Sinclair duplexer notch drifts

[WRMD298]

I have two duplexers that are in mint condition (physically) but I do not know the past history. I inherited them from an agency who did away with VHF high band. They honestly look like they have never been used but no guarantees.

The first is a Sinclair Q2220E and the second is a Q202.

The Q2220E gives slightly over 75 dB of rejection at the notches and has around 1.4 dB of insertion loss. The Q202 has slightly better specs, around 85 dB of rejection with the same insertion loss as the previous, only with somewhat better selectivity. Return loss and VSWR through all ports are better than spec on both. All specs exceed the datasheets.

Each duplexer will tune smoothly. Pass plungers are tight, but not too tight. Just the perfect amount of physical resistance you would expect. Likewise, all notch trimmer capacitors are smooth in tuning and just enough tension for the right "feel." All brass caps were in place on the capacitors and the units are clean.

I started with the first, the Q2220E, with frequencies of 147.00 and 147.60 MHz. Repeater worked great at 75W output with no desense to the RX. Several days later, I noticed receive signals were down. I checked, and desense was in excess of 10 dB. I checked the duplexer, and both the high and low pass notches were WAY off. I re-tuned, and all was well. Days later, the same happens. This goes on for several days and each time I check, the notches are slightly off frequency. The duplexer pass is fine, but the notches won't stay stable.

I then tried the Q202, which uses the same identical coupling loops, and I get the exact same thing. They will NOT stay stable.

I pulled all the coupling loops out, and examined each capacitor. They seem to be Johansen 1-30 pF caps. They are very clean, no burn marks, no cracks in the ceramic. I have physically examined all parts of the cavity and find nothing wrong. I can't imagine ALL of the capacitors are bad. I put all the cores back in and retuned, and the same thing happens. This has been going on for several weeks.

I am using an Anritsu S820 for a tracking gen/VNA. The building is climate-controlled at a constant 72 degrees and no, the AC vent is not blowing directly on the cavities. The VSWR on my antenna is around 1.3:1 at the duplexer input.

Any thoughts?

 

[prcguy]

Are you tuning the duplexer in one place then transporting to another? They can be very sensitive to vibration and should be tuned on site in the rack if possible. Do you have all unused ports terminated when tuning? Do you tune looking at just insertion loss and notch depth or do you also look at return loss? Return loss method is best while also looking at insertion loss, making slight adjustments there then smoothing things out in return loss. Tuning for best return loss will also be less sensitive to external cable and equipment changes and I've seen some duplexers tuned for insertion loss into loads change when connected to different cables and the repeater.

I always smack each cavity with a tool or piece of wood after final tuning to make sure the adjustments stay put. If you don't do that minor movement or mechanical vibration can change things and especially the notches since they are narrower than the pass.

BTW. 75dB isolation is not good enough for a 50-100w repeater but you may get by if only running 10-20w. I would say 85dB is the minimum for a 4 cavity Bp/Br type duplexer and 100dB or more for a 6 cavity Bp/Br.

 

[prcguy]

Looking at the Q2220E it appears to be a wee little 4" cavity job. If so that's not going to work well on a 600KHz split and duplexers of that type are typically rated minimum 1.5MHz split. If you had four 6" Bp/Br cavities you can get reasonable isolation at 600KHz.

 

[WRMD298]

Duplexer is being tuned in the rack that it operates in and is bolted down with zero movement. I've tried wiggling everything around, tapping on it, beating it (slightly, out of frustration) with a block of wood, and all is stable while doing so. I've checked all the Delta tee-connectors, use phase-stable cables while tuning, and terminating the unused port while using the tracking gen/VNA. I do check the return loss through each cavity and tune for best return loss/lowest SWR and then compare that to the pass sweeps and they match perfectly. I'm using the same interconnect cables each time, they are 1/4" superflex with silver-plated connectors. Tuning is done with an insulated tool although it doesn't seem to make a difference what I tune them with. I look at the return loss INTO the transmit/receive cavities while connected to a 50 ohm load versus the antenna - only a very slight difference in the sweep.

I borrowed a Wacom WP-639 out of desperation from a friend and it has been working perfectly for the past week. This is the only duplexer I know of that used the Rexolite tuning notch capacitors. The thing looks like garbage and there's green corrosion on the threads and connectors but it's quiet as a mouse and doesn't drift.

