Once I realized that you were stating the frequencies as input/output
(I'm used to output/input; Mea Culpa!), the answer became obvious:
Third-order intermodulation interference.
If we consider that Repeater A transmits on 146.70 and receives on
146.10, and Repeater B transmits on 147.30 and receives on 147.90, we
have a 2A-B (third order) product on 146.10. That is, 2 times 146.70
minus 147.30 = 146.10 MHz, the RX frequency of Repeater A. It also
works the other way, 2B-A, with the product falling on the RX frequency
of Repeater B. Exactly the same combinations exist for the other two
repeaters. What a mess!
What's happening here is that the TX carrier from Repeater B is able to
sail right through the TX cavities on Repeater A, because pass/notch
duplexers (the most common type) have very little bandpass effect. The
notch in Repeater A's TX cavities is tuned to notch out 146.10 MHz, so
it has little effect on the incoming 147.30 carrier. The intruding
147.30 carrier mixes with the native 146.70 carrier in a 2A-B manner,
and a new 146.10 MHz carrier is generated. Unfortunately, the notch
depth of Repeater A's TX cavities is probably in the 85 to 100 dB range,
not nearly enough to attenuate this spurious carrier. It simply crosses
over to the receiver and swamps the desired RX carrier. The same
process exists on the other three repeaters.
The straightforward solution is to install a dual ferrite isolator on
each repeater's transmitter. This will prevent intruding carriers from
reaching the power amplifiers and causing intermod.
As another poster mentioned, the coordinating body should not allow such
IM-vulnerable pairings to occur within close proximity. I must assume
that one of each conflicting repeater pair is uncoordinated.
73, Eric Lemmon WB6FLY
Q wrote:
Trying to troubleshoot an intermod??? problem between two repeaters. The
146.10/.70 repeater's receiver gets blasted by the 147.90/.30
transmitter but only when they are both transmitting. Yes, the transit
freqs are 600 kHz apart and they are only 5 miles apart. Would a
circulator help this problem? Also have similar problem between the
146.07/.67 and the 147.87/.27 repeaters when they are both keyed up.
Ideas?
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