3.2 Line of Sight, Fresnel Zone, Polarization and MIMO

Physical line of sight is not RF line of sight

Cedar Harbor's roof-to-garage hop has a clear view. A technician on the roof can see the garage dish. The first week of rain, throughput still collapses. A mature oak sits beside the visual line, not on it. That tree is inside the first Fresnel zone.

Physical line of sight (LOS) means an optical path: nothing fully blocks the straight line between the two antennas. RF line of sight means the three-dimensional ellipsoid around that line—the Fresnel zone—is clear enough that the wave does not lose large energy to diffraction and extra multipath. Eyesight can be perfect while RF LOS is not.

The first Fresnel zone

Radio energy does not travel as a zero-width thread. Around the visual line sits a series of nested ellipsoids called Fresnel zones. The first zone is the most important for outdoor WLAN and PTP planning. Energy that grazes objects in that ellipsoid bends and can arrive out of phase with the direct path, cancelling wanted power at the receiver.

A compact midpoint formula for the first-zone radius in meters is:

r ≈ 8.656 × √(D / f)

where D is path length in kilometers and f is frequency in GHz. At a point that is not the midpoint, the more general form is r = √(λ × d1 × d2 / D), with λ the wavelength and d1, d2 the distances from each end.

Cedar Harbor's 180 meter (0.18 km) hop at 5.8 GHz:

• r ≈ 8.656 × √(0.18 / 5.8) ≈ 8.656 × √0.0310 ≈ 1.5 meters at the midpoint
• 60 percent of 1.5 m is about 0.9 m

If the oak canopy sits 0.7 m off the visual line at mid-span, it is still inside the first zone. The technicians who looked at the path missed the ellipsoid.

Outdoor planning rule of thumb: keep about 60 percent or more of the first Fresnel zone clear of trees, rooftops, earth bulge, and rooftop clutter. One hundred percent clearance is nicer and is sometimes used on long microwave hops; CWNA-depth WLAN teaching uses the 60 percent figure as the typical outdoor target, not a claim that the 40th Fresnel zone must be empty. Indoor short links still have Fresnel geometry, but walls, people, and clutter dominate, so you rarely draw the ellipsoid on a hallway drawing.

Earth bulge matters on multi-kilometer paths. Cedar Harbor's 180 m hop can ignore bulge. A 12 km rural hop cannot: the earth curves into the zone, so antenna height is part of RF LOS, not just a mounting convenience.

Beamwidth

Beamwidth is the angular width of the main lobe, usually the half-power beamwidth (HPBW): the angle between the points where power is 3 dB down from the peak. Antennas have an azimuth beamwidth (around the horizon) and an elevation beamwidth (up-down). A high-gain dish might be 8 degrees by 8 degrees. A hallway patch might be 70 degrees by 50 degrees. A vertical omni is about 360 degrees in azimuth and a much narrower elevation doughnut.

Narrower beams:

• put more energy on the intended receiver (passive gain),
• collect less energy from interferers off the bore sight,
• are harder to aim,
• leave nulls beside the beam where clients disappear.

Cedar Harbor should not hang a 4-degree dish in a lobby. Clients walking 10 degrees off bore sight fall out of the main lobe even though they are close in meters.

Passive gain focuses energy

Passive gain is not an amplifier. The antenna has no extra DC watts that magically appear as RF. A director/reflector structure or a larger aperture takes the same radiated power and concentrates it into a smaller solid angle. Compared with an isotropic radiator, that concentration is the dBi number in the EIRP formula.

Consequences you will be tested on:

• Higher dBi usually means a narrower beam. You trade coverage angle for distance along the bore sight.
• Passive gain helps both transmit and receive on that same pattern (reciprocity of a passive antenna).
• A 10 dBi omni is still an omni in azimuth; the doughnut gets flatter, so floors above and below get less energy. That is not a bug in the catalog. It is the pattern.
• You still subtract cable loss. A 13 dBi patch with 8 dB of jumper loss can radiate worse than a 6 dBi patch on a 1 dB pigtail.

Do not call a passive antenna an amplifier. Amplifiers add active gain and also add noise figure; they belong in a different troubleshooting story.