 

[WRMD298]

Are you tuning the duplexer in one place then transporting to another? They can be very sensitive to vibration and should be tuned on site in the rack if possible. Do you have all unused ports terminated when tuning? Do you tune looking at just insertion loss and notch depth or do you also look at return loss? Return loss method is best while also looking at insertion loss, making slight adjustments there then smoothing things out in return loss. Tuning for best return loss will also be less sensitive to external cable and equipment changes and I've seen some duplexers tuned for insertion loss into loads change when connected to different cables and the repeater.

I always smack each cavity with a tool or piece of wood after final tuning to make sure the adjustments stay put. If you don't do that minor movement or mechanical vibration can change things and especially the notches since they are narrower than the pass.

BTW. 75dB isolation is not good enough for a 50-100w repeater but you may get by if only running 10-20w. I would say 85dB is the minimum for a 4 cavity Bp/Br type duplexer and 100dB or more for a 6 cavity Bp/Br.

Isolation isn't the problem. The isolation is good, as there is no desense at all on the repeater directly after I tune the Q2220E, period. Motorola MTR2000, 75W. It will do 100W with ZERO desense. I think the isolation is sufficient so long as the transmitter doesn't affect the receiver. The Q2220E is a small cavity duplexer, true, but it's a full-length duplexer, not a helical shortened duplexer. It IS spec'd for 70 dB of isolation minimum at a 500 KHz split. The isolation is sufficient. I just can't keep the notches stable. They seem to tune themselves.

 

[prcguy]

I have what I believe is a very similar 4” square 4 can Bp/Br from EMR rated for 1MHz minimum split and there is no way it could ever do a 600KHz split. I currently have it on a 2.655MHz portable repeater split where it’s working fine. I would seriously consider getting a better performing duplexer.

 

[prcguy]

Duplexer is being tuned in the rack that it operates in and is bolted down with zero movement. I've tried wiggling everything around, tapping on it, beating it (slightly, out of frustration) with a block of wood, and all is stable while doing so. I've checked all the Delta tee-connectors, use phase-stable cables while tuning, and terminating the unused port while using the tracking gen/VNA. I do check the return loss through each cavity and tune for best return loss/lowest SWR and then compare that to the pass sweeps and they match perfectly. I'm using the same interconnect cables each time, they are 1/4" superflex with silver-plated connectors. Tuning is done with an insulated tool although it doesn't seem to make a difference what I tune them with. I look at the return loss INTO the transmit/receive cavities while connected to a 50 ohm load versus the antenna - only a very slight difference in the sweep.

I borrowed a Wacom WP-639 out of desperation from a friend and it has been working perfectly for the past week. This is the only duplexer I know of that used the Rexolite tuning notch capacitors. The thing looks like garbage and there's green corrosion on the threads and connectors but it's quiet as a mouse and doesn't drift.

The Wacom has 5” cavities and that makes a lot of difference. The Wacom will do 80dB isolation at 600KHz where the Sinclair will not. And you really want 85dB minimum for a 50-100w repeater.

 

[WRMD298]

I have what I believe is a very similar 4” square 4 can Bp/Br from EMR rated for 1MHz minimum split and there is no way it could ever do a 600KHz split. I currently have it on a 2.655MHz portable repeater split where it’s working fine. I would seriously consider getting a better performing duplexer.

View attachment 166157

OK.. The Q202 behaves the same way as I mentioned in my original post. It's a 6-inch, 4-cavity duplexer with 85 dB isolation.

My Q2220E that the manufacturer themselves specifically say will do down to a 500 KHz split with 70 dB of isolation is working fine for me when it is first tuned. The problem is it will not stay this way. I don't have an isolation problem to begin with, I have an isolation problem because the notches drift.

Can purchase a new Telewave TPRD1556 for about $3K. The isolation will exceed what I need by 30 dB or more. I will have spent $3K for a new duplexer and still have two Sinclair duplexers that the notches drift on, which was my original problem anyway.

I am just trying to figure out why the notches drift on what should be excellent quality duplexers.. that's all.

73..

