What repeater spacing is used for a VHF tunnel radio system with leaky feeder cable?

In the worked underground railroad design example, repeaters are placed every 300–500 m along the tunnel. For a 500 m repeater spacing at 150 MHz with a 5 W (37 dBm) transmitter, 3 dBi antenna gains, and leaky feeder cable loss of 10 dB per 100 m, the received power at the next repeater works out to −67.05 dBm — 17.95 dB above the −85 dBm receiver sensitivity, confirming reliable communication.

How much extra attenuation should be budgeted for underground tunnel propagation?

Tunnel propagation is modeled empirically by adding a tunnel constant K, typically 20–30 dB, on top of free-space path loss to account for guided-wave attenuation in the tunnel environment. This is in addition to leaky feeder cable loss, which runs about 10 dB per 100 m for VHF systems and 12 dB per 100 m for UHF systems in the article's design examples.

What is the formula for free-space path loss used in radio system design?

FSPL (dB) = 20·log(d) + 20·log(f) + 32.45, where d is distance in kilometres and f is frequency in MHz — a simplified practical form derived from L_p = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c). In the UHF fire department design example, this formula gives 71.51 dB of free-space path loss between repeaters spaced 2,000 m apart at 450 MHz.

What Fresnel zone clearance is needed for near-line-of-sight radio performance?

At least 60% clearance of the first Fresnel zone is required for near-LOS performance, with the first Fresnel zone radius given by r_n = √(nλd₁d₂ / (d₁ + d₂)), where n is the zone number, λ is wavelength, and d₁, d₂ are the distances from the obstacle to the transmitter and receiver.

What is the minimum C/I ratio for reliable P25 digital voice versus analog FM?

Minimum carrier-to-interference (C/I) requirements are approximately 12 dB for analog FM voice and 5 dB for P25 digital at the 5% BER threshold, meaning P25 can maintain usable voice quality at a lower carrier-to-interference ratio than analog FM. This C/I ratio is calculated as C/I = P_r / P_i, where P_i is the total interfering signal power at the receiver.

Engineering·28 min read·June 18, 2026

📻 Designing UHF and VHF Radio Communication Systems

A comprehensive guide to designing UHF and VHF radio communication systems — from fundamentals and key calculations to full design examples for underground railroad VHF and fire-department UHF systems, including BOMs and installation guidance.

📖Radio Communications — Illustrated Guide (Full Access)

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Introduction

Designing a radio communication system requires a structured approach: defining system requirements, selecting the appropriate frequency band, calculating link budgets and path losses, choosing components, and validating coverage through testing. This guide walks through the full process — from foundational theory to worked design examples for underground railroad (VHF) and fire-department (UHF) systems — and covers installation, testing, and inspection best practices.

Topics covered

UHF radio designVHF radio designradio communication systemlink budgetpath lossFresnel zoneSNRnoise figureleaky feeder cablerepeater spacingunderground railroad radiotunnel radio systempublic safety UHFEIRPfade marginantenna gainfree-space path lossDAS tunnelVHF 150 MHzUHF 450 MHzBOM radio system