A microwave radio advertises 300 Mbps. The path profile looks clear, the receive level is comfortably above the receiver threshold, and the link comes up at full speed. Then heavy rain arrives and the radios step down to 75 Mbps.
The link is still connected. If the service needs 200 Mbps, it has nevertheless failed its requirement.
Plan against the minimum capacity and availability the service needs in both directions. The radio’s fastest mode and its most robust mode carry different amounts of traffic, so each needs its own budget.
The example below follows a 12 km, 18 GHz path from the site checks through to commissioning.
The Short Answer
Microwave link planning combines a terrain profile, antenna-clearance checks, radio and antenna data, a link budget, propagation predictions and frequency coordination to design a fixed wireless path. The acceptance criterion is the required service at the required availability.
Work through the design in this order:
- Define the traffic, minimum capacity and availability target.
- Confirm the sites, mounting positions and path profile.
- Shortlist suitable bands, channels, radios and antennas.
- Check clearance under the applicable propagation assumptions.
- Calculate both directional budgets and performance at each radio mode.
- Resolve interference, coordination and installation constraints.
- Commission against the design and retain a measured baseline.
The noIM₃ Link Planner brings the path profile, budget and predicted availability into one project. The checks below explain what to look for in the results.
1. Define the Service Before Choosing the Radio
Start with a requirement such as 200 Mbps of usable capacity in each direction at 99.99% annual propagation availability. It is much more useful than asking for a 300 Mbps radio.
State whether the capacity is simultaneous in both directions, the minimum that must survive fading, or a peak that can fall during bad weather. Also specify whether the availability target applies to each direction separately or to the simultaneous two-way service. Distinguish Ethernet payload throughput from the gross radio bit rate. Add the traffic priorities, latency target and growth allowance that affect the design.
Also define what the availability figure covers. A propagation prediction describes a particular source of outage. It does not include every failure of a power supply, switch, radio, backhaul connection or support structure.
For a 365-day year, the arithmetic behind common annual targets is:
| Availability | Unavailable fraction | Equivalent annual time |
|---|---|---|
| 99.9% | 0.1% | 8 hours 45 minutes 36 seconds |
| 99.99% | 0.01% | 52 minutes 34 seconds |
| 99.999% | 0.001% | 5 minutes 15 seconds |
Those are time equivalents, not a forecast of how many events occur or how long one event lasts. If the business needs an end-to-end service target, allocate a budget to propagation and to the other failure causes rather than giving the radio path the entire allowance.
2. Confirm the Sites and Inspect the Actual Path
Use the coordinates of the intended antenna mounting positions. A site address or the centre of a compound can be several hundred metres from the mast, enough to change a path through a narrow terrain gap.
Keep three heights distinct: ground elevation, antenna height above that ground, and antenna elevation above the common vertical reference. A 30 metre mast on a hill and the same mast on the valley floor are different geometries.
Extract the profile, then inspect the obstacles that matter. Check buildings, trees, new earthworks and structures near either end. Find out whether the elevation source represents bare ground or a surface that already includes some vegetation and buildings. The Terrain Data Viewer helps inspect and compare source profiles; the radio line of sight guide explains the curvature assumptions.
A field survey is especially valuable where one uncertain crest or structure decides the antenna heights. Increasing the number of samples from an existing elevation raster does not improve the survey accuracy of that raster.
3. Choose the Band, Channel and Antennas Together
There is no universal best frequency for a microwave link. The useful options depend on distance, rainfall, available channels, permitted equipment, antenna size, site loading and the required capacity.
Changing band changes more than the free space loss. It changes rain attenuation, antenna gain for a given physical aperture, beamwidth, radio performance and the applicable spectrum arrangements. Compare complete candidate designs rather than changing frequency in an otherwise frozen budget.
Use the actual equipment data for each candidate:
- Transmitter power at the proposed modulation, bandwidth and configuration.
- Receiver thresholds and net capacity for each supported mode.
- Antenna gain, polarisation and radiation pattern at the operating frequency.
- Feeder, branching, connector and other installation losses.
- Supported channel widths, duplex arrangements and equipment options.
A radio’s maximum transmitter power may apply to a robust mode rather than its fastest mode. Using that power with the sensitivity and capacity of a different mode creates a budget the equipment cannot deliver.
