UHF Yagi Installation with Long Coax and Low MER
A viewer 32 km from a UHF transmitter installs a 12-element Yagi with 12 dBd gain on a 6 m mast. The downlead is 30 m of RG-6 coaxial cable with a published loss of 0.25 dB/m at 600 MHz, plus four connectors estimated at 0.5 dB each. At the wall outlet, a DTT receiver reports signal strength −72 dBm and signal quality (MER) 18 dB with a pre-BER of 2×10−4. Some multiplexes pixelate badly, especially in rain, while others are stable. The receiver needs at least 22 dB MER for reliable reception and can accept −80 to −20 dBm. The antenna is aimed at the main transmitter, but a nearby hill is just inside the first Fresnel zone. The user has a 15 dB masthead amplifier and a 4-way passive splitter. The splitter adds 7 dB loss, and the receiver is on a 10 m further cable. The user wants to know whether to add the amplifier, move the antenna, or replace the cable.
1. Calculate the approximate signal at the antenna terminals and assess whether the system is noise-limited or interference/reflection-limited.
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The 30 m RG-6 run has 30 × 0.25 = 7.5 dB loss. Four connectors add about 4 × 0.5 = 2 dB, so the total downlead loss is about 9.5 dB. The receiver sees −72 dBm at the wall, so the signal at the antenna terminals is approximately −72 + 9.5 = −62.5 dBm. That is a strong signal: it is 17.5 dB above the receiver’s −80 dBm sensitivity floor and well below the −20 dBm overload limit. Therefore the system is not primarily noise-limited. An MER of 18 dB with such a strong level points to multipath, reflections from the hill inside the Fresnel zone, or interference rather than weak signal. The rain-related pixelation suggests the link margin is marginal and reflections change with the environment. Amplifying a strong but distorted signal will not restore MER; it may even overload the receiver.
2. Evaluate the proposed 15 dB masthead amplifier and 4-way passive splitter. What will improve and what will not?
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A 15 dB masthead amplifier would raise the level at the wall to about −57 dBm, but the input is already about −62.5 dBm at the antenna. Amplifier gain increases both the wanted signal and any interference, and it cannot repair multipath. If the amplifier is overloaded by strong local signals, MER can fall further. The 4-way passive splitter adds 7 dB loss, so the receiver would see about −79 dBm after the splitter even with the amplifier, which is only 1 dB above sensitivity and leaves little rain margin. Without the amplifier, the splitter would put the receiver at about −79 dBm from the existing −72 dBm, also marginal. A masthead amplifier is appropriate when the antenna signal is near the noise floor, not when it is strong and MER is poor. The splitter should be replaced by a low-loss distribution amplifier or the number of outlets reduced.
3. Recommend a practical plan to achieve reliable reception.
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First fix the antenna environment: raise the Yagi, move it sideways or backwards to clear the hill from the first Fresnel zone, and re-aim using a signal meter that displays MER, not just level. Check the balun, the boom-to-mast bond, and all connectors for water ingress; replace the RG-6 with RG-11 if the run must stay long. Add an LTE/4G filter if the site is near a cellular base station. Do not install the 15 dB amplifier unless measurements at the antenna show the signal near the noise floor; with −62.5 dBm at the antenna, the amplifier is likely to hurt more than help. For two or more outlets, use a low-loss active splitter or a distribution amplifier with adequate headroom. Target at least 22–25 dB MER with a level between about −60 and −40 dBm at the receiver, then test all multiplexes in rain and record the margin.