Hello Tom, I think that the clue here is that you had desense with both the RG-213 and the LMR-400. My guess would be that there is some mismatch between the antenna and the duplexer. The duplexer may be tuned for a 50 resistive ohm load, but the antenna system is presenting some other impedance/reactance.
Can you tell us more about the duplexer? An 8 cavity duplexer seems a little overkill for a 1.6Mhz split. What is the TX-RX isolation? Is it bandpass/band reject? I use a 4 can BP/BR duplexer on my 6 meter repeater with more than enough isolation for a 1Mhz split 52.41/53.41Mhz. I would not get too concerned on feedline loss on 6 meters. The difference between RG-213 and LMR-400 at 6 meters is less than .5dB loss. Look at this link for reference: <http://ocarc.ca/coax.htm> LMR-400 has a definite potential problem with repeater use. I think you read the posts abut it, plus Motorola put out a white paper not recommending this type of cable for duplex operation. We pulled out hundreds of LMR cable jumpers at my old job because of intermod problems. The intermod seems more related to poor connector installation and durability of the connectors being moved around. Noise was more of a problem in the cable itself, especially where the cable was subject to movement on the tower, or getting to the tower. I know many people will say that they have used LMR for years with no problems. This can be true, but the potential of a problem is still there. 73, Joe, K1ike Tom Elmore wrote: > I recently put a six meter repeater (52.810/51.110) on the air here in > Anchorage, Alaska. I am feeding it with about 60 feet of LMR-400 > cable and am experiencing quite a bit of desense. I did a search for > "LMR-400 in duplex operation" and came across several posts from users > of this list and decided to sign up and investigate some more. I am > running a GE Master Pro at 100 watts into a 8 cavity Sinclair > duplexer. The antenna is a Diamond co-linear mounted about 35 feet > above ground at the present time. I live on a hill here in town and > currently have the repeater mounted at my home qth. When I terminate > the duplexer into a dummy load and look at it with the spectrum > analyzer it performs very well with no desense. Connecting up the > antenna is another story altogether. I have been pulling my hair out > over this one thinking it must be a duplexer problem. Originally I fed > the antenna with RG-213 which I know is not the best choice for > repeater use but it is what I had handy at the time. I was getting > desense with the RG-213 so I switched to the LMR-400 since I had a > roll someone had given me. I actually think I had slightly less > desense with the RG-213. Is the 400 really not that suitable for > duplex operation even at 6 meters? What would be a good alternate choice ? > > > > > Thank You > Tom Elmore KA1NVZ > Anchorage, Alaska >
