By David (VA7SZ) with notes from Jesse (VE7LYD) and Wilf (VE7OHM)
Summary
During a visit to a commercial repeater on Silver Star in September 2022, Wilf (VE7OHM), made a quick detour to RSS to check its general condition. He ran some basic tests and noted an abnormally high loss of power between the repeater power amplifier and the antenna, with the duplexer being the main suspect. This power loss would have barely been noticeable except to hams operating in areas of marginal reception.
A full maintenance visit was arranged and another club duplexer was tuned for RSS frequencies as a replacement if the existing unit couldn’t be fixed on site. Fortunately, the existing duplexer was successfully retuned and all other systems were found to be fully functional.
The output from the duplexer was increased from 8.15 Watts to 19 Watts and all systems are deemed in good order for winter.
If you are unfamiliar with duplexers or want to know how RSS functions, I have included a basic overview at the end of this report.
The new Environment Canada weather radar dominates the summit. VE7RSS is in the building on the left.
Preparations for Site Visit
Armed with the knowledge that there was potentially a duplexer problem at RSS, the tech committee located another identical NORAC duplexer. Wilf cleaned it up, and with help from Ralph Olds (VA7NU), it was tuned for RSS. However, there seemed to be a minor issue as performance didn’t quite meet typical specs. That said, it was better than the duplexer currently operating at RSS, and if the current RSS duplexer couldn’t be retuned this would serve as a replacement. Wilf kindly gave us a demonstration of repeater tuning at his home and also demonstrated how intermodulation from multiple RF sources can lead to repeater problems.
Site Visit
The team, Ritchie (VA7RLX), Wilf (VE7OHM), Lorne (VE7LWK), Jesse (VE7LYD) and David (VA7SZ) met on Pinnacle Drive in Silver Star Village. Ritchie had obtained prior permission from Silver Star management, and security opened the gate for us to access the trail. The trail to the summit was dusty due to the unseasonably warm weather and the temperature at the summit at 10 am was already in the low teens due to an inversion. We had 4WD vehicles but 2WD with moderate ground clearance would be sufficient in these conditions.
The summit is a busy communications hub with many towers. It is now dominated by Environment Canada’s new weather radar dome and tower structure. The repeater is housed in the loft of a Silver Star building and access requires an eight-foot ladder to get into the loft through a trap door.
The Daniels repeater with controller on top.
The loft was clean and tidy, with no evidence of recent rodent activity. Jesse and Wilf hauled some impressive looking test equipment into the loft and the rest of us stood back and let them get to work. Installing and maintaining mountain top repeater sites is Jesse’s day job so he knew exactly what to do. The 210C4 antenna and feed line were scanned (Heliax runs from the shack to the antenna) and were found to be fine, as was the pass band filter between the duplexer and receiver. All connecting cables also tested fine so this narrowed down the power loss to the duplexer. It was probable that one of the tuning rods had been accidentally moved – their positions are held with locking nuts but an impact could have dislodged one.
Wilf and Jesse successfully retuned the duplexer in situ. Output from the duplexer was increased from 8.15W to 19W. With 24.5W leaving the transmitter this is an insertion loss of approximately 1.1dB – an excellent result!
Receiver sensitivity was measured at -119dBm with no de-sensitivity from transmit. The site’s noise floor was measured at 4.5dB above the thermal noise floor of the receiver. On-air testing confirmed a hint of noise from an unknown external source, but this isn’t unusual in such a dense RF environment.
While this work was going on the batteries were given a load test with approximately 25Ah being discharged. At this point there was more than 95% capacity remaining. Ritchie topped-up some of the cells with distilled water. The battery charger provided a 10.5A charge rate. The repeater power supply and diode bridge were working properly.
We packed up and left the repeater around 1 pm, and Ritchie called his management contact to confirm our departure.
Jesse runs some diagnostics
Overview of VE7RSS Repeater Operation
For those of you who are unfamiliar with repeater operation, here is a short primer.
VE7RSS uses an old but robust Daniels repeater. It functions in duplex mode – receiving on 146.28 MHz and simultaneously rebroadcasting on 146.88 MHz. It does this using the same antenna, with only 600 KHz separating transmit and receive. Without excellent filtering, the transmit signal (approximately 25W from the PA) would overpower and likely damage the receiver. A duplexer is a sophisticated series of filters that isolate these two frequencies when using a common antenna.
Duplexers are physically large devices because they use resonant cavities that are approximately ¼ wavelength in size – that’s around 50cm for a 2m repeater. The RSS repeater is a six-cavity design that provides excellent isolation between the two frequencies. Resonant cavity filters are used rather than coils and capacitors primarily because they can handle large amounts of power, have low insertion losses (typically 1.5 dB) and are highly selective.
Three of the cavities are used on the receive side, and three on transmit. On the receive side, the cavities are configured in series and tuned to pass 146.28 and reject 146.88. On the transmit side, cavities are also configured in series, and 146.88 is passed and 146.28 is rejected or notched out.
Tuning a cavity involves raising or lowering a rod in the center of the cavity for pass band tuning and adjusting a capacitor for tuning the notch. Tuning is an iterative process and requires expensive test equipment to get the best results. It’s an art as much as a science!
RSS also has a cavity band pass filter between the duplexer and receiver. Its function is to keep out unwanted signals from other communications equipment on the mountain. These unwanted signals could otherwise mix in the receiver’s circuitry and degrade performance.
RSS uses a 210C4 antenna, which is a stacked array of four, folded dipoles. This design has a lot of gain towards the horizon to maximize reach and is located at the top of the tower.
A controller is used to operate the repeater functions such as the familiar voice and Morse identification, double beep tone for battery power and other control functions via DTMF tones.
The repeater has its own power supply and should this fail, a lead-acid battery bank will provide backup for a limited time. This battery bank has an independent charger and if there is still 110V power to the building the charger will keep the batteries topped up and run the repeater.
VE7RSS' duplexer.