Repeater Coverage Complaint After Antenna Change
A 2 m repeater uses a 150 W amplifier, a duplexer, and a 100 ft run of 7/8 inch heliax to a 4-bay folded dipole antenna at 200 ft. Users report that coverage to the north has degraded, and the SWR at the transmitter reads 1.8:1. The duplexer was recently retuned, and the antenna was replaced with a similar model. The repeater's receive sensitivity seems normal. A nearby commercial FM station at 100.1 MHz is 0.5 miles away. The site has a new LTE cell site 300 ft away.
1. What are the likely causes of degraded north coverage and elevated SWR?
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Possible causes:
- Antenna pattern distortion from the new antenna's mounting or nearby structures (the LTE site or tower changes the pattern).
- Feedline or connector damage during antenna replacement, raising SWR.
- Duplexer detuning after retuning, causing increased insertion loss or mismatch.
- Intermodulation or desensitization from the nearby LTE site, affecting receive but not necessarily SWR.
- Antenna damage (water in the connector, bent elements).
Since receive sensitivity seems normal, the problem is more likely transmit-side loss or pattern change. A 1.8:1 SWR at the transmitter may reflect duplexer loss or feedline issues. Check the antenna with a time-domain reflectometer (TDR) or sweep the feedline; inspect connectors and the antenna for damage.
2. How do you determine whether the LTE site is causing receiver desensitization or intermodulation?
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Desensitization: Strong off-frequency signals overload the receiver front end, reducing sensitivity. Test by adding a bandpass filter or cavity filter ahead of the receiver and see if sensitivity improves. Also check the receiver's noise floor with the antenna disconnected vs connected.
Intermodulation: Two or more strong signals mix in a nonlinear component (receiver front end or antenna system) to produce a signal on the repeater input. Test by monitoring the input frequency with a spectrum analyzer and looking for IM products. If the LTE site is the source, increase isolation with a duplexer/cavity filter or relocate the antenna.
Also check the commercial FM station at 100.1 MHz: its second harmonic (200.2 MHz) is far from 2 m, but strong FM can cause front-end overload. A bandpass filter and ferrite chokes on the feedline can help.
3. What steps would you take to restore coverage and verify the fix?
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- Inspect the antenna and feedline: check connectors, weatherproofing, and antenna orientation; use a TDR to locate faults.
- Sweep the duplexer and confirm tuning; check insertion loss and isolation.
- Measure SWR at the antenna (not just at the transmitter) to separate feedline loss from antenna mismatch.
- Add filtering: a bandpass filter or cavity filter on the receive side to reject LTE and FM signals; ferrite chokes on the feedline.
- Check the antenna pattern with a field-strength meter or by driving the area; compare north coverage before and after.
- Document changes and monitor user reports. If the pattern is distorted by the new structure, consider relocating the antenna or using a directional antenna to restore north coverage.