Microwave links
A radio link joins two fixed points with antennas aimed at each other. Calculating it means adding gains and subtracting losses to see whether enough signal reaches the receiver, and checking that the path is really clear.
01Where a radio link is used
Joining two buildings, a plant with its office, a town with the fiber node. It is installed in days and does not depend on right-of-way permits.
Carrying the traffic of a cell tower to the core network, or connecting radio and TV repeaters.
It is the realistic way to bring internet to a farm or a remote settlement: a chain of links between hills, with low-cost equipment in license-exempt bands.
| Band | Typical range | Characteristics |
|---|---|---|
| 2.4 GHz (unlicensed) | 10 to 30 km | Very congested in cities. Large antennas for the same gain. |
| 5 GHz (unlicensed) | 10 to 50 km | The usual choice for low-cost links: less crowded and good small antennas. |
| 7 to 15 GHz (licensed) | 10 to 60 km | Professional links with interference protection. |
| 18 to 38 GHz | 1 to 10 km | A lot of capacity, short hops: rain becomes the dominant factor. |
| 60 to 80 GHz | < 2 km | Gigabit capacity. Oxygen in the air absorbs strongly at 60 GHz. |
02Line of sight and the Fresnel zone
Seeing the other end with your eyes is not enough. The wave does not travel along a line but through a volume shaped like an ellipsoid: the first Fresnel zone. If an obstacle enters it, attenuation appears even though there is a direct view.
A 10 km link at 5.8 GHz, with a 12 m tall stand of trees right in the middle of the span. The antennas are 20 m above the terrain, which is flat.
- Radius of the first zone at the midpoint: r = 17.32 × √(5 × 5/(5.8 × 10)) = 17.32 × √0.431 = 11.4 m.
- Required clearance, 60 %: 6.8 m below the straight line.
- The straight line between the two antennas passes 20 m above the ground at the middle of the span. The treetops are at 12 m: 8 m remain clear.
- 8 m > 6.8 m: the link passes, although with little margin. If the trees grow two more meters, it no longer passes.
That last point is real and always forgotten: trees grow and buildings appear. A link is calculated with the terrain of the coming years.
03The link budget
It is an addition and subtraction of decibels, from end to end. If the result exceeds the receiver sensitivity with enough margin, the link works.
| Item | Value | Cumulative |
|---|---|---|
| Transmitter power | +20 dBm | 20 |
| Cable and connector loss (Tx) | −1 dB | 19 |
| Antenna gain (Tx) | +24 dBi | 43 |
| Free-space loss (15 km, 5.8 GHz) | −131.2 dB | −88.2 |
| Antenna gain (Rx) | +24 dBi | −64.2 |
| Cable and connector loss (Rx) | −1 dB | −65.2 dBm |
| Receiver sensitivity | −83 dBm | — |
| Margin | 17.8 dB | |
FSPL = 32.44 + 20·log(5800) + 20·log(15) = 32.44 + 75.27 + 23.52 = 131.2 dB. A margin of almost 18 dB is a healthy link: it withstands rain, misalignment and equipment aging.
- Less than 10 dB: fragile link. It works on a nice day and drops with rain or wind.
- 15 to 20 dB: reasonable for a service link.
- More than 25 dB: critical links, or high bands where rain hits hard.
- And the fade margin due to multipath must also be subtracted, which on long spans over reflective terrain —water, salt flats, flat fields— can be 20 dB.
04Rain, availability and fading
It depends enormously on the frequency. Below 6 GHz it is almost negligible; at 11 GHz it already matters; at 23 GHz or more, it is the dominant factor in the design: a heavy storm can add tens of dB.
That is why high-band links are short hops, and why in areas of intense rainfall it is preferable to move down a band even if it costs bandwidth.
It is expressed as a percentage of the year:
- 99.9 % → up to 8.8 hours per year out of service.
- 99.99 % → 53 minutes per year.
- 99.999 % → 5 minutes per year.
Every extra nine costs margin, and margin costs bigger antennas or taller towers.
On long spans over smooth surfaces, part of the signal is reflected and reaches the receiver out of phase. If it arrives in antiphase, it cancels. It is fought with space diversity —two antennas separated vertically by a few meters— or by choosing heights that put the reflection outside the critical zone. It is the reason why two identical links can behave very differently.