Polarization

An electromagnetic wave has an electric-field orientation. WLAN antennas are commonly linearly polarized: vertical or horizontal. Some links use circular polarization (the field rotates). For two linearly polarized antennas, matching polarization transfers power well. A 90-degree mismatch (vertical talking to horizontal) produces cross-polarization loss. In textbooks the isolation can look enormous; in the field you still see roughly 20 dB or more of extra loss on a badly mismatched pair, which can kill a marginal hop even when the path looks short.

Practical rules:

• Mount both ends of a PTP hop for the same linear polarization unless the design intentionally uses dual-pol MIMO (two polarizations as two chains).
• A ceiling omni that is side-mounted on a wall may rotate its polarization relative to client laptops.
• Rain, reflections, and people tilt polarization slightly; dual-pol access points reduce that fade. That is polarization diversity, taught below—not a reason to ignore matching on a single-pol dish pair.

Cedar Harbor's first garage install used a vertically polarized patch on the roof and a horizontally polarized panel on the garage because the mount holes were easier. RSSI looked mysteriously weak. Rotating one antenna 90 degrees recovered on the order of 20 dB. That was not a new radio. It was polarization.

Antenna diversity types

Diversity means the receiver (and sometimes the transmitter) has more than one look at the fading channel so a single fade is less likely to erase the frame. CWNA-depth types:

Diversity is not the same as spatial multiplexing. Diversity spends extra antennas on reliability. Spatial multiplexing spends extra chains on different data when the channel can support it. A two-antenna AP in a 2005-era switched-diversity mode is not doing 2-stream multiplexing.

Radio chains

A radio chain is one complete analog path: transmit and/or receive electronics plus the antenna (or antenna port) it drives. Marketing shorthand such as 3×3:3 means 3 transmit chains, 3 receive chains, and up to 3 spatial streams. A 4×4:3 radio has four chains but the MAC/PHY may use at most three streams of unique data. Chains cost silicon, power, and antennas. Streams need a channel that can separate those paths.

If a chain's pigtail is damaged, that stream's SNR collapses. A survey that only plots one RSSI number can hide a broken third chain.

MIMO at CWNA depth: three jobs, not one buzzword

Multiple-input multiple-output (MIMO) uses multiple radio chains on each side of the link. For CWNA-109, keep three techniques separate. Do not dump OFDMA (Wi-Fi 6 resource units) into this antenna section; that is a PHY-access topic in a later domain.

Spatial multiplexing sends different bits on different streams at the same time. Throughput can scale with the number of streams when SNR is high enough and the paths are independent (indoor multipath actually helps here). If the channel is a single clear outdoor path with little scattering, extra streams may not materialize; the radio falls back to fewer streams. Multiplexing is a rate play.

Transmit beamforming sends the same information from multiple chains with coordinated phases so the waves add constructively at the intended receiver. The far end sees a stronger signal (better SNR), not automatically extra unique data streams. Beamforming is a link-budget play. It still uses multiple chains; it is not a passive dish.

Space-time block coding (STBC) sends coded copies of the same information across antennas and time slots so a fade on one path is less likely to erase the block. STBC is a robustness play. It does not, by itself, multiply unique payload streams the way multiplexing does.

Cedar Harbor indoor clinic APs use 3×3:3 indoor MIMO: multiplexing when laptops sit in a cluttered ward (rich scattering), beamforming to lift a weak tablet behind a crash cart, STBC when a single stream must just get through. The garage PTP hop uses dual-pol chains mainly as two polarizations on a directed path. Treating that hop as if it needed a 4-stream multiplex the way a packed lobby does is the wrong mental model.

Aiming and pattern traps

1. Calling eyesight RF LOS.
2. Ignoring a tree beside the visual line that still sits in the first Fresnel zone.
3. Expecting a high-gain antenna to widen coverage.
4. Treating passive gain as a PoE amplifier.
5. Crossing linear polarization on a single-pol PTP pair.
6. Calling switched diversity spatial multiplexing.
7. Assuming 4×4 silicon always delivers four unique data streams outdoors.
8. Dragging OFDMA resource units into an antenna-characteristics item.