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Re: Desense is driving me crazy
?I often battle insomnia. Tonight at 3AM I decided to look at some photos I took prior to taking the loops out of the duplexer to clean with alcohol, along with the tee connectors. I take photos more for reference of placement of things really. It's a trick I've learned over the years because I've been taking things apart since I was a little kid. Works well for long term auto restoration too.
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I was already cleaning the tee connectors prior to taking the loops out. So when I did take them out they went straight to cleaning. Instead of examining them prior. Mainly because I figured since I seen some silver flakes on the tee connectors, that I'd just do all the connectors to the duplexer for good measure. Afterwards I was looking at them with the microscope to see how they looked, they looked good then.?
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?Ok so back to the pre removal photos. I started looking at my photos to see if there was any correlation to loop placement. By looking at the tuning capacitor relative to mounting screw locations. Because there's no degree marks on these for any reference points. Then as I zoomed way in I started noticing there was some heavy silver flakes down inside some of the loop N connectors. So there was a bit of an ah ha moment there. When the loops are in the cans, inside the cabinet. I can't see down inside those connectors that easily. The cabinet doesn't allow my head to be inside there to look down. Plus getting older I need glasses to see far away & close up now. But thankfully the camera was able to capture this for me to see & realize.?
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Im going to get some sleep hopefully. Then finish working on this more later today. Set the loops for .5db IL per cavity & line everything up again. Hopefully this solves the noise issue in the duplexer. Then I can start focusing on what's causing noise on the tower exactly. Wish me luck.?
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Here's a picture of what I seeing down in the loop N connectors. It's been zoomed in. Hopefully it shows the flakes clearly & the picture doesn't get too degraded in post.
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Desense is driving me crazy
Desense is driving me crazy. Might be a short ride there. ?
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Warning: This will be long & descriptive. In hopes I cover everything for getting good advice in return.
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?Last year I bought a new Yaesu DR-2X & a new Hustler G7-144. I'm using some older Andrew's 1/2 heliax. The duplexer was purchased from a wholesaler as new old stock. It's a Sinclair Q201GC 6 cavity (7" diameter) set in it's own cabinet. The data sheet says it left Sinclair in 2015. Originally tuned to 170Mhz.?
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?This makes the 4th repeater I've messed with in the 26+ yrs of being a ham. I've done lots of tower & antenna work over the years. I even used to work for a tower company many years ago climbing. One thing I never messed with was tuning duplexers. So this time I wanted to do everything myself. One reason was I didn't want to haul this large duplexer many miles away to get it tuned. I didn't have a safe way to haul it sitting upright (vertical).?
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So I decided it was time to invest in some test equipment. For tuning & other ham radio projects. Originally I already had a Lite VNA 64. I used it to check the duplexer cavities individually to see if they will tune down from 170MHz to 145MHz. They looked like they would. So I decided to move forward with this set. Now, as I mentioned, I was told these were "new old stock". They look great! Still shiny. They had a cardboard box cover over the cabinet for shipping. Plus as stated the data sheet from Sinclair inside. One thing that strikes me as odd, was all 6 of the brass dust covers for the trim capacitors were missing. I'm to understand those are important to keep the caps clean.?
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? I've gotten some great help from people here on repeater builder to get started. I may have even gotten on some nerves asking so many questions lol.?
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It was mentioned I will need to make a new harness for the duplexer to get it down to 145.330. I was able to get information on the correct lengths for the cables. Using RG214U & silver N hex crimp-on connectors. I made each cable 13.5 inches long. I used silver plated amphenol tees. I had to buy the tees used. As they were impossible to find new at a reasonable price. With the cable, connectors & tools I had nearly $300 dollars tied up in just materials to make the harness.?
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?I needed some more test equipment to begin alignment of the duplexer. Keep in mind this is my first time tuning a duplexer & getting equipment for it. I was originally going to invest in a used VNA like HP, Keysight etc. But the turn off there was age of equipment vs cost. Then being worried about buying old equipment that might be a problem later with failing parts due to age of components. Then finding parts to repair them. Plus most everything that was suggested still only had about 100db of dynamic range. Which wasn't going to be enough for this duplexer. I started looking around at other equipment that's newer. Plus more in my budget.?
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?I decided to buy a LibreVNA. It's advised as 90 to 100db of dynamic range & seemed to be better than my Lite VNA 64. It still wasn't going to get me past 100db for final tuning. But it was great for all the individual cavity tuning. To final tune I decided to buy a Siglent SSA3021X + spectrum analyzer with tracking generator. I used this stuff to tune the duplexer. It appeared to have about 120db of rejection.?
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?When I hooked everything up & started testing out the repeater it all worked OK. But I'm not one to just stop there. So I decided to start testing for desense. I also have a TinySA ultra. This has a spectrum analyzer plus a signal generator. I'd generate a low power signal with that & listen to the DR-2X receiver speaker by ear. Get the signal down low & listen. Then engage the transmitter & listen for any changes to how the receiver sounds via the speaker. Sure enough I was hearing changes aka desense.?
