marcspaz

Hmm. I'm not sure what you are describing is all that unusual. Every radio that is on the ragged edge of a repeater can cause that 'oscillation' condition if the duplexer is not tuned correctly. There really isn't anything you can do beyond a properly tuned duplexer. Now when I am testing, I do run repeaters with the receive input not connected to anything, a dummy load on the transmit side and no duplexer. I have never had a transmit signal from the repeater desense my test HT to the point where there was a failure to operate correctly. I'm not sure you can create that condition unless something is horribly wrong somewhere.

It stands for "Oh, Shit! Over!"

It's a Smith Chart SWR Circle. It's a circle that is drawn centered at the origin frequency, passing through the plotted impedance point. This circle represents a constant SWR through the range. The value where the SWR circle intersects the positive real axis (right side) of the chart indicates the SWR. The closer the red dot is to one of the lines radiating from the right side, the closer to 1:1 the SWR is on that frequency. I tried the TinyVNA and it did not work. It was way off... very inaccurate. A friend of mine (who is a retired RF engineer) uses a SAA-2N VNA when he is in the field. He has great results with it, which is why I went with that specific VNA. It's "good enough" that I would not encourage people who are doing this as a mild hobby to buy a commercial Signal Analyzer. If professionals are using it, the SAA-2N VNA is good enough for me. I have not compared mine to a commercial grade SA, but I have tuned 5 duplexers (UHF and Commercial VHF) with the SAA-2N VNA since I bought it and have had amazing results. With a 2m repeater only having a 600KHz offset, accuracy is extremely critical. My 2m repeater is covering over 8 miles with 5w handheld radios and well over 25 miles with 50w mobiles, and the antenna is only 100' above the ground, 200 feet above average terrain. Those numbers are both further than the theoretical/calculated RF LOS. So, I personally have confidence in the SAA-2N VNA.

To answer question about tools used and how to adjust a duplexer, here is a quick overview. Tools I used: Windows PC SAA-2N VNA VNA View software Box Wrench Screw Driver Brass Brush To tune the duplexer, you have to follow a couple of steps. Calibrate the VNA for the frequency you are testing (via the PC or on the VNA, depending on if you are using the PC or not) I typically set the center frequency to the frequency I am tuning for, with a 10MHz spread. From there, we are going to tune either the low side or high side, separately. For this example, we can start on the low side. Connect the S11 cable to the "antenna" port. Connect the S21 cable to the "LOW" port. Put a dummy load on the "High" port. Refer to Image 1. Set the Trace 1 format to S21 Thru. With a mobile UHF cavity duplexer, you can disable the other traces, but if you want to track your SWR at the same time, set Trace 2 format to S11 SWR, and then disable the remaining traces. Adjust your scaling so you can see at least -110dB Adjust each adjustment screw to create the deepest notch possible, to the transmit frequency. In this example, 467.600MHz. only adjust one tuning screw at a time, loosen the jam nut with the box wrench, just enough to allow the tuning screw to move, and while making adjustments, hold the jam nut with the wrench. Typically, once you set the screw to the proper depth, it can be a bit tricky to set the jam nut without turning the screw a bit at the same time. A trick I learned is that I put the screw just a few degrees off, counter clockwise, so as I tighten the jam nut, it snugs the tuning screw into the correct location. See Image 2 and Image 3 From there, switch the S11 cable to High, and the dummy load to the Low side. Repeat the tuning steps for the high side, but tuning the notch to the receive frequency. In this example, 462.600MHz. This guy has a great video on using a VNA to adjust a duplexer. I have no idea who he is, but it it's good content. The only thing I would add is, he stated he is using 200 sweep points. I would use the highest sweep setting you can that provides a good enough resolution of the frequency spread you are tuning. On the VNA itself, I use 401 points, but on the PC, I use 1024 points. https://www.youtube.com/watch?v=TbEK4v_3Xuo With this duplexer, specifically, the tuning screws are brass. Either the manufacturer or B-Tech spray painted the tuning slugs. I had to spend a bit of time cleaning the paint off the screw threads and heads with a brass cleaning brush to make it so the duplexer could be tuned. If you have a situation like this and want to repaint the screws for cosmetic reasons, be sure to use non-metallic paint, such as Krylon Fusion for Plastic. It must be non-metallic, polyurethane free, and lead free. I am including a few extra pictures displaying the results. Image 1 Image 2 Image 3

That's a good question, but not the way receivers get rated. The sensitivity is the 'receiver' sensitivity. A couple points about how the duplexer work and why the receive sensitivity isn't impacted. The duplexer filters out the receive frequency on the transmit side, and the transmit frequency on the receive side. There is no direct impact to the signal due to tuning. The only real 'reduction' would be the overall reduction in signal from insertion loss. However, you may see a difference/improvement due to the filtering for another reason. The cavity filters have a bit of a floating echo effect, with the dB of suppression wildly varying. Your duplexer, for example, floats between -85dB and -110dB per cavity pair. With a varying isolation of -170dB to -220dB. Prior to tuning, the duplexer isolation floated between about -45dB and -70dB. This can impact how much desense the receiver experiences, giving you the impression that receive sensitivity improved. To summarize, while the receive sensitivity doesn't improve with improved duplexer tuning, the improved filtering removing more of the transmitter signal and reducing desense, will allow the appearance of improved sensitivity.

