Re: Desense is driving me crazy
Charles, the return loss isn¡¯t really important on the rx side. 10 to 14 dB is fine and won¡¯t affect system performance to any noticeable degree. The return loss on the rx side does not increase the tx rejection to the rx unless you are drastically changing the pass response shape of the rx side.?
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Re: Desense is driving me crazy
Slightly different yet related subject, I've never tested any of the little VNAs, but have always wondered how close they are to being on frequency???
Mick - W7CAT
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----- Original Message ----- From: "N4FOX via groups.io" To: [email protected]Sent: Sunday, May 25, 2025 11:09:23 AM Subject: Re: [repeater-builder] Desense is driving me crazy Jason, I'm not sure how many db of desense exactly. Just listening to it by ear for the crackling noise to appear.
The TinySA ultra in signal generator mode does allow me to generate
an audible tone to listen too. I've used that in the past when experimenting.
The noise being generated can overcome or make it hard to hear a weak
rx station trying to talk through the repeater sometimes. But part of that is the extra noise I think that's on the hardline or antenna. That 1/2 hardline has been up 15 years or so. It was on my aprs digipeater once upon a time. Then it was left up there after that was taken down. I suspect water may of gotten in because it wasn't sealed up that great afterwards. Prior to putting the repeater on that line, I had used it with the Hustler G7 I put up last year. Testing it to talk off of for a few months. Because the repeater was on back order. It seemed OK. But often when duplexing more issues can show up. So now it's part of the suspecting problems. Along with the Hustler & so many assembly parts. I've heard good & bad things from people about those. So I figure it's worth checking out as well. Perhaps the hot/cold & winds have caused something to get a little loose on it.
Right now I'm just trying to cure the noise within the duplexer. To
where I can engage the transmitter & not hear any changes. I have a 100 watt PA that I'd like keep using. But right now it makes things worse even into the dummy load.
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Re: Midland 91-1110 Basetech III intermittent receive
Considering the relative obscurity of these machines, you might try reaching out to Midland directly, and see if you can speak with Roger French. I think he's about the only person in the US that knows anything about the BTII/III line of radios.
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He might be able to help you. He helped me get set up to do firmware updates on the BTII, wasn't an easy task. Both it and the III require specific MCU programmers to do firmware updates, but that's outside the scope of this.
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Point is - he's extremely helpful, *if* you can reach him.
--
Chris Baldwin, CETSr. (KF6AJM)
Trustee - MetroNET Cal. Intertie (KB3PX)
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Re: Desense is driving me crazy
Take the T out of the antenna port of the duplexer and put dummy loads at those ports to see if there is any desense. That would remove and question of coupling between the cans some how. Then if there isn¡¯t desense, put in a good new T connector in retest into the dummy load. Try different dummy loads as well.?
how much isolation does Yaesu say is needed for a duplexer?
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are you using double shielded silver plated copper coaxial cables everywhere?
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Also, are you using a barrel connector to get the duplexer output through the cabinet? If so, try eliminating that connection.?
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Re: Desense is driving me crazy
Yes, 23 db is a good RL,? but are all three cans tied together, after individually measuring RL. The connecting cables may change the total RL when connected to a 50 ohm load.
Sometimes, they can be slightly tweaked to get a little more. I don't know how that will affect the slight desense you are speaking about, but in commercial systems we don't worry about very very slight desense.
The reason for less RL, especially on the receive side of the duplexer is to attenuate more rejection of the TX frequency.
Charles Adams
Industrial Communications Co.
Mobile 610-360-0050?
Office 610-253-1214?
cadams@...
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Jason, I'm not sure how many db of desense exactly. Just listening to it by ear for the crackling noise to appear.?
?
The TinySA ultra in signal generator mode does allow me to generate an audible tone to listen too. I've used that in the past when experimenting.?
?
?The noise being generated can overcome or make it hard to hear a weak rx station trying to talk through the repeater sometimes. But part of that is the extra noise I think that's on the hardline or antenna. That 1/2 hardline has been up 15 years or so.
It was on my aprs digipeater once upon a time. Then it was left up there after that was taken down. I suspect water may of gotten in because it wasn't sealed up that great afterwards. Prior to putting the repeater on that line, I had used it with the Hustler
G7 I put up last year. Testing it to talk off of for a few months. Because the repeater was on back order. It seemed OK. But often when duplexing more issues can show up. So now it's part of the suspecting problems. Along with the Hustler & so many assembly
parts. I've heard good & bad things from people about those. So I figure it's worth checking out as well. Perhaps the hot/cold & winds have caused something to get a little loose on it.?
?
Right now I'm just trying to cure the noise within the duplexer. To where I can engage the transmitter & not hear any changes. I have a 100 watt PA that I'd like keep using. But right now it makes things worse even into the dummy load.?