 

[speedway_navigator]

Does this have lots of TX time that might cause a heating issue if the internal parts have some defect? Something along the lines of the dendrites that grew in tuning coils in the Mostar mobiles back in 80s and 90s.

Maybe hit it with freeze spray when it's out and see if it comes back.

>>I always smack each cavity with a tool or piece of wood after final tuning to make sure the adjustments stay put.
Wife did the same thing to me when I hesitated signing her nagging permit, I mean marriage license.

 

[prcguy]

I would install some directional couplers either side of the duplexer and see if you can get somewhat of an isolation measurement with VNA and the repeater connected. Then with a freshly tuned duplexer meeting specs and the VNA disconnected, fire up the transmitter for a few minutes then quickly attach VNA and measure isolation for any changes. If no change do it again with a 10min-30min key down to determine if transmitter power is causing a problem. If no problem do it for an hour or two or more to simulate a couple of days of repeater traffic. I would also monitor the tuning cap area with an IR non contact temp probe to see if there is any capacitor heating.

What this can do is tell you if transmit power is causing the problem and if it gets incrementally worse with more transmitter time. The temp probe will help verify if heating is happening and therefore part of the problem. If none of that pans out then don’t give back the loaner duplexer.

 

[WRMD298]

Does this have lots of TX time that might cause a heating issue if the internal parts have some defect? Something along the lines of the dendrites that grew in tuning coils in the Mostar mobiles back in 80s and 90s.

Maybe hit it with freeze spray when it's out and see if it comes back.

>>I always smack each cavity with a tool or piece of wood after final tuning to make sure the adjustments stay put.
Wife did the same thing to me when I hesitated signing her nagging permit, I mean marriage license.

This is something I should probably check. I don't know (exactly) how much transmit time it gets. I do know that if I tune the set and don't hook them to anything, they stay stable during the course of the week. I can only assume they drift when TX power is active, and if that's the case, then it must be temp sensitive. Freeze spray is a good idea, and maybe even a hair dryer on low temp hitting the top of the coupling loop assembly. I would think that if TX power caused it to drift due to temp, it would eventually drift back to the previous setting after it is cooled, unless something is shifting. I'll try heating and cooling the top of the cavities next week when I get back to the site. I was always under the impression that these caps were pretty tolerant but maybe (for some reason) I have some faulty ones.

 

[prcguy]

This is something I should probably check. I don't know (exactly) how much transmit time it gets. I do know that if I tune the set and don't hook them to anything, they stay stable during the course of the week. I can only assume they drift when TX power is active, and if that's the case, then it must be temp sensitive. Freeze spray is a good idea, and maybe even a hair dryer on low temp hitting the top of the coupling loop assembly. I would think that if TX power caused it to drift due to temp, it would eventually drift back to the previous setting after it is cooled, unless something is shifting. I'll try heating and cooling the top of the cavities next week when I get back to the site. I was always under the impression that these caps were pretty tolerant but maybe (for some reason) I have some faulty ones.

If they are Johansen piston trimmer caps they are very rugged unless you exceed their HV rating and they arc. The only other minor problem with Johanson caps is if you use solvents to clean them it will remove the special lube put on at the factory and they can seize up. I think the factory lube is made by Apiezon.

 

[davidgcet]

have you pulled the plungers and checked the finger stock? i have had more than one set of sinclairs where the plunger and finger stock would pit. normally i'd say a work around would be retune it and flip sides, but if this is the issue i'm not sure that would work with 600k split. another option since you have 2 sets would be to tee into 1 bottle on the rx side so you have 3 bottles to isolate out the tx from it.

 

[prcguy]

have you pulled the plungers and checked the finger stock? i have had more than one set of sinclairs where the plunger and finger stock would pit. normally i'd say a work around would be retune it and flip sides, but if this is the issue i'm not sure that would work with 600k split. another option since you have 2 sets would be to tee into 1 bottle on the rx side so you have 3 bottles to isolate out the tx from it.

The OP says the notches are drifting and nothing about the pass drifting. On many duplexers the notch will follow the pass and the pass on a 4” can is not well defined, so it’s possible the pass is drifting slightly and going unnoticed while taking the notch with it but more likely the problem is confined to the notch.