05From calculation to installation
- Survey both ends: coordinates, available height, obstacles, power supply and access.
- Plot the terrain profile between them, with contour lines or with a profile tool, and include the curvature of the Earth.
- Check Fresnel at the critical points and define the tower heights.
- Choose the band and equipment, and calculate the link budget with margin.
- Install and aim: with a compass and GPS for the coarse work, then fine-tuning with the equipment's own level indicator, moving a little at a time and waiting for the reading.
- Document: azimuth, elevation, received level, SWR, photos of the alignment and of the sealing.
It is one of the most dangerous tasks in the trade: height, electrical storms, radiation from other active antennas on the same structure. Never climb alone, use a harness with a double lanyard, check the condition of the tower and of the guy wires, and ask for the nearby transmitters to be switched off before working in front of their antennas. With wind or a storm in sight, do not climb.
06In the lab
Choose two real points in the city or the area, obtain their coordinates and heights, plot the profile and calculate the Fresnel zone at the most critical point. Determine the minimum height of the antennas and make the link budget with specific commercial equipment and its prices.
With two WiFi devices and directional antennas, set up a 100 to 200 m link inside the school and measure the received level. Then place a metal surface at different points —near one end and in the middle— and note the drop. The attenuation is much greater when the obstacle is in the middle: that is where the Fresnel zone is widest.
With the link established, misalign one antenna one degree at a time and note the drop in level. Plot level against angle: you obtain the antenna's radiation pattern and see how critical aiming is for high-gain antennas.
Record the received level of the link over several days, including a rainy day. Compare the drop with what is expected for that band. With 5 GHz equipment the drop is small, and that observation —made with your own data— explains why that band is chosen.
07Common mistakes
| Symptom | Usual cause |
|---|---|
| There is a direct view and the link performs poorly | The Fresnel zone is obstructed even though the other end can be seen. |
| The link degrades over the years | Trees grew or a building appeared along the span. |
| It drops in the rain | Insufficient margin for the chosen band. In high bands, rain dominates. |
| It works in the morning and drops at noon | Fading due to multipath or atmospheric refraction, typical over flat terrain or water. |
| Received level far below the calculated one | Coarse aiming, crossed polarization, or actual cable loss greater than assumed. |
| The antenna was aimed at a side lobe | When the antenna is moved, several maxima appear: look for the highest, not the first one that shows up. |
| Interference in an unlicensed band | Another user on the same channel. The spectrum has to be surveyed before choosing the channel. |
08Self-assessment
Why is a line of sight not enough?
Because the wave occupies a volume, the Fresnel zone. An obstacle inside it attenuates the signal even if it does not block the direct view. The practical rule is to clear 60 % of the first zone.
An 8 km link at 5 GHz with an obstacle in the middle: what is the radius of the first zone?
r = 17.32 × √(4 × 4/(5 × 8)) = 17.32 × √0.4 = 10.95 m. At least 6.6 m must be cleared.
Where is the Fresnel zone widest?
In the middle of the span. That is why an obstacle at the center is much more of a nuisance than one near either antenna.
What margin is considered healthy on a service link?
On the order of 15 to 20 dB. Below 10 dB the link drops with rain, wind or any degradation.
Calculate the free-space loss for 20 km at 5.8 GHz.
FSPL = 32.44 + 20·log(5800) + 20·log(20) = 32.44 + 75.27 + 26.02 = 133.7 dB.
Why are 23 GHz links short hops?
Because rain attenuates enormously at those frequencies: a storm can add tens of dB. It is compensated by shortening the span and increasing the margin.
What does an availability of 99.99 % mean?
That the link can be out of service for up to 53 minutes per year. Every additional “nine” requires more margin, and therefore bigger antennas or taller towers.
How is fading from ground reflection fought?
With space diversity: two antennas separated vertically, so that when one is in a null the other is not. Also by choosing heights that shift the reflection point.
When aiming, several signal maxima appear. Which one is the correct one?
The absolute maximum: the others are side lobes. Aiming at a side lobe gives a link that works but with many dB less, and drops with any degradation.
What must be documented when the installation is finished?
Azimuth and elevation of each antenna, received level, SWR, configured channel and power, photos of the alignment and of the connector sealing. Without that, the next intervention starts from scratch.