said, it has been remarkably close in its projections- in my personal experience, anyway.
The reality of your situation is that you do not have sufficient real estate or tower height to construct a workable repeater with separate TX and RX antennas. I strongly suggest that you give up on the two antenna idea and start looking for a good used 220 MHz duplexer. My own 220 MHz repeater uses a Telewave TPRD-2254 duplexer, and has been desense-free. Although this duplexer is available new for about $1,120 with a Ham discount, I have seen this exact duplexer on the used market for less than $500. More info about the TPRD-2254 duplexer is here: <www.telewave.com/pdf/TWDS-6026.pdf> 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Monday, August 30, 2010 5:45 PM To: [email protected] Subject: Re: [Repeater-Builder] Duplexers Thank you for your response. The problem is that the repeater is located on top of a building and the tower on that building is only about 20 feet tall. We can move the two antennas apart horizontally, but only 20 feet vertically. Duplexers are way too expensive and hard to find for the 200 Mhz band. We are running about 20 watts and the frequency separation is 1.6 mhz. Sometimes a week signal comes in and sometimes the transceiver is desensitizing the receiver and covers it up. Any suggestions? Thanks Chris In a message dated 8/30/2010 8:36:53 P.M. Eastern Daylight Time, [email protected] writes: Chris, You do not have to use a duplexer, but it makes building a repeater SO much easier! Keep in mind that "antenna separation" usually means vertical separation, not horizontal separation. Moreover, the same isolation provided by 1000 feet of horizontal separation might be provided by 10 feet of vertical separation. The amount of isolation you need is based generally on the transmit power, frequency separation between TX and RX, and the sensitivity of the receiver. The receiver bandwidth and antenna types also play a factor. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> [mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Mackey Sent: Monday, August 30, 2010 4:44 PM To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> Subject: [Repeater-Builder] Duplexers Our club was recently given a 220 repeater. We have two seperate antennas. We do not have a duplexer. My question is do we have to have a duplexer? How can we keep the transmitter from desensitizing the receiver? The antennas are apart but can be moved farther. Thanks Chris Kg4bek