Sure! On the way out right now, but I'll share what im using for this job. Its easy to do and the tools are very affordable.

Thank you for the kind words. It was my pleasure! I'm glad I was able to help.

@WSIK532 the entire process from setup and initial testing to the finalized range test and cleanup was about 2 hours. The actual tuning itself only takes a few minutes.

Hey folks! For people looking for an affordable duplexer that works well, I may have a solution for you. For those of you who have been following the thread, you may be aware that our friend @LeoG had a problem with a duplexer in his B-Tech repeater. They exchanged it for him, but I offered to take a look at the duplexer, hoping that we could get a spare available, or a second for another machine. I have to say, I am impressed. The duplexer is a SGQ-450D. It is a traditional six cavity mobile duplexer, made in China, and the cost is about $115-$120. When I first got the duplexer, I connected it to a VNA. The unit looked like it was tuned perfectly, the notch and SWR were great... it was just on the wrong frequency, preventing it from working on GMRS. I connected it to a radio and a dummy load, checking both sets of filters, and it worked perfectly. The downfall with duplexers setup like the ones installed in the B-Tech repeaters isn't the duplexer itself, but rather the tuning. What companies do to make these wideband (and what I found with this duplexer), they tune the 3 receive cavities to slightly overlap instead of perfectly align. Also, the 3 transmit cavities to slightly overlap instead of perfectly align. What you end up with is about 500KHz-700KHz of filtering at about 45dB-50dB on each side, for a max of about 100dB of isolation. While not terrible, it leaves a lot of room for improvement, which is easily accomplished if you align all 3 cavities to work on the same frequency. @LeoG asked to have the duplexer aligned to 462.600/467.600. On the high side, I was able to get a minimum of -85dB notch centered at 462.600MHz. On the low side, I was able to get a minimum of -87.4dB notch centered at 467.600MHz. That is better than -172dB of isolation... almost double what the previous tune was. Here are the final tune-up results. Isolation = > -172dB Transmit SWR = 1.08:1 Receive SWR = 1.18:1 Loss due to SWR = 0.0064dB Total Insertion Loss is <1dB For a test, I hooked it up to my portable repeater while at my house. I got the same range as with my personal duplexer. For comparison, these specs are on par with my SinTech/Sinclair duplexer, which is priced 4 times higher than the SGQ-450D. The duplexer will be shipped back to Leo next week. I am looking forward to his real-world feedback after the tune-up. Below are a few images of the duplexer and the tuning results.

The only other thing I can think of is the duplexer is not tuned correctly on either side and the protective limiter circuit and the transmit radio is stopping it from transmitting so it doesn't break. You are going to need a VNA at a minimum to test. At this point I don't trust the duplexer. I'll kick some thoughts around, but without being right in front of it it's going to be hard to troubleshoot from this point.

Just added that to my cart. I'm gonna test it next week.

I am thinking you need to swap radios on the duplexer ports. Like, I think they are labeled backwards. It would make sense that the case of the HT is letting enough RF leak in to trigger a receive light, but there is not enough of a signal to create usable audio output to trigger the vox operation. If its not labeled wrong and it is wired correctly, you could just be washing out the systems due to being to close to each other and too close with the test handheld. I have seen some HTs so sensitive to RFI from other HTs, that we had to separate them by 75 to 100 yards before the RFI and desense was small enough for the radios to work correctly.

I need to ask some questions... You are using a cable to go from the receiver audio out to the mic audio in, and set to vox? Correct? If you simply remove the antenna cables from the duplexer, everything works (as well as to be expected without a duplexer, anyway)? When you hook up the duplexer, the receiver receives a signal, gets a green light, but the transmit radio doesn't go into transmit mode, even though vox works fine without the duplexer? Or does the transmit light/display indicate that the transmitter is actually transmitting, but there is just no output from the duplexer?

I think both radios need to be the same brand or use the same compander tech AND compander has to be enabled on all radios wishing to benefit from the tech. Otherwise, it sounds worse instead of better. It's been awhile since I read about it and I don't know anyone personally who uses it due to the compatibility issues.

@WRUE951 that sounds good. I'm glad you've had good luck with them. My experience isn't terrible, just not ideal results. Since most of mine are EmComm related, I never really ran one for more than a few days at a time, and mostly left them off until I/we need to use them. I know what you mean about learning the hard way. I struggled so much to avoid spending the money on a proper repeater that I probably spent 3 or 4 times more than if I just went for a regular repeater to start. I finally built a full-blown potable repeater system... it can do 2m, 70cm, crossband or act as a base station. It has a built-in 50 amp hr battery system and can run on solar or AC. Retail, it is about $17k to build, but thankfully some stuff was donated and I was able to get great deals on a used repeater and commercial VHF duplexer, which cut the cost down to about 10% of new/retail.