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I tried this with the repeater hooked to the coax & antenna. Then I'd try it into a 250 watt TX/RX dummy load. It was worse with the coax & antenna hooked up. But it was also still slightly present into the dummy load that bypassed the hardline & antenna. Now for the coax & antenna I suspect the Hustler G7 could be making noise. Since there's so many assembly parts to it. Plus that hardline is older & may have some water damage. But for repeater & duplexer into the dummy load. I totally suspect it's likey something I've done. Or there's dust in those tuning capacitors that didn't have dust covers on them. I'm leaning towards my own fault somewhere. I mean it's my first time tuning a duplexer. It's 6 cavities (4 would be easier). It's my first time making a duplexer harness. Plus my first time using test equipment for this. I do like to learn & do things though. I've always been a bit of a techy geek my whole life.?
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Now since having these issues I've tried to get some better tools to go with the equipment I bought. I invested in several 3 & 6ft RF test set cables. I bought some used but good shape N calibration sets. Agilent Keysight male set & Anritsu female N set. Plus some older amphenol silver barrels (male & female) for through calibrations. I bought new old stock of Narda male & female N 50 ohm loads. As well as Narda shorts & opens. Plus some Midwest microwave 50 ohm loads that were new in package. I picked up various name brand antennuators as well. I even decided to buy a Measall KC901J dual port VNA. Just because I wasn't liking the LibreVNA & having to be tied to a PC to use it.
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?I've retuned this duplexer several times now. Just to try different insertion loss settings with the loops. From .4 to .7db per cavity. Nothing seems to totally remedy the issue completely. As well as trying out different devices to tune it with. I even tried using older tuning methods I've read & have been told about. Where you use a low power signal & a receiver on the other end. I've even tried using two devices when tuning. To check one vs another in a sense. This was actually beneficial because I've noticed the LibreVNA can often give me different readings vs the Spectrum analyzer & KC901J VNA. Then later on I was reading some other people having issues with the accuracy of their LibreVNAs. So for now I just use my KC901J & the Siglent SSA3021X + for making adjustments.?
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I've tried examining the cables I made for the harness. I'd key up & get a low signal into the receiver & engage the TX to hear the desense in the receiver speaker. Then wiggle each cable on the harness. To see if I hear any changes to indicate one of the cables was being problematic. Nothing there made any difference. I have also checked the cables for electrical length & distance to fault. They all came up the exact same length 13.5 inches & didn't show any faults there.?
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?This weekend I took the loops out & looked at them to see if anything looks odd. I also cleaned all the tee connectors with 90% isopropyl alcohol. Plus examined them with a microscope. To see if there were any issues with the insulators or anything that looked odd. I haven't put the duplexer back together yet. It's gotten to the point I don't enjoy messing with it almost. But I don't like being beat. So I won't give up. Everything looks OK inside the cans the best I can tell. I've asked others to give an opinion on those pictures for 2nd opinions.?
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In my mind I keep going back to 3 possibilities. Its either the harness I made. Or it's a problem with the dust caps not being there & dust has gotten into the tuning caps. When I'm adjusting the rejects it can be a pain. They will jump around a bit. Sometimes you'll be turning it to the left & it suddenly jumps right instead, then goes left but jumps ahead after more turning. But that can make it hard to get the notch to be exactly where it needs to be. I've read that can be dirty caps. Where dust inside is making contact with areas of the blades in the caps. Im to understand that dirty caps can cause desense issues from things I've read. Or a 3rd idea I had that's probably unlikely but would explain somethings. This duplexer had a problem. A tech took all the dust caps off to make adjustments. Found that things were not simply solved. Didn't put the dust caps back on. Then they got another identical duplexer & put the box it came in on this set & eventually it got sold to a wholesaler & I got it. I know..that version likey isn't true. But your mind wanders when you can't seem to make sense of things. It's almost amusing what my imagination comes up with.?
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As for the harness I made. I've never used crimp-ons before. I had to buy the tools for it. I looked around for some stuff that had good ratings/reviews. I watched several videos before hand to learn what to do & what not to do. I even used heat shrink that has a glue residue inside, that helps bond things together. The center pins are soldered on as well. Maybe it's because I bought used Tees??
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?Aside from all that I've even went a little further. I have read that the internal RF cables in Yaesu repeaters are marginal. So I replaced them with RG400 with silver plated connectors. I am also using RG400 from the repeater to the duplexer. I have also tried using RG214U from the repeater to the duplexer. I have purchased a RFI surge outlet for everything to plug into. I also tried using a battery powered inverter in one test to totally remove the house electrical from the equation. Just doing other processes of elimination.?
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The only thing I haven't done yet is take these cans completely apart. I can't see the bottom of the plunger tubes & the fingers down there just looking in from the top loop holes. I didn't think it to be nessesary at first. But when you get to your wits end of trying to figure & rule things out on your own, you might find yourself doing things you previously wouldn't choose too.?
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?Ideas & constructive criticism welcome. Hopefully I've covered most everything that would be helpful.?? |
N4FOX
