My 70CM Repeater (Abandoned)
While locked in the house during the Covid-19 outbreak, I built a 70CM repeater out of two TYT TH-9000D 70CM radios and an ID-O-Matic IV controller.
This whole project started as a result of an Ebay listing that came recommended to me by Ebay. The listing was for repeater cables with DB-9 output for TYT TH-9000D radios by the seller radiodan-w7rf. Apparently these radios have a a header on the main board that provides PTT, audio in and out, and COS (Carrier Operated Signal). They also have a knock out in the back for a DB-9 connector. How convenient.
These radios are monoband and 3 variants are sold: 2 meters, 1.25 meters and 70 centimeters. I chose the 70CM model since A) there are supposedly no more 2M repeaters pairs in the area and B) you can get mobile duplexers for 70CM cheaply and are roughly the size of an iPad. I ordered the radios and cables and assembled them as instructed by the Ebay seller. I also disconnected both speakers since they would not stop making popping noises wheneven the squelch was opened or the radios were turned off or on.
The next task was to get a repeater controller. The Ebay listing recommended an ID-O-Matic IV and had all the information needed to connect the radios so I went with this. The seller did offer the 2 adapters for the radios for $30 or for $50, you could get the adapters with a DB-9 cable hacked up and color coded for the ID-O-Matic. I chose the cheaper route and bought a $4 serial cable and identified the correct wires with the help of the listing. Saved myself $20. :) The seller even threw in a free plastic box for the controller.
After all this was done, I was able to test the repeater minus the duplexer that I didn't have yet. I hooked the receive radio up to my Comet GP-3 in the attic and the transmit side into my old magmount on my closet floor. It provided the isolation needed between the two radios but wouldn't offer much transmit range. RX was at 30 feet and TX was at roughly 20 feet. I ended up ordering a 50 watt 6 cavity UHF duplexer straight from China. I was planning on ordering one as an experiment back in late 2019 but I never did.
These duplexers have mixed feedback from people since they are from China. However, there seemed to be more positive evidence that these work quite well with people easily getting 110db of isolation out of them. The only drawback is that the seller's free tuning should be taken with a grain of salt and always double checked. When I went to check mine, there wasn't much isolation on the Low side (TX frequency) and was set to roughly a 4 MHz split even though I specified a 5 MHz split. Since I didn't have access to the club's spectrum analyzer, I broke out my Nano VNA and got to work. At my first attempt, I could only get 60 db of isolation on the High side (RX frequency) and roughly 50 DB on the Low side. I also found that when I tuned the Low side, the High side got messed up.
Obviously I was doing something really wrong so I spent some time researching on how to tune duplexers. The following examples assume the repeater transmits on 442 MHz and receives on 447 MHz. When tuning the Low side (TX side), you need to be notching out the receive frequency of 447 MHz and NOT 442 MHz like I did on my first attempt. When tuning the High side (RX side), you need to notch out 442 MHz and NOT 447 MHz like I was doing. I was still struggling with getting the supposed 75+ dB of isolation I was supposed to be getting from the duplexer.
I ended up having a video call with the club's station trustee, Dwayne, KA9KQH, where I was informed that I was really getting over 110dB of isolation from the duplexer. I learned that the isolation rating is derived from the amount of isolation that each side of the duplexer provides. So the ~60 dB on the High side and the ~50 dB on the Low side was giving me an excellent isolation of 110+ dB. These duplexers are rated at 75 dB or better of isolation so that part of these duplexers is true. I am planning on doing some fine tuning on a spectrum analyzer to see just how much isolation I can get out of it. The Nano VNA will certainly get you darn close to an excellent tune on a duplexer but it's too slow to do the really fine/final pass. I was finally able to get the entire repeater on my Comet GP-3 and there was no sign of desense with 2 handhelds around the house and immediate area around the neighborhood.
Some limited range testing was performed the next day. I was able to get roughly three-tenths of a mile away from my house before I would get no receive audio, only the courtesy tone which leads me to the conclusion I have some major desense issues. When I had the transmit radio on a second antenna at 20 feet, I was able to get almost a mile out of the repeater and on top of that, when I was using the duplexer, I was able to get the courtesy tone back full scale over 2 miles away with only 10 watts so I am 99% sure it's a desense/duplexer issue.
I ended up mounting everything including a power supply onto a vented 2U rackmount shelf and strapped a small fan to the transmit radio. Works and look great!
In the middle of June, I was able to get access to the club's spectrum analyzer. We threw the duplexer on the spectrum analyzer and found that my tuning was almost dead on. We made some minor adjustments to lower the insertion loss and I went home with some other things to try. I also got to climb my first antenna tower too which was a lot of fun. ;) When I got home, I set about testing the repeater. Once again, I was only able to get to the end of my street before I'd loose receive audio. I tried connecting the duplexer in reverse in case I was doing something wrong but it really made it worse so that's not the issue. I also tried putting some distance between myself and the repeater in case it was somehow desensing my mobile radio. I ended up 3 miles away from the repeater and still had the same issue. I only managed the courtesy beep on both an HT and my mobile radio but at least I was receiving the repeater full scale so there's a positive.
As of now, I've written the Ebay duplexer off. I ended up asking a local contact and was able to procure a vintange Phelps Dodge duplexer, later Celwave and now RFS. The model number is PD526-4-2 for the curious. I went back to Dwayne's house where we spent well over 5 hours screwing around with it. He most of the work. :) We were able to get roughly 100 db of isolation out of it. We decided to check the SWR on the transmit side and that's when we found out the middle transmit cavity is bad. So we bypassed it and retuned it and now I have a 5 cavity duplexer. There's a 2.28 db loss on the transmit side, not the best but not the worst. It's funny, even in this state, this duplexer is better than the set that the SVRC uses on 443 MHz. We're tempted to retune them and see if they would be more immune to the intermod on that repeater since mine has 3 cavities on the receive side where as the set up there has only 2.
I tested again and this time I was able to get farther away but not by much. At this point, it's possible the antenna is desensing the receive radio. Once I get the repeater antenna installed, it should hopefully have more isolation. Fingers crossed.
The antenna I'm using is a Tram-Browning 6157 broadband UHF antenna with 25 feet of LMR-400 mounted in the middle of the attic, roughly 30 feet in the air.
I am toying with the idea of getting an MMDVM board to convert this into a local DMR repeater for public service events and the like. Stay tuned for that.
Update: The repeater controller died in July, I've scrapped this project. I acquired a used Motorola GR500 repeater that was in service at St. Johns Hospital for an ambulance company. I'm making a new page on this.
This is the supposed coverage of my repeater.
