Will a 2 m Simplex Path Work? A Link Budget

Before buying an amplifier or a bigger antenna, it is worth asking whether a VHF path can work at all. A link budget adds up everything between two stations: power, antenna gains, cable losses and path loss. Free space gives the best case; the terrain decides the rest. This guide checks a 2 m FM simplex path between two home stations 45 km apart.

The stations

Station A runs 50 W into a 10 dBi Yagi at 10 m height, with 2 dB of cable loss. Station B has a vertical with 2.15 dBi gain at 15 m height and 1.5 dB of cable loss. Both use FM at 146.52 MHz, the North American national simplex calling frequency, and their receivers need about 0.2 µV, −121 dBm, for usable audio.

Step 1: free-space path loss

The free-space path loss calculator gives 20 log₁₀(45) + 20 log₁₀(146.52) + 32.45 = 108.8 dB. The dBm converter turns 50 W into 47.0 dBm. The received power at B is 47.0 + 10 + 2.15 − 2 − 1.5 − 108.8 = −53.2 dBm, about S9 + 40 dB on a VHF meter calibrated to the −93 dBm standard. The margin over the receiver’s sensitivity is 67.8 dB. On paper, the link is enormously strong; a 5 W handheld with a rubber antenna at each end would still have about 51 dB of margin in free space.

Step 2: is there a line of sight?

The radio horizon over a smooth Earth, allowing for normal atmospheric bending, is about 4.12 × (√h₁ + √h₂) km with heights in metres. For antennas at 10 m and 15 m, that is 4.12 × (3.16 + 3.87) = 29.0 km. The stations are 45 km apart, so even over perfectly flat ground they are beyond each other’s horizon. Raising A’s antenna to 30 m extends the horizon to 38.5 km, still short. The signal must therefore diffract over the Earth’s bulge or over intervening ridges, and diffraction losses of 20 to 40 dB are common on such paths. That eats a large part of the 68 dB margin but may leave enough: this path will probably work with the Yagi, and the handhelds will probably not.

Step 3: clearing the Fresnel zone

Even on a line-of-sight path, obstacles near the direct line matter. The first Fresnel zone at the midpoint of a 45 km path at 146 MHz has a radius of about 152 m; keeping at least 60%, about 91 m, clear of hills and buildings avoids significant extra loss. On VHF, that clearance is rarely available over long distances from home stations, which is why operators favour hilltops for contests and why repeaters sit on mountains and tall buildings.

What to change

If the path fails in practice, compare the options in decibels. Doubling A’s power to 100 W adds 3 dB. Replacing B’s vertical with a 10 dBi Yagi adds about 8 dB and helps both directions. Changing B’s cable to a low-loss type might save 1 dB. Raising either antenna can turn a diffracted path into a clear one, worth 10 to 20 dB or more, which is why height is almost always the best investment on VHF. A common mistake is to spend money on power, which helps only one direction and costs the most per decibel.

Common mistakes

• Treating free-space margin as a guarantee. It is the best case on a clear path.
• Forgetting the horizon. Check the line-of-sight distance before anything else.
• Counting gain twice. Use dBi throughout, or dBd throughout, not a mixture; a dipole is 2.15 dBi.
• Ignoring the other direction. Power helps only one way; antenna gain and height help both.
• Leaving out cable loss. On VHF it is often several decibels.

Link budget checklist

1. Convert transmitter power to dBm and list antenna gains in dBi.
2. Add cable and connector losses at both ends.
3. Calculate free-space path loss for the distance and frequency.
4. Compare the received power with the receiver sensitivity to find the margin.
5. Check the radio horizon and Fresnel clearance, and allow for terrain.
6. Improve height first, then antenna gain, then power.