Sunridge FIS: feedline loss, a power upgrade and a summer duct
Sunridge Flight Information Service covers a valley aerodrome from a hilltop site at 640 m AMSL. The VHF installation is a 25 W transmitter on 118.550, 60 m of RG-213 feeder (manufacturer data: 3.5 dB loss per 30 m at 130 MHz), 1.0 dB total loss for connectors and the lightning arrestor, and a single collinear with 6 dBd gain mounted 20 m above the equipment hut. Pilots report that they hear the FIS clearly at 80 NM, but that their own transmissions are not received beyond about 35 NM, and coverage in the northern valley is patchy below 2,000 ft. In summer, when an anticyclone sits over the region, the FIS also hears a distant station on the same frequency from roughly 250 km away, and the interference lasts for hours. The station manager proposes buying a 100 W amplifier. The regulator has asked for the engineering basis of any change.
1. Calculate the effective radiated power at the antenna, showing each step. State whether the installation is transmit-limited or receive-limited.
Read the worked answer
Transmitter: 25 W = 10 log10(25/0.001) = 44.0 dBm.
Feeder: 60 m at 3.5 dB per 30 m = 7.0 dB. Connectors and arrestor: 1.0 dB. Total feedline loss: 8.0 dB.
Antenna: 6 dBd gain.
ERP = 44.0 − 8.0 + 6.0 = 42.0 dBm
Convert: 42.0 dBm = 10^4.2 mW ≈ 15,800 mW ≈ 15.8 W ERP, i.e. 12.0 dBW.
The installation is receive-limited. Two of the reported facts point the same way: the ground station is heard at 80 NM while the aircraft is heard only to 35 NM, and the northern valley is weak at low level. Since the transmitter and receiver share the same feeder and antenna, 8.0 dB is being lost on receive as well as on transmit - an aircraft's signal is attenuated by 8 dB before it ever reaches the receiver front end. Roughly 63% of the transmitter output is being dissipated as heat in the feeder, and the same proportion of every received signal is lost.
2. Compare the proposed 100 W amplifier with moving the transmitter to the masthead and fitting a short jumper. Which is the better engineering decision and why? Give figures.
Read the worked answer
Option A - 100 W amplifier. 100 W = 50.0 dBm. ERP = 50.0 − 8.0 + 6.0 = 48.0 dBm ≈ 63 W. That is a +6 dB gain, obtained only by quadrupling transmitter power, and it does nothing for reception. It may also require a licence variation for the higher power.
Option B - masthead transmitter. Fit the transmitter in a weatherproof enclosure at the antenna, leaving only a 3 m jumper (≈0.35 dB) plus the 1.0 dB connector/arrestor allowance. Loss falls from 8.0 dB to about 1.35 dB, a gain of 6.65 dB, with the existing 25 W transmitter: ERP = 44.0 − 1.35 + 6.0 = 48.65 dBm ≈ 73 W.
Option B gives slightly more transmit gain than quadrupling the power, at lower cost and with no increase in licensed power. Decisively, it also improves the receive path by 6.65 dB, which is where the complaint actually lies. Add a receive preamplifier only after the feeder is fixed, and consider a higher-gain antenna with electrical downtilt to fill the valley without lifting the horizon.
A 6 dB power increase also increases the station's interference footprint toward the distant co-channel user, so it attacks the second problem in the wrong direction.
3. Explain the summer interference in propagation terms, and set out the operational and engineering measures available.
Read the worked answer
The anticyclone produces subsidence and a persistent temperature inversion, with warmer, drier air over cooler, moister air. The resulting sharp refractive-index gradient traps VHF energy in a tropospheric duct, refracting it back towards the ground far beyond the normal radio horizon. The same mechanism explains both symptoms: the ground station's coverage extends to several hundred kilometres, and a distant station on the same 118.550 assignment is carried into the valley with a signal strong enough to be heard for hours. Ducting is seasonal and weather-driven, not a fault.
Measures:
- Frequency coordination: establish who the distant user is; if the assignment is shared, agree a coordination distance or a sub-channel. This is the only measure that addresses the interference at source.
- Reduce incoming energy: a directional antenna with downtilt and a null toward the horizon, or a vertical pattern that suppresses low-angle arrival, will attenuate the ducted signal.
- Do not simply reduce power: that narrows your own coverage without reducing the strength of the interfering signal at your receiver.
- Operational fallback: agree a documented alternate frequency or relay arrangement with the parent ATS unit for ducting periods, notify pilots of degraded coverage, and log the episodes with times and duration so the regulator can see the pattern when the case for a new assignment is made.