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Re: Desense is driving me crazy
Harold,
?Thanks for sharing your insights. I appreciate that. I do see 3 humps in the return loss when the cans are all hooked together. It's always kinds bugged me, but I was told it's normal by several folks. So it put my mind at ease a little.?
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I have tried listening for desense by taking the duplexer out of line completely. It's not present then. It sounds exactly how I'd imagine it to sound. No change in the receiver at all when the transmitter is engaged, no matter what power setting is used.
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Re: Desense is driving me crazy
Jason, I'm not sure how many db of desense exactly. Just listening to it by ear for the crackling noise to appear.?
?
The TinySA ultra in signal generator mode does allow me to generate an audible tone to listen too. I've used that in the past when experimenting.?
?
?The noise being generated can overcome or make it hard to hear a weak rx station trying to talk through the repeater sometimes. But part of that is the extra noise I think that's on the hardline or antenna. That 1/2 hardline has been up 15 years or so. It was on my aprs digipeater once upon a time. Then it was left up there after that was taken down. I suspect water may of gotten in because it wasn't sealed up that great afterwards. Prior to putting the repeater on that line, I had used it with the Hustler G7 I put up last year. Testing it to talk off of for a few months. Because the repeater was on back order. It seemed OK. But often when duplexing more issues can show up. So now it's part of the suspecting problems. Along with the Hustler & so many assembly parts. I've heard good & bad things from people about those. So I figure it's worth checking out as well. Perhaps the hot/cold & winds have caused something to get a little loose on it.?
?
Right now I'm just trying to cure the noise within the duplexer. To where I can engage the transmitter & not hear any changes. I have a 100 watt PA that I'd like keep using. But right now it makes things worse even into the dummy load.?
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Re: Desense is driving me crazy
The return loss dips do not align and add in depths when all the pass cans are tuned to the same frequency and connected together. ?This is more noticeable when tuning 2 or more pass cans in series. ?When 3 or more pass cans are tuned in series, the pass band response shape depends on the cable lengths and the insertion loss (loaded Q) and the inner cans are tuned for less loss to get a wider flatter pass response. ?That¡¯s how wider pass bands are achieved. ?
I listened to your video and it¡¯s a little difficult to tell how much degradation there is with a tone present. ?I don¡¯t think the tiny SA can do that.?
Did you try that test without the duplexer - test the radio to see that no desense is occurring inside that assembly? Tx directly into dummy load with double shielded cable and same with the receiver.?
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Re: Desense is driving me crazy
Return loss at equipment port can vary by how the cans are tuned and interacting with cable lengths. You could see 3 dips in the return loss response. Those dips are relating to the pass tuning. ?20dB is good.?
you don¡¯t need to use pads when using good VNAs which are 50 ohms. ?Pads were used in the past when old signal generators and tracking analyzers were used. ?A lot of the old tuning instructions were based on old equipment and not today¡¯s VNAs.?
BTW, I have worked at Sinclair in the past as the application systems engineer and have tuned up hundreds of systems in other lives.?
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Re: Desense is driving me crazy
Don't know if I missed it, but did you determine how many DB of desense is there when testing on an antenna?? Tracking you hearing it on the speaker when keying up but if your hearing a 1KC tone around the noise ( -119 / -121) signal and are still receiving this when you turn TX on - that's to my notes working. If that tone/signal went away (versus you only hearing a change) and you had to crank your signal generator up 3db before it came back, thats another story Jason
Charles, with all the cavities hooked together it has about 23.5db of return loss per side. I was told anything over 20db is good. Does that sound correct??
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Re: Desense is driving me crazy
Charles, with all the cavities hooked together it has about 23.5db of return loss per side. I was told anything over 20db is good. Does that sound correct??
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Re: Desense is driving me crazy
Jeremy I haven't used 6db. I have a couple identical 10db ones I've used. As they are the only matching pair I have. I have various singles of 3db, 6db, 30db, 50db ones.?
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I also have a Wiltron VSWR Bridge 10 Mhz - 1Ghz.?
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I just recently a few days ago picked up a variable RF sampler.?
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Re: Desense is driving me crazy
How is the RL on each set connected together, and to the load or antenna?
Charles Adams K3HKZ
Industrial Communications Co.
Mobile 610-360-0050?
Office 610-253-1214?
cadams@...
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Re: Desense is driving me crazy
Charles,?
Could you elaborate on your procedure? I've always adjusted the passes for best RL, but never the notches. I've never used RL as a measurement to adjust a notch cap, and would imagine any "touch up" would be to the detriment of the notch frequency, depth, and alignment.?
Apologies to hijack the thread, but this has me both perplexed and curious.?