Shortlist the regulatory route at this stage as well. A frequency that looks excellent in a propagation calculation is not necessarily a channel available at those sites.
4. Check Fresnel Clearance and Earth Curvature
Visible line of sight is only a geometric test. A radio path also occupies a Fresnel zone around that line, and atmospheric refraction changes the effective curvature used to check obstructions.
The first Fresnel radius at an obstacle is:
r₁ = √(λ × d₁ × d₂ / (d₁ + d₂))
Use metres for wavelength and distances to obtain a radius in metres. A 60% Fresnel-clearance check is a useful preliminary screen, but it is not the complete microwave planning procedure.
For a non-diversity path, ITU-R P.530-19 §2.2.2.1 starts with a full first-zone clearance at the appropriate median k-factor, using 4/3 where no data is available. It then checks an adverse effective k-factor with criteria that depend on climate and obstruction type, and takes the larger height requirement. Above about 13 GHz, the obstacle-height estimation accuracy is also added to the clearance. ITU-R P.530-19
Worked clearance check: 12 km at 18 GHz
Assume two sites with ground elevation 100 metres, both using 30 metre antennas. The antenna centres are therefore at 130 metres. A ridge halfway along the path has a ground elevation of 122 metres. These are illustrative inputs, with a common vertical reference and no additional trees or buildings on the ridge.
At 18 GHz, the wavelength is about 0.01666 metres. At the midpoint:
r₁ = √(0.01666 × 6,000 × 6,000 / 12,000) ≈ 7.07 m
With k = 4/3 and a physical Earth radius of 6,371 km, the midpoint Earth bulge is about 2.12 metres. The ridge therefore sits at 124.12 metres on the curvature-adjusted profile, leaving:
Clearance = 130 − 122 − 2.12 = 5.88 m
That clears 60% of the first zone, 4.24 metres, but falls short of the full radius, 7.07 metres.
| Equal antenna height at both ends | Midpoint clearance at k = 4/3 | Clears 60% of F₁? | Clears 100% of F₁ before a height-accuracy allowance? |
|---|---|---|---|
| 30 m | 5.88 m | Yes | No |
| 32 m | 7.88 m | Yes | Yes |
| 34 m | 9.88 m | Yes | Yes |
The 32 metre case passes the median full-zone check at this ridge by only 0.81 metres. If the study uses a 2 metre obstacle-height allowance, the midpoint alone requires about 33.19 metres at both ends; 34 metres clears that particular check. The 2 metres is a sensitivity-test assumption here, not the stated accuracy of an elevation product.
This still leaves the adverse-k check and every other point on the profile to assess. The table is not approval to build at 34 metres. It shows why a simple clearance percentage can conceal the input that actually controls the design.
5. Build the Link Budget for the Required Radio Mode
Continue with the 12 km path at a nominal 18 GHz. For the budget, assume the clearance requirements have been met and additional obstruction loss is zero. Use 39 dBi antenna gain at each end, representative of a possible 0.6 metre antenna specification, and 18 dBm transmitter power. These are chosen inputs, not the specification of a named product or an assigned frequency channel.
Free space path loss, with frequency in MHz and distance in km, is:
L_FSPL = 32.45 + 20 log₁₀(18,000) + 20 log₁₀(12) = 139.1 dB
Include a stipulated 0.5 dB clear-air atmospheric loss for the arithmetic. A real study obtains gaseous attenuation from the frequency and atmospheric inputs, for example with ITU-R P.676, rather than assuming every 18 GHz path loses that amount.
| Budget item | Contribution |
|---|---|
| Transmitter power | +18.0 dBm |
| Transmit feeder and connection loss | −1.0 dB |
| Transmit antenna gain | +39.0 dBi |
| Free space path loss | −139.1 dB |
| Clear-air atmospheric loss, assumed | −0.5 dB |
| Receive antenna gain | +39.0 dBi |
| Receive feeder and connection loss | −1.0 dB |
| Predicted clear-air received power | −45.6 dBm |
Now suppose the selected radio offers these two operating points at the configured bandwidth. The capacities are assumed net capacities per direction, available simultaneously in both directions, and 18 dBm is assumed available in both modes.
| Illustrative mode | Net capacity | Receiver threshold | Fade margin at −45.6 dBm | Meets a 200 Mbps capacity requirement? |
|---|---|---|---|---|
| Higher-capacity mode | 300 Mbps | −64 dBm | 18.4 dB | Yes, when this mode is supported |
| Robust fallback mode | 75 Mbps | −78 dBm | 32.4 dB | No |
The link has 18.4 dB of fade margin for the capacity that meets the brief. The larger 32.4 dB figure belongs to a mode that does not carry enough traffic.
