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 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.
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
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.
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.
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
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.
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.
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.
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.