 

[WRMD298]

If they are Johansen piston trimmer caps they are very rugged unless you exceed their HV rating and they arc. The only other minor problem with Johanson caps is if you use solvents to clean them it will remove the special lube put on at the factory and they can seize up. I think the factory lube is made by Apiezon.

OK... now we are getting somewhere with the capacitor properties. I DID in fact clean the capacitors, the pistons themselves and their bases, with what is called "QD electronic cleaner." I have used this numerous times on all kinds of things to remove grease, dirt, etc from fragile electronic parts. It doesn't leave a residue and dries clean, however, after spraying a surface, the surface becomes cold and moisture condenses.. a little "air in can" to dry them off and set them aside should have returned them to a normal state, but now I am thinking the the spray cleaner could have possibly changed the properties of the capacitors. They still move freely and smooth, but did the cleaner somehow leave a coating on the surfaces that change when RF hits it.. Again, this spray is great for delicate surfaces. It removes oil, like a solvent, however, it doesn't destroy plastic like a regular solvent would.. Point taken.

 

[WRMD298]

have you pulled the plungers and checked the finger stock? i have had more than one set of sinclairs where the plunger and finger stock would pit. normally i'd say a work around would be retune it and flip sides, but if this is the issue i'm not sure that would work with 600k split. another option since you have 2 sets would be to tee into 1 bottle on the rx side so you have 3 bottles to isolate out the tx from it.

I've put an automotive camera down into the cavities and everything looks beautiful, but I have not pulled the plunger completely out to check the contact sides of the finger stock. The pass curve isn't drifting, just the notches. I can set the tracking gen to a VERY sensitive dynamic range and check the pass curve and the return loss and they are dead on within a hundredth of a dB each time.

 

[WRMD298]

I would install some directional couplers either side of the duplexer and see if you can get somewhat of an isolation measurement with VNA and the repeater connected. Then with a freshly tuned duplexer meeting specs and the VNA disconnected, fire up the transmitter for a few minutes then quickly attach VNA and measure isolation for any changes. If no change do it again with a 10min-30min key down to determine if transmitter power is causing a problem. If no problem do it for an hour or two or more to simulate a couple of days of repeater traffic. I would also monitor the tuning cap area with an IR non contact temp probe to see if there is any capacitor heating.

What this can do is tell you if transmit power is causing the problem and if it gets incrementally worse with more transmitter time. The temp probe will help verify if heating is happening and therefore part of the problem. If none of that pans out then don’t give back the loaner duplexer.

These are good ideas. It would be fairly easy for me to dead key the machine for long periods of time then disconnect the cables and check with the tracking gen again, but the 30 dB directional couplers on each port would allow for the tracking gen to see the notches move in real-time if heating or RF is making them move. That would be a 60 dB decrease in signal overall for the tracking gen to check the notches, meaning 75 dB notches would look more like 135 dB notches and the tracking gen plots get noisy around -115 dB so I will have to see how that works. The infrared thermometer will be nice, also. I would love to trade the loaner duplexer for one of these other ones but I doubt he will do it now. Anyone have a Wacom WP-641 they would like to trade for two Sinclairs? Hi hi hi...

 

[davidgcet]

the pass will affect the notch in my experience, it does not take much change to make the notch move a little. at 600k spacing that minor move can cause desense. i have had this happen more than once. and like i said sometimes it is easier to just do a full retune and reverse the sides so your high/low flip flops. if that solves the issue then the problem most likely is in the pass. worst case add a 3rd bottle on the rx to notch it further, this really will not hurt the rx levels but could drastically reduce desense.

 

[prcguy]

Anyone have a Wacom WP-641 they would like to trade for two Sinclairs? Hi hi hi...

I have a spare Telewave TPRD-1586, six 8” cans that will do 100dB isolation at 400KHz and at 600KHz it’s got more isolation and only 1.2dB insertion loss. Plus it’s rated for 350w! But I don’t have any use for the Wacom, sorry.

 

[WRMD298]

Problem was solved by swapping the high/low pass sides with each other. This shifted the capacitor positions on each side. Apparently, there was a "touchy" few turns on two of the capacitors at just a certain depth. Has been running for a month straight with zero issues. Checked with tracking gen 10 times during the past month.