Also, a question for the OP: Are you using 6db pads when making your SA/TG measurements? A while back, I was having similar issues and found that impedance mismatch was my issue.?
Jeremy K1LFK?
HI
In your very detailed and lengthy procedure I did not see any mention of?
RL (return loss) measurements. If you have a good NVA you should touch up the three notch caps on each of the TX and then the Rx cavities after connected together for the best matching RL.
I have been doing this for several years now with improvement over not doing it.
Charles Adams K3HKZ
Industrial Communications Co.
Mobile 610-360-0050?
Office 610-253-1214?
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Re: Desense is driving me crazy
I recorded this a while back. I thought it might be useful when asking for help about things. Over the years I've strayed away from even asking for help online. The world seems to be filling up with rude folks. Often quick to criticize others just to prop themselves up to fill their own voids. Sad thing is I can remember a time when you could get help from people with similar interests. They were happy to share the wealth of knowledge among fellow enthusiasts. It brought people together.?
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Re: Desense is driving me crazy
You are correct I didnt bother to mention return losses. They are roughly 26db per cavity.?
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Re: Desense is driving me crazy
HI
In your very detailed and lengthy procedure I did not see any mention of?
RL (return loss) measurements. If you have a good NVA you should touch up the three notch caps on each of the TX and then the Rx cavities after connected together for the best matching RL.
I have been doing this for several years now with improvement over not doing it.
Charles Adams K3HKZ
Industrial Communications Co.
Mobile 610-360-0050?
Office 610-253-1214?
cadams@...
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Re: Desense is driving me crazy
Wow, you are over thinking things. ?For vhf, 90 dB should be sufficient at 600khz (for most radios) so needing 100dB or more is nice but not really necessary - you just tune it to see noise width in the notch pattern.?
A little dust in the capacitor isn¡¯t a big deal- there is an o-ring in there that might get affected. ?The jumping of the tuning of the notch might be due to non-centric capacitor plates caused by overheating the loop and causing misalignment or it might even become cold soldered and partially cracked. Those caps are fragile.?
those flakes of metal should be removed. Use forced air to do that from a compressor.?
Have you looked at the receiver side of the duplexer with a spectrum analyzer to see the noise is real??? ?Try using a separate receiver connected to the duplexer to also check for desense and if it is real.?
Dummy loads can cause noise as well.?
Transporting it horizontally should not be an issue.?
Eliminate the radio as the source of desense.?
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Re: Desense is driving me crazy
I'll take a look again later when I mess with it. Some of that appearance of distortion is actually a bit of clear dielectric grease. I noticed that same look on the tee connectors prior to cleaning them. It's one of the other reasons I decided to clean all the connectors. I wanted to try and remove it. I noticed it was causing things like small hairs to stick to them.?
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Re: upgrade cdm 750 dash to 1250 remote
Back in the day I was able to convert HT750 to HT1250 and CDM750 to CDM1250 by just replacing the EEPROM chip and then adding the appropriate housing with display and keypad. Learned from another tech that was cloning quite a few radios. Reprogramming the existing EEPROM with a personality binary file data taken from a real 1250 radio EEPROM would also work. Just read the EEPROM from a real CDM1250 and then write that data to a blank chip, then replace. Remember to save a backup of your original 750 binary file, just in case. Of course if you had access to an appropriate flashing adapter and software this could also be done without replacing the actual chip, but back when those where very difficult to get, not to mention expensive, and in the wrong hands could easily brick a radio.
Memory and CPU in the radio where the same, so a converted radio was fully capable and functional with expanded channel capacity, but the serial number would change to the one from the original cloned radio. Just be aware of a few hardware limitations, as only same band should be cloned, and also ideally only same or adjacent sub-band models should be cloned otherwise VCO might not lock or only have a limited tuning range. Also the HT1250 will need a battery and holder retrofitted for the clock to keep time when the main battery is removed, but usually nobody cared much about this. Keep in mind to back up all your channel, signaling and other radio settings as they will no longer be present after the cloning process and will need to be reprogrammed.
Replacing the EEPROM was also often the solution to repair many a failed radios due to Firmware corruption. Notably in the CDM series this would often lead to the radio not transmitting or not powering up, it was a classic fault. Also if after power up you saw the dreaded and not too uncommon "EEPRM HW ERROR" or EEPRM CS ERROR" on the LCD with a short low tone, the only way to fix it was to replace the EEPROM chip with one with cloned data.
The 25128 (128K) blank EEPROM SOIC-8 chips are still very common type and easy to source. After replacing it you could edit and write the radio codeplug with CPS as a real 1250 series radio and without needing special tricks or any missing fields. Many inexpensive chip programmers out there will do the task, for example GQ-4X(4) and TL866 with the appropriate SOIC8 adapter, and many others as well.
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Chris Baldwin
N4FOX