The 2 m Repeater Coverage Complaint
You are the trustee of a 2 m repeater on 146.940 MHz (input 146.340 MHz, negative offset, 100 Hz CTCSS). Users report that coverage has shrunk: mobiles that used to reach the repeater from 25 miles now drop out at 10 miles, and the repeater occasionally 'hears' noise that opens the squelch. The antenna is a 5/8-wave vertical at 200 ft on a commercial tower, fed with 150 ft of 7/8-inch foam-dielectric coax. The duplexer is a 4-cavity notch type. You have a service monitor, a wattmeter, and a dummy load. Receive sensitivity at the repeater site is normal when tested with a signal generator into the antenna port.
1. Given that receiver sensitivity measured at the antenna port is normal, where do you look next, and why?
Read the worked answer
If the receiver is sensitive with a generator at the antenna port, the receiver and duplexer are not the primary problem - the loss is between the antenna port and the antenna, or the antenna itself. Check, in order: 1) Feedline and connectors - 7/8-inch foam coax is moisture-sensitive; water in a connector or a cracked jacket raises loss and can create noise. Measure through-loss with the service monitor or a wattmeter (forward/reflected) at the antenna end if accessible. 2) Antenna - a 5/8-wave vertical's matching network or radials may have corroded; a change in VSWR or pattern explains reduced coverage. 3) Duplexer drift - even with good sensitivity, a detuned duplexer can add loss on transmit and desense on receive; verify notch depth and insertion loss. The 'noise that opens squelch' suggests an intermittent connection or arcing, consistent with a feedline/connector fault rather than a receiver fault.
2. How would you use the wattmeter and dummy load to distinguish feedline loss from antenna mismatch, and what result would confirm each?
Read the worked answer
Test A - transmitter/duplexer output: Connect the transmitter through the duplexer to the dummy load; measure forward power. This gives the power delivered to a 50-ohm load and checks the duplexer and PA. Test B - at the antenna end: If you can access the antenna, measure forward and reflected power there (or use the service monitor to measure through-loss of the feedline by comparing a known signal injected at the shack end with the level at the antenna end). Interpretation: - If Test A is normal and feedline through-loss is high (several dB more than the coax's spec), the feedline/connectors are lossy - replace/repair. - If through-loss is normal but reflected power at the antenna is high, the antenna is mismatched - repair the antenna/match. - If both are normal, revisit the duplexer (insertion loss/notch) and site noise. A wattmeter alone cannot separate the two; you need a measurement at both ends, or a known-signal through-loss measurement.