Ken, In some respects, it's better to have two high-power repeaters very close in frequency, rather than more than 1 MHz apart.
Here's my real-life scenario: Two MTR2000 100 watt UHF stations, with Decibel Products DB408L antennas about 12 feet apart. These are elliptical-pattern dipole antennas, and both of their major lobes are facing in the same direction, with their minor lobes looking at each other. The transmit and receive frequencies are 150 kHz apart between the two repeaters, and the TX-RX split is the standard 5 MHz. Both MTR2000 stations come standard with a single circulator, but I am adding a Telewave "high performance repeater panel" to each machine. This comprises a four-cavity bandpass/bandreject duplexer, a dual isolator, and a dual 8-inch cavity bandpass filter in front of a GaAsfet preamplifier. The key to making this work is that the notching inherent in each repeater's duplexer is just wide enough that it greatly attenuates the TX carrier from the other repeater, if it is close enough, and the bandpass cavities ensure that nothing remains to desense the receiver- even with the preamp. The interfering signal from another repeater is a lot easier to eliminate when it comes over 12 feet of air, than when it is in the same coaxial cable. The triple circulators will prevent intermodulation between the transmitters. Besides, the primary reason for having two repeaters, two duplexers, and two antennas is to make them separate and completely redundant. The last thing in the world I want to do is to invest in a transmitter combiner, where I throw away half of the power from each transmitter, and now have single points of failure (the antenna, the feedline, the hybrid and the hybrid's dummy load) in a position that can bring down both repeaters. My installation may be unusual, but the design rule from the git-go was that there must not be any shared components between the two repeaters. The rule calls for separate solar panels, charge controllers, battery racks, antenna masts, etc. A few years ago, I personally dismantled a Government radio system that was once used for the Space Shuttle program at Vandenberg AFB, which program was later canceled. There were eight 100 watt Micor VHF repeaters combined into one transmit antenna, with the eight receivers being fed from a multicoupler connected to a separate receive antenna. I tested every component after removal, and I found that three of the four 150 watt dummy loads- absolutely CRITICAL components in this combining scheme- were open! It also happens that I was one of the military users of this radio system at the time (mid 80's), and I remember that it never measured up to my expectations for coverage. No wonder; the great mismatch caused by the open loads was making the Micor PAs go into SWR shutdown, and one PA burned up. Instead of 12.5 watts, only 1 or 2 watts was making it to the single antenna. It is ironic that the cause of this system's demise lay abandoned and gathering dust for 15 years until it was discovered purely by accident! 73, Eric Lemmon WB6FLY Ken Arck wrote: > > Hi folks > > Imagine this scenario. 2 quality, UHF repeater colocated at the same site. > Both repeaters are higher power ( > 50) out of duplexers, duplexers are > high quality BP/BR types. Here's the kicker - they're spaced 25 Khz away > from each other but not inverted from each other. > > Now, theory is one thing as to the interference potential without resorting > to mulicouplers and combiners. But what is the real world problem > potential? Anyone want to take a stab at this one? > > Ken > ------------------------------------------------------------------------------ > President and CTO - Arcom Communications > Makers of state-of-the-art repeater controllers and accessories. > http://www.ah6le.net/arcom/index.html > We now offer complete Kenwood TKR repeater packages! > AH6LE/R - IRLP Node 3000 > http://www.irlp.net > > > Yahoo! Groups Links > > > > 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/
