Re: Desense is driving me crazy
I saw a used Agilent 8753d with all the goodies in it (6ghz and tdr) sitting on the grass at Dayton this year for $850. I commented to him that it should be respected more and put on a table¡.. ?:) ?
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Re: Desense is driving me crazy
There are also a lot of RF connections being made with all those T connectors as opposed to a one piece crimped and soldered together harness.?
In commercial and public safety systems, a 3dB degradation was acceptable. ?In fact, on VHF, site noise was a major factor before any transmitters were keyed. I¡¯ve seen some sites as high as 18dB and they wondered why those were deaf.?
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Re: Desense is driving me crazy
Mick,
?I've wondered the same thing. I started off with a Lite VNA 64. It was regarded as the best small handheld inexpensive VNA at the time of purchase a few years ago. It was only about $150. It's really cool for what you get vs cost. Handy..
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When I decided to get into tuning these cans & to have some more equipment I looked into other devices. From 30yr old professional equipment with floppy drives & CRT displays to newer VNA within the budget. The 2nd device I picked up was called the LibreVNA for about $600. It's a two port VNA that hooks to the PC & software as a means to view things. That gave me 90/100db of dynamic range. Plus it was easier to navigate & set things up like markers & more over the Lite VNA 64. I knew the duplexer would exceed the dynamic range of that device. So I ordered a Siglent SSA3021X + Spectrum analyzer with TG for about $1800. To try & help get the rejects dialed in. Plus I figured it would be useful for the hobby. I used those to tune the duplexer. Then here just a couple months ago I picked up a Measall KC901J dual port VNA for about $1300. It's pretty nice & has lots of extra features as well. It's still only 100db of dynamic range. It's appeal to me over the LibreVNA was It's a self contained unit with It's own display.? Plus it was highly regarded by several people as being good quality for the cost. I've actually seen it go past 100db of dynamic range in these lower VHF ranges of 2M.?
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I know all together I spent a good bit for all 3 items. I could of probably got a pretty good used old VNA. I don't really regret it though. I have some poor luck. I'd probably buy an old one then have something fail on it that's hard to get replaced. ??
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Re: Desense is driving me crazy
The older bulkhead barrel connectors that had glass and hermetically sealed had intermod generating material in the pass through so that temperature changes didn¡¯t break the glass. ?
Try a different load that can handle the power. ?
another method but used for intermod testing instead of noise was to put good low intermod lossy long cable between the load and the device under test to act like a 10 to 20dB pad before the load and that reduced the intermod generated by the load. ?But that takes quite a bit of cable at VHF.?
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Re: Desense is driving me crazy
I never testing the one I have, but have wondered how close many of them may be.
Mick - W7CAT
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----- Original Message ----- From: "Harold Farrenkopf via groups.io" To: [email protected]Sent: Sunday, May 25, 2025 11:43:00 AM Subject: Re: [repeater-builder] Desense is driving me crazy The tiny VNAs are pretty accurate in frequency- at least the ones I¡¯ve used and if they are off 5khz, it wouldn¡¯t affect it much.
-- Untitled Document
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Re: Desense is driving me crazy
Harold,
?I will give that method a try with taking the tee out of the antenna port. Then using different dummy loads. The picture below is the main dummy load I've been using for reference (TX/RX). The others I have are the finned smaller screw on type. Then some other lower wattage (1-5wt) ones I use when testing things with the VNAs. Pics below.
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Im not certain what isolation yaesu recommends. The repeater is 5 to 50 watts. I prefer to run it at 5 watts into a motorola msf5000 100 watt PA. Because those amps are real workhorse that doesn't hardly break a sweat. For reference though in the video the PA was bypassed so it was just the 5/20/50 watt settings of the repeater itself.?
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Im using RG214U cables for the repeater harness. Then RG400 from the repeater to the duplexer. I have also tried RG214U from the repeater to the duplexer. There was no difference between them other than a tiny difference in db loss per cable. Also using RG214U from antenna port to the dummy load. As for antenna it's all 1/2 Andrew's heliax.?
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I do have bulkhead N barrels on the cabinet door. But I have not used the door yet. I've been bypassing that door & have it removed for now.?
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Re: Desense is driving me crazy
The tiny VNAs are pretty accurate in frequency- at least the ones I¡¯ve used and if they are off 5khz, it wouldn¡¯t affect it much.?
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Re: Desense is driving me crazy
Charles,
?Yes it's 23+ (on each side tx/rx) of return loss with all the cans tied together. Individual cavity tuning, I was seeing between 26/27db of RL.?
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Re: For Sale/Trade: (2) Kenwood TKR-840 UHF Repeaters
Still available...
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Chris
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--
Chris Baldwin, CETSr. (KF6AJM)
Trustee - MetroNET Cal. Intertie (KB3PX)
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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.
-- Untitled Document
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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.?
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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.?
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?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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N4FOX
Chris Baldwin