For about two years, K1IR has been home for a ham radio repeater. It’s part of the NEDECN DMR network. The repeater is on the 2 meter band and is accessible with these settings:
- Callsign: K1IR
- Frequency: 146.47000MHz
- Offset: -1.5MHz
- ColorCode: 0
The Challenge
The coverage area for the repeater has not been the best. That’s because we have been using a two-antenna system. The receive antenna is a Diamond vertical located at about 105 feet on top of the tower. That antenna works quite well. But, we’ve been using a second antenna only about 30 feet above ground level for transmitting. This antenna, another Diamond vertical, simply doesn’t perform very well at this low height. The result has been a very unbalanced coverage area for the repeater. Not very attractive to potential users. We needed to make a change.
![](https://144.202.15.12/wp-content/uploads/2021/04/K1IR-Repeater-Antenna-1-1024x562.jpg)
Our first effort to improve the coverage was to put up a new vertical at about 70 feet on the tower. But, that project was never completed because getting a second low-loss feedline in place to feed that antenna proved quite difficult.
Adding a Duplexer – A Real Solution
Using a duplexer is another way to solve this problem. What is a duplexer? It’s a three-port device where the feedline to the antenna is connected to one port, a second is connected to the repeater receiver input, and the third is connected to the transmitter. The duplexer creates two low insertion-loss paths that have extremely high isolation from each other.
- Antenna port to receiver port
- Transmitter port to antenna port
![](https://144.202.15.12/wp-content/uploads/2021/04/rptrblockdiagram.gif)
The duplexer that has some great advantages. It allows you to use the same coaxial feedline and antenna for both transmitting and receiving, with no compromise in performance.
![](https://144.202.15.12/wp-content/uploads/2021/04/dupfig15.jpg)
To achieve these high-Q curves and deep isolation nulls, the long-proven approach is to build filters with resonant cavities. These cavities must be carefully tuned with specialized test equipment.
Duplexers are fairly expensive – and not in the K1IR repeater budget! But, they do become available on occasion as repeaters are upgraded or decommissioned. So, I put the word out to my network to be on the lookout for a surplus duplexer. That was at least a year ago.
![](https://144.202.15.12/wp-content/uploads/2021/04/MVD6136_detail.jpg)
About a month ago, long-time friend, Dom Mallozzi N1DM, told me he might just have a lead on a VHF duplexer coming out of service following a repeater upgrade in a public safety application. The frequencies for VHF public safety are close to the 2 meter band, so retuning would work just fine. That duplexer – a Sinclair Q201 – was made available as surplus just this week, so Dom got his hands on it as quickly as possible and we did the installation this morning.
Assembly and Tuning
![](https://144.202.15.12/wp-content/uploads/2021/04/20210425_094642-1024x498.jpg)
Tuning is easy when you have the right gear. And, Dom came fully equipped. He pulled out his Rigol DA815-TG Spectrum Analyzer with built-in tracking generator. It didn’t take more than a few minutes to ensure that the tuning was within spec on the new frequencies.
![](https://144.202.15.12/wp-content/uploads/2021/04/20210425_095007-1024x498.jpg)
With the cavities pre-tuned, we built up a couple of cables and connected the duplexer in line with the repeater and antenna, and started our final testing.
![](https://144.202.15.12/wp-content/uploads/2021/04/20210425_110955-498x1024.jpg)
Initial tests showed low power output from the repeater. With a little micro-adjustment of one cavity on the transmitter side, power came up to exactly what it should be – and we were in business!
Coverage Testing – It’s a Big Win!
Final testing was a coverage area test. Dom packed up his stuff and headed south back towards his home QTH in Natick. I got my HT setup in the car, connected to a trunk lip mounted 5/8 wavelength whip. As we drove in opposite directions, it quickly became clear that coverage was greatly improved.
![](https://144.202.15.12/wp-content/uploads/2021/04/K1IR-Repeater-Coverage-Test-3.jpg)
Again . . . huge thank you to Dom Mallozzi for finding and making the duplexer available and for his skills in getting it running on the Sudbury, MA 146.47 DMR machine.
![Jim Idelson](https://144.202.15.12/wp-content/uploads/2019/11/Jim-K1IR.jpg)
Jim is a technologist, an innovator and a business builder. He most enjoys helping teams take on big challenges to achieve big goals.