Check the reverse direction with its own transmitter, receiver, losses and paired frequency. An asymmetric installation can have a different limiting direction. Our link budget guide covers the individual terms, and the Link Budget Calculator lets you check them separately.
6. Predict Availability at the Capacity You Need
To convert fade margin into predicted availability, the propagation model needs the path geometry, climate and radio performance.
For rain, ITU-R P.838 supplies the specific-attenuation relationship γ_R = kR^α, with γ_R in dB/km and rainfall rate R in mm/h. The coefficients depend on frequency, polarisation and path geometry; this k is a rain coefficient, distinct from the effective Earth-radius factor used for clearance. The rainfall statistics come from an appropriate source such as ITU-R P.837. A terrestrial-link method accounts for the effective rain path; multiplying one intense rain rate by the entire physical hop is not automatically the correct statistical prediction.
Clear-air multipath needs its own assessment. A dry day can still produce a fade, especially where the direct signal and reflected or refracted energy combine unfavourably. Our rain fade guide and fade margin guide explain those mechanisms further.
Use the planner’s capacity and availability results for each radio mode. A very high availability at 75 Mbps cannot establish 200 Mbps availability. Likewise, an annual average throughput above 200 Mbps does not establish that the minimum is held through the fades that matter.
At this stage, the higher-capacity mode has 18.4 dB of margin. Run the availability calculation with the actual coordinates, climate and equipment to decide whether it meets 99.99%. Revise the design if it falls short.
What changes when the margin is insufficient?
| Change to investigate | What it can improve | What else needs checking |
|---|---|---|
| Larger antennas | Receive level and off-axis discrimination | Mount loading, alignment, pattern and permitted equipment |
| Shorter hop or relay | Path loss and exposure to fading along each hop | Additional sites, power, maintenance and route availability |
| Another band | Rain and equipment trade-offs | Antenna gain, spectrum access and the complete revised budget |
| Another radio mode or bandwidth | Required receiver threshold | Whether the remaining net capacity still meets the brief |
| Diversity or an alternate route | Resilience to the failure mechanism it addresses | Correlated weather, shared infrastructure and the protection design |
At the same frequency and efficiency, doubling a dish diameter adds about 6 dB of antenna gain. Doing that at both ends adds about 12 dB to the received level if transmitter power and other losses stay unchanged and radiated-power limits allow it. In this example, replacing the assumed 39 dBi antennas with assumed 45 dBi antennas raises the received power to −33.6 dBm and the higher-capacity margin to 30.4 dB. Those gains still need confirmation from the actual antenna data.
There is a mechanical cost to that improvement. A larger dish has a narrower beam, and wind loading and mount movement can become controlling considerations. Receiver maximum input and permitted radiated power also need checking after a substantial gain increase.
7. Resolve Interference and Frequency Coordination
A healthy wanted-signal budget does not prove that another service will not interfere with the receiver, or that the new transmitter will not affect a neighbour. Consider antenna discrimination, wanted-to-unwanted levels, adjacent channels, receiver selectivity and other transmitters sharing the structure.
For Australian fixed microwave services, ACMA RALI FX 3 sets out frequency-coordination procedures and band-specific requirements. Sharing with fixed-satellite earth stations can also bring RALI MS 45 into the assessment. Use the arrangements that apply to the chosen band and sites.
The Link Planner’s register lookups and interference screens help prepare the design. They do not issue a frequency assignment certificate or an ACMA licence. Resolve the coordination and licensing route before treating a candidate channel as available for installation.
8. Commission Against the Design
The installed system needs to reproduce the planned configuration: frequency pair, bandwidth, polarisation, antenna orientation, mode limits and transmit power. Check cable and connector losses, mount rigidity, weatherproofing, power and backhaul interfaces as part of the installation.
