RF Line of Sight (LOS): From Fresnel Physics to Field-Proven Deployment

Introduction

If your wireless link must “just work,” Line of Sight (LOS) is the first lever you can pull. Clear LOS improves throughput, lowers latency, and stabilizes availability. In this guide, I focus on RF LOS—not optical sightlines—and give you a practical path from physics to planning, installation, acceptance, and compliance. You’ll also find selection tables, quick checks, and internal links to ready-to-ship assemblies so you can move from theory to purchase and pilot fast.

Want the formal definition of LOS propagation? Start here and then come back for the engineering “how”: Wikipedia: Line-of-Sight Propagation.

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1) What RF LOS Really Means (and why NLOS often disappoints)

At RF, LOS means the energy leaving one antenna reaches the other without a blocking obstacle in the main path and with sufficient Fresnel-zone clearance. In practice:

• Clear path: No buildings, trees, ridgelines, cranes, roofs, or masts cutting through the direct ray.
• Fresnel clearance: Aim for ≥60% of the first Fresnel zone to be obstruction-free (engineers often target ≥80% for high availability).
• Stable medium: Over water or hot roofs, multipath and thermal ducts can distort the signal despite geometric LOS; you’ll budget extra margin.

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For a friendly primer on transmission lines and practical RF propagation, a good companion read is this tutorial on real-life RF signals from All About Circuits: https://www.allaboutcircuits.com/textbook/radio-frequency-analysis-design/real-life-rf-signals/what-is-a-transmission-line/.

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2) The Physics You Actually Need (FSPL, Fresnel, Horizon)

2.1 Free-Space Path Loss (FSPL)

FSPL grows with frequency and distance. Doubling distance adds ~6 dB loss; doubling frequency adds ~6 dB loss. That’s why 6 GHz links demand tighter alignment and lower-loss feedlines than 900 MHz.

2.2 First Fresnel Zone

The first Fresnel radius is largest near mid-path. If obstacles encroach too far, diffraction and destructive interference raise your fade rate. Clearing 60–80% of the zone dramatically reduces deep fades.

2.3 Radio Horizon & Earth Curvature

Over long spans, Earth curvature hides the target even if maps look flat. Use a K-factor (standard atmosphere ~4/3 Earth) or terrain-profile tools to decide whether to raise antennas or insert a relay.

For rigorous, globally recognized methods, see ITU-R guidance (e.g., diffraction and path modeling families): https://www.itu.int/en/ITU-R/. These recommendations underpin many national engineering rules.

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3) Quick Intent Check (answer these before you design)

Interactive questions (Yes/No):

1. Can you draw a straight line between sites without any roofline, ridge, or tree intersecting it?
2. Is the first Fresnel zone ≥60% clear at every point along the path?
3. Will the path cross water, hot rooftops, or valleys that can trigger multipath/ducting?
4. Do you know your regulatory EIRP and any DFS/coordination constraints for your band?
5. Can your feedline (length, loss, connectors) keep the total link margin ≥15 dB under worst-case weather?

If you answered No to any item, you’ll want to raise antenna height, change band/antenna, shorten or upgrade the feedline, or add a relay. (You can send us your band, coordinates, target capacity, and tower limits—we’ll return a quick link budget and BOM. See CTA.)

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4) Planning Toolkit: from paper to predictable links

4.1 What to calculate

• FSPL at your band(s) and distance(s)
• Fresnel radius & clearance (entire path)
• Radio horizon / Required height with terrain and K-factor
• Rain fade / multipath risk (especially >10 GHz or over water)
• Link margin with realistic cable/connector losses

4.2 What to document

• Antenna heights, azimuths, and tilt
• Feedline type/length and terminations
• Grounding and waterproofing method
• Photos at both ends (before/after)
• Acceptance test snapshots (RSSI, SNR, MCS/throughput, latency, jitter)

For LOS propagation fundamentals (helpful when explaining to stakeholders), see again: Line-of-Sight Propagation on Wikipedia.

For national methodology on exposure and site evaluation (U.S.), consult FCC OET Bulletin 65: https://www.fcc.gov/general/oet-bulletins-line. Internationally, the ICNIRP 2020 guidelines provide widely adopted exposure limits: https://www.icnirp.org/en/faq/rf-faq.html. For EU device coexistence and channel-access behaviors (2.4 GHz reference), see ETSI EN 300 328: https://www.etsi.org/standards.

