Outdoor Wireless Links: Point-to-Point, Fresnel Zones, and DFS

An outdoor wireless link replaces a cable with radio: two (or more) fixed antennas exchange traffic across open air, bridging networks between buildings, towers, or sites where trenching fiber is impractical. A point-to-point link connects exactly two fixed locations over a single dedicated radio path; a point-to-multipoint design connects one central radio to many remote sites. Making these links reliable requires more than visual line of sight — the Fresnel zone around the beam must be clear, connectors and feedlines must be sound, and on 5 GHz channels the radios must obey DFS (dynamic frequency selection) rules that force channel changes when radar appears.

Point-to-point versus point-to-multipoint

A point-to-point (PtP) link is the wireless equivalent of a single cable: one radio and directional antenna at each of two fixed locations, aimed at each other, with no intermediate devices relaying traffic. When the task is “connect the wired networks of these two buildings with one dedicated link,” PtP is the design that fits — it dedicates the full capacity of the radio channel to that one path. Both ends typically use high-gain directional antennas (parabolic dishes or grids for long hops; see Wi-Fi antenna types for the hardware).

A point-to-multipoint (PtMP) topology puts a wide-beam sector antenna at a central site — a tower or rooftop — serving many subscriber radios spread across its coverage arc. This is the standard architecture for a WISP (wireless Internet service provider) delivering broadband to scattered homes and businesses: airtime on the sector is shared among all subscribers, trading per-site capacity for economical reach.

Neither of these is a mesh. A wireless mesh forwards traffic hop-by-hop through intermediate nodes; PtP and PtMP are single-hop designs between fixed endpoints. If a scenario explicitly rules out intermediate relaying devices, it is ruling out mesh and pointing at a bridge topology.

Characteristic	Point-to-point	Point-to-multipoint
Endpoints	Exactly two fixed sites	One central site, many remotes
Antennas	High-gain directional at both ends	Sector at the hub, directional at each remote
Capacity	Full channel dedicated to one path	Shared airtime across all subscribers
Typical use	Building-to-building bridge, tower backhaul	WISP last-mile access, campus distribution

The Fresnel zone: why “I can see the other tower” isn’t enough

Radio waves do not travel as an infinitely thin ray. The usable signal occupies a football-shaped (ellipsoidal) volume around the straight line between the antennas called the Fresnel zone, fattest at the midpoint of the path. Any object intruding into that volume — even without touching the visual sight line — diffracts and attenuates the signal.

The practical rule: keep at least the inner 60 percent of the first Fresnel zone free of obstructions. The zone’s radius grows with path length and shrinks with frequency, so long links need surprisingly generous clearance at mid-path — for a multi-kilometer 5 GHz hop, several meters above and around the sight line.

This produces one of the most instructive failure patterns in outdoor wireless. A PtP microwave link is commissioned with clear line of sight and runs at rated speed for years — then throughput sags and the link starts dropping, and a fresh site survey finds a stand of trees near the midpoint has grown just tall enough to poke into the beam path without blocking the view. That is Fresnel zone encroachment: partial obstruction of the ellipsoid degrades the link long before anything visibly blocks it. Foliage is a repeat offender because it grows, sways, and attenuates far worse when wet or in leaf. The fixes are raising the antennas, relocating an endpoint, or removing the obstruction.

DFS: sharing 5 GHz with radar

Large portions of the 5 GHz band are shared with primary users — chiefly weather radar and airport radar systems. Regulators (the FCC in the United States, with equivalents elsewhere) permit Wi-Fi and fixed wireless on those channels only if radios implement DFS (dynamic frequency selection): the radio must continuously monitor for radar signatures, and on detecting one it must vacate the channel — typically within seconds — and stay off it for a non-occupancy period, commonly 30 minutes. Before using a DFS channel at all, the radio performs a channel-availability check, listening for about a minute before transmitting.

For an outdoor operator, DFS behavior looks like this from the logs: the radio abruptly switches channels on its own, the link drops or degrades during the move and re-check, and service resumes on a new frequency. A recurring real-world pattern: a 5 GHz PtMP link that drops for roughly ten minutes at a time, more often after thunderstorms, with logs showing automatic channel changes just before each outage — that is DFS reacting to weather-radar activity, which intensifies when storms are being tracked. It is not equipment failure and not rain physically absorbing the signal; the giveaway is the logged channel switch preceding each drop.

Mitigations include selecting non-DFS channels where legal and clean, or accepting DFS channels with a planned fallback channel ready.

VSWR and reflected power: the feedline can betray you

Between the radio and the antenna sits a feedline system — coax, connectors, lightning arrestors — that must present a consistent impedance (typically 50 ohms) end to end. Any discontinuity, such as a connector that is not fully seated, a corroded joint, water in the cable, or a damaged conductor, creates an impedance mismatch. At a mismatch, part of the transmitted RF energy is reflected back toward the transmitter instead of radiating from the antenna.

The measure of this condition is VSWR (voltage standing wave ratio) — the ratio describing how much energy reflects versus how much is delivered. A perfect match is 1:1; real installations aim for roughly 1.5:1 or better. High VSWR means weaker radiated signal, and severe reflection can overheat or damage the transmitter’s output stage. Field technicians hunt these faults with a cable/antenna analyzer, checking VSWR (or its cousin, return loss) after every tower install — which is why “connector not fully seated on a new outdoor antenna” is the textbook trigger for a reflected-power problem. The related concept of feedline attenuation simply eating gain is covered in antenna gain and polarization.

How the N10-009 exam tests this

• Name the topology. Two buildings, one dedicated wireless link, no intermediate relaying devices — point-to-point. A central tower serving many scattered subscribers — point-to-multipoint. Distractors usually include mesh (relies on relaying) and ad hoc.
• Diagnose the slowly degrading link. Clear visual line of sight but poor throughput and drops, with vegetation or a new structure near mid-path just intruding into the beam — Fresnel zone obstruction. The trap is assuming line of sight equals a clean path.
• Recognize DFS behavior. Intermittent outages on 5 GHz with logs showing automatic channel changes, often correlated with storms/radar activity, or a definition-style question about the regulatory feature that forces radios to detect and vacate radar-shared channels — the answer is DFS.
• Name the reflection condition. A poorly seated connector causes an impedance mismatch and some transmit energy bounces back toward the radio; the term being tested is VSWR (or reflected power/return loss, depending on answer wording).

Fresnel zones, DFS, and VSWR are niche enough that most candidates first meet them in practice questions — better there than on exam day.

Quick reference

• Point-to-point = two fixed endpoints, one dedicated link, directional antennas both ends, no relays.
• Point-to-multipoint = one sector hub serving many remote radios; classic WISP access architecture.
• The Fresnel zone is an ellipsoid around the sight line; keep at least ~60% of the first zone clear.
• Growing trees at mid-path are the classic cause of a good link going bad — obstruction without blocking visual line of sight.
• DFS is mandatory on radar-shared 5 GHz channels: detect radar, vacate the channel, wait out a non-occupancy period (~30 min).
• Logged automatic channel switches right before dropouts point to DFS events, not hardware failure.
• Impedance mismatch (bad/loose connector) reflects RF energy back toward the transmitter; VSWR quantifies it.
• Target VSWR near 1:1 (≤ ~1.5:1); verify with a cable/antenna analyzer after every outdoor install.