Align using the antenna main beam and the equipment’s commissioning procedure. Compare the measured receive level in both directions with the predicted level, allowing for the actual test conditions and measurement uncertainty. A large unexplained difference is a reason to investigate the configuration or installation before accepting the link.
Measure the service as well as the signal. Check usable throughput under the agreed traffic conditions, latency, packet loss, alarms and the intended protection behaviour. Store the measured receive levels, mode state, equipment settings and test conditions as the baseline for maintenance.
A short acceptance test can demonstrate installation performance on the day. It cannot observe a year’s propagation statistics in an afternoon. Retain the propagation prediction and monitor the operational link so later faults can be compared with both the design and the commissioned baseline.
Planning a Route with More Than One Hop
An extra relay can solve an individual hop while adding another place for the route to fail.
For a simple series route in which all hops must work, independent hop availabilities multiply. Three hops each at 99.99% give:
A_route = 0.9999³ ≈ 99.9700%
That is about 2 hours 38 minutes of equivalent annual unavailability in a 365-day year. It is not 99.99% for the route.
This calculation assumes independent failures. Shared power, shared structures and weather can correlate failures; protected routes need a separate assessment. Set the route target first, then allocate hop performance and protection accordingly.
Frequently Asked Questions
What is microwave link planning?
Microwave link planning designs a fixed radio path using site geometry, terrain, antenna systems, radio performance, propagation predictions and spectrum requirements. It checks whether the path can deliver the required capacity at the required availability in both directions.
Is line of sight enough for a point-to-point radio link?
No. The path also needs appropriate Fresnel clearance, a suitable radio budget and an assessment of fading and interference. A path can look clear while failing its clearance criterion or its required service availability.
How much Fresnel clearance should a microwave link have?
A preliminary check often uses 60% of the first-zone radius. For non-diversity microwave planning, ITU-R P.530-19 uses a fuller procedure involving median and adverse effective Earth-radius conditions. Apply the relevant procedure and height-accuracy allowance rather than approving the design from one percentage.
What is a good fade margin for a microwave link?
A good margin is one that the propagation assessment shows is sufficient for the required capacity and availability on that path. There is no universal decibel target that establishes the same availability for every frequency, climate, path length and radio mode.
Does adaptive modulation guarantee the required throughput?
No. It allows a radio to use more robust modes as conditions deteriorate, usually at lower capacity. Check availability at the lowest mode that still carries the required service, not only at the most robust mode that keeps the radio connected.
Do larger dishes always solve a fading problem?
Larger dishes can improve the budget and antenna discrimination. They do not remove terrain obstructions or replace an interference assessment, and they add alignment and structural considerations. Recalculate the complete design with the actual antenna data.
Does a Link Planner interference screen mean the channel is licensed?
No. An interference screen supports planning and preparation. Frequency coordination, any required certification and the licensing decision are separate steps under the applicable regulatory arrangements.
Plan the Path in noIM₃
Use the Link Planner to place the sites, inspect the profile, compare antenna heights, select equipment and review capacity and predicted availability by mode. Keep the engineering report with the input data and equipment revision used for the design.
For individual checks, use the Fresnel Zone Calculator, Link Budget Calculator, Rain Fade Calculator and Parabolic Antenna Calculator. The free space path loss guide explains the baseline loss before the rest of the path is added.
Notes on Sources
The path geometry, receive levels, fade margins, annual time equivalents and three-hop availability are our own arithmetic. The example uses stipulated site elevations, equipment performance and a 0.5 dB atmospheric allowance. The 2 metre height allowance is a sensitivity check. These inputs are not measurements, a manufacturer product specification or a completed availability prediction for a real path.
- ITU-R P.530-19, Propagation data and prediction methods required for the design of terrestrial line-of-sight systems, especially §2.2.2.1 for the non-diversity clearance procedure. Retain the edition and digital inputs used with a reproducible propagation study.
- ITU-R P.525, Calculation of free-space attenuation, for the baseline free space relationship; ITU-R P.676, for atmospheric-gas attenuation.
- ITU-R P.838 and ITU-R P.837, for rain specific attenuation and rainfall statistics. These are inputs to a terrestrial-link prediction, not interchangeable with the resulting annual availability.
- ACMA RALI FX 3 and RALI MS 45, for the Australian coordination context.
