Jeff, You did an excellent job of explaining the complex interrelationships among 2m repeaters. However, not all 6m repeaters have a 1 MHz split; my 6m repeater on Tranquillon Peak follows the California band plan and has a 500 kHz split. The duplexer has four cans about 12" in diameter and five feet tall.
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jeff DePolo Sent: Friday, July 30, 2010 7:30 AM To: [email protected] Subject: RE: [Repeater-Builder] Repeater transmit levels at the receiver? <snip> How many thousands of 2m repeaters are out there running 100 watts at 600 kHz offset without desense? Let's be generous and say they have 100 dB of isolation in the duplexer. +50 dBm TPO - 100 dB = -50 dBm transmit carrier hitting the receiver. No big deal. And that's on 2m. The offset on 2m is only 0.4% (0.6 MHz / 146 MHz), whereas on 6m, it's 1.9% (1 / 53 MHz), making isolation requirements that much more stringent on 2m. Now let's look at a 6m example. You have a 6m repeater on a 1 MHz split? Let's say it's on 53.99-, highest channel in the band, putting your receiver on 52.99. Some other ham is working simplex on 52.525, using 100 watts into a unity-gain antenna, and he's 40 miles away. His signal into your receiver, assuming unity gain on your end too, and line-of-sight, is -53 dBm (that's what the free-space path loss works out to: 103 dB for 40 miles on 6m, check my math). Would you expect this guy 40 miles away talking on 525 to desense your repeater? If so, then you should expect *every* ham who transmits on 525 (or potentially any other frequency within 1 MHz of your receiver) within a 40 mile radius of your repeater to cause you desense; those that are closer than 40 miles are just going to desense it even worse. <snip> --- Jeff WN3A
