blisswheeler wrote: > I've run into a problem that baffles me. > > I have two sets of WACOM WP-641 4 can Duplexers. They have been tuned > to TX on 146.96 mHz and RX on 146.36. One set was tuned in a > Professional Lab, the other I tuned myself. They tune very well with > excellent rejection ~90db. When I duplex into a dummy load running > 100W, I can inject a signal with an iso-tee and there is vertually > zero desense to the receiver. When I hook it to an antenna it > instantly experiences 30db of receiver desense. I can lower the power > to 3 watts or less and still have 20db desense. I have a GE Mastr II > repeater, but I've used a separate Transmitter and receiver and > experience the same results.
You say "desense" but the description sounds like external or nearby noise, perhaps generated on-site via Passive Intermod. Confirm for us that you're saying the noise is only present (while you're injecting a test signal weakly via the Iso-Tee) when your transmitter is on-air. If you turn the transmitter off, the weak signal gets stronger, correct? I like the 600 KHz switch mode power supply theory, but you could also be fighting PIM from the good old "rusty joint" problems, etc. Are you at a high-RF site with a lot of other transmitters? Lots of other antennas hung near yours in your near-field? If so... do you see anything "loose" up there, in the near-field of your antenna? Rusty? Not grounded? How's the tower grounding/bonding? Are all feedline shields bonded properly with hardline grounding kits? How about some cheap/crappy antenna near your good commercial quality one that might have fallen apart internally? (You're not using a comet or some other low-grade antenna prone to having things physically fall apart inside are you?) (To figure out if that's the problem, a wooden broom handle and some medium-gentle tapping on the "stuff" around your antenna can be used to see if the noise suddenly gets worse when things are vibrated/shaken. Of course, something more violent than a "tap" can be employed when you find out it's some idiot's low-grade antenna... WHACK... if only to make you feel better. Haha... I would never recommend DAMAGING their gear, but well... the thing is falling apart anyway, right? Heh heh... just kidding. Kinda. Grrr.) How difficult would it be to temporarily hang (even at ground level) another antenna to see if you physically move where your transmitter is, if the problem gets better/worse? It will help show if your transmitter is "lighting up" a problem area up on the tower. If it's PIM, you'll be "enjoying" the hunt for the source(s) for quite a while, I'm afraid... I've seen some very smart folks throw up their hands in frustration over PIM issues on heavily-loaded towers with lots of transmitters on them, surrounded by lots more towers and tons of high-RF broadcast and other transmitters. It'll drive you crazy hunting for them in that environment. Especially if the resulting mixes are RIGHT ON your input frequency and additional band-pass or notch filtering can't alleviate it. Describe your antenna system and feedline... and any others around/near you. That'll start to tell the tale, if it's not a noise source you can easily find. Currently I'm hunting a "buzz" (and we've been hunting for a long time) on a VHF input at 147.825 - receiver and pre-amp work great... had a user with a 50W mobile check into a net Sunday night from over 80 miles out... but... there's this damn weak signal mixing with everyone who doesn't fully capture the receiver. Annoying. Especially since it constantly buzzes behind the multitude of new ham HT users running HT's and rubber ducks. Nate WY0X
