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Radiating cable calculator.

Enter the datasheet values for your cable and for the radio. The tool calculates the system loss, the received level at the worst-case point of the run, and the maximum length you can feed in the configuration you choose.

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How it works

The calculation model

A radiating cable has two losses that are independent of each other. One grows with the distance traveled along the cable, the other does not. IEC 61196-4 measures them together, and their sum is the system loss.

TUNNEL BOREEQUIPMENT ROOMBASESTATIONAMPLIFIERP_txSPLITTERjumpers and connectorsLUMPED LOSSESLONGITUDINAL ATTENUATIONα × z / 100RADIATING CABLELOAD50 Ωcoupling loss+ free-space lossdRADIOWORST-CASE POINT
Base station and amplifier
The power you enter in the calculation is the amplifier output, before the passive components.
Jumpers, connectors, and splitter
These are the passive components ahead of the cable. The sum of their losses goes in the “lumped losses” field.
Radiating cable
It attenuates along the route: that is the longitudinal attenuation, in dB every 100 m, multiplied by the distance z from the feed point.
50 Ω load
It closes the far end and absorbs the remaining power. It does not radiate and does not enter the calculation, but without the load the signal comes back and skews the measurement.
Cable-to-radio path
This is where the datasheet coupling loss applies and, if the radio is beyond the reference distance, the free-space loss.
Feeding from one end only: the worst-case point is the far end, where z equals the run length. Feed from both ends and the worst-case point moves to the middle, halving z.
  1. 01

    Longitudinal attenuation

    The loss of the signal traveling inside the cable, in dB every 100 m. It rises with frequency. Multiplied by the distance from the feed point, it gives the part of the loss that depends on length.

  2. 02

    Coupling loss

    The ratio in dB between the signal leaving the radiating cable and the signal received by a half-wave dipole. It does not depend on how much cable the signal has run through. Manufacturers state it at 50 % and at 95 %; in a tunnel you design on the 95 % value.

  3. 03

    System loss

    The sum of the two above. IEC 61196-4 measures them in free space, with the cable 2 m above the ground and the dipole 2 m from the cable. It is the figure tender specifications use to compare one cable with another.

  4. 04

    Free-space loss

    If the radio is farther away than the 2 m of the measurement, you add the free-space loss between the two distances, equal to 20·log10(d / d_ref) dB. Below the reference distance the term is zero: the standard does not characterize the near field, so we do not apply a reduction the standard does not measure.

  5. 05

    Lumped losses

    The losses on the passive components ahead of the cable: branching, connectors, splitters, and jumpers. They are subtracted once, before the signal enters the radiating run.

  6. 06

    Worst-case point and margin

    Feed from one end and the worst-case point is the opposite end. Feed from both ends and the design length halves, with the worst-case point at mid-run. The link margin is the difference between the received level and the sensitivity of the radio.

Formula applied

P_rx = P_tx − L_lumped − (α × z / 100) − L_coupling − 20·log10(d / d_ref)

z is the distance from the feed point to the worst-case point, α the longitudinal attenuation in dB/100 m, d the distance between cable and radio, d_ref the datasheet reference distance. The last term applies only when d exceeds d_ref: below that it is zero.

The calculation

Link budget for the radiating cable run

Equipment

Power before the passive components: the lumped losses are subtracted below.

Losses on the passive components ahead of the cable: branching, connectors, splitters, and jumpers.

The minimum level the radio needs. Take it from the manual, not from the tender spec.

Radiating cable

Datasheet cable attenuation at your operating frequency. It changes with frequency: use the row for your band.

Datasheet coupling loss, measured to IEC 61196-4. In a tunnel you design on the 95 % value.

The distance the coupling loss is measured at. IEC 61196-4 uses 2 m.

Length of radiating cable laid in the bore, downstream of the passive components.

Feeding from both ends moves the worst-case point to mid-run. The calculation assumes the same power at each end.

Coverage scenario

Distance between the cable and the radio at the worst point in the cross-section.

Reserve you require above sensitivity, to cover vehicle traffic, the operator’s body, and component aging.

Frequently asked questions

Common questions about radiating cable

Where do I find the longitudinal attenuation of my cable?
On the manufacturer’s datasheet, in the table by frequency. The same cable has very different values at 400 MHz and at 2100 MHz, so the row for your band is the one that counts, not an average. A 7/8 inch cable and a 1 5/8 inch cable give noticeably different values: at the same frequency the larger diameter attenuates less.
Why does the coupling loss have a percentage next to it?
Because it is a statistical quantity: along the cable the level varies from point to point. The 50 % value is the median of the measurements. The 95 % value is the one not exceeded at 95 % of the points, that is the unfavorable case. In a tunnel you design on the 95 % value: it is the one that describes the worst-case point.
What is the difference between system loss and total losses?
System loss is the sum of longitudinal attenuation and coupling loss, the two quantities IEC 61196-4 measures on the cable. Total losses add what depends on the installation and not on the cable: the lumped losses on the passive components and the free-space loss beyond 2 m.
When is it worth feeding from both ends?
When the run is longer than the maximum length you get from a single end. Feeding from both sides halves the design length, so you cover roughly twice the bore. You do need two feed points, so two equipment rooms reached by the signal. One thing to watch: the calculation assumes the same power at each end. If you split a single transmitter in two, add the splitter loss, about 3.5 dB, to the lumped losses.
What design margin should you use in a tunnel?
The margin has to cover at least three things: the attenuation caused by the body of the person holding the radio, the vehicles filling the cross-section, and the aging of cable and connectors. A reasonable starting value is 10 dB, then you verify it with the field measurement. Some tender specifications set it explicitly: in that case the tender spec wins.
Does the calculation account for the end antennas?
No. The tool sizes the radiating run only. The areas served by an antenna, typically the portals and the emergency recesses, have to be calculated separately with the classic free-space link budget.
Why does the result not match the field measurement?
Because the datasheet coupling loss is measured in free space, to IEC 61196-4, with the cable 2 m above the ground. In a real bore the cross-section, the fans, the cable ducts, the vehicle traffic, and the position of the cable relative to the crown all change the coupling. The calculation is there to size the run, the measurement to validate it.
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We have been designing, building, and commissioning in-tunnel radio coverage systems for over twenty years. Our technicians measure attenuation and coupling on the installed cable and issue the acceptance test report.

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