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5) Antenna Choice: map your goal to a beam

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  5.1 What really changes with antenna type

• Parabolic dish: Highest gain, narrowest beam, best for long spans and noisy environments. Requires precise alignment; wind loading matters.
• Panel: Medium-high gain, moderate beamwidth; fast alignment; strong PTP/PTMP for short-mid distances.
• Yagi/LPDA: High directivity at VHF/UHF/900 MHz; excellent when penetration/near-LOS and modest bandwidth are required.

5.2 Quick selection matrix (distance vs band vs antenna)

Target Distance	Band (typical)	Recommended Antenna	Notes
≤5 km	2.4/5/6 GHz	Panel or small parabolic	Fast install, urban rooftops
5–20 km	5/6 GHz	Parabolic (med. dish)	Narrow beam cuts interference
10–50 km	6–13 GHz	Parabolic (large dish)	Watch rain fade & alignment
5–20 km	700–960 MHz	Yagi/LPDA	Better diffraction, lower capacity

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6) Feedlines & Connectors: the silent killers of link margin

Even perfect antennas won’t save a link if the feedline eats your budget. Keep high-frequency, long-distance links on low-loss coax and weather-sealed connectors.

6.1 Attenuation by frequency (indicative; verify datasheets)

Cable	100 MHz	1 GHz	2 GHz	5 GHz	Bending/Notes
RG58	~0.20 dB/m	~0.64 dB/m	~0.93 dB/m	~1.46 dB/m	Flexible; short runs
RG316	~0.40 dB/m	~1.20 dB/m	~1.80 dB/m	~3.00 dB/m	High-temp pigtails
LMR-200	~0.11 dB/m	~0.35 dB/m	~0.51 dB/m	~0.83 dB/m	Small O.D.
LMR-400	~0.07 dB/m	~0.22 dB/m	~0.32 dB/m	~0.52 dB/m	Outdoor workhorse

Recommended internal options you can buy now:

• Low-loss outdoor jumper (ready to ship): LMR400 N-male ↔ SMA-RP male coaxial assembly
• Short, flexible pigtail for inside the enclosure: SMA female bulkhead ↔ SMA male RG316 jumper

6.2 Connectors that survive outdoors

• Prefer N-type for outdoor 2–6 GHz runs (robust threads, good sealing).
• Use SMA where space is tight (inside radio enclosures/pigtails).
• Seal everything: heat-shrink boots + self-amalgamating tape + proper torque.

Internal, LOS-friendly connector choices:

• N-female bulkhead waterproof crimp for LMR400
• Waterproof N-type female ↔ N-male adapter
• For certain cable families, FME remains popular in telematics/outdoor patch-throughs: Waterproof FME male crimp for H155

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7) Outdoor reliability: design for sun, salt, and storms

What ages links: UV exposure, thermal cycling, water ingress, salt fog, wind-induced vibration, and movement at the connector/bulkhead. Use UV-stable jackets (PE), IP67/68 hardware, and strain-relief. For a relatable read on how infrastructure survives the elements (contextual, not RF-specific), see MIT’s engineering Q&A: https://engineering.mit.edu/engage/ask-an-engineer/how-do-electricity-transmission-lines-withstand-a-lifetime-of-exposure-to-the-elements/.

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  Field checklist (extract):
• Cable jacket: PE/UV-rated?
• Drip loops, strain-relief, and bend radius respected?
• Heat-shrink + self-amalgamating tape layered correctly?
• Torque recorded (per connector family)?
• Photos of each termination and weatherproof stage saved?

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8) Regional compliance you can’t ignore (EIRP, exposure, coexistence)

Your antenna, feedline, and height decisions intersect with exposure, EIRP, and coexistence rules. Plan early to avoid rework.

• ICNIRP 2020 (international exposure guidance, adopted widely): https://www.icnirp.org/en/faq/rf-faq.html
• U.S. FCC OET Bulletin 65 (site RF exposure evaluation): https://www.fcc.gov/general/oet-bulletins-line
• EU ETSI EN 300 328 (2.4 GHz device requirements incl. channel access/coexistence): https://www.etsi.org/standards
• National planners like Ofcom (UK) and ACMA (Australia) publish band and licensing guidance. Start on their official portals:
  • UK Ofcom: https://www.ofcom.org.uk/
  • AU ACMA: https://www.acma.gov.au/

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9) Installation & acceptance: make success repeatable

9.1 Alignment SOP (summary)

1. Mount antennas loosely; pre-aim per survey azimuth.
2. Coarse align while watching live RSSI/SNR.
3. Fine align (tiny azimuth/elevation nudges) to maximize SNR/throughput.
4. Tighten to spec torque; capture photos and metrics.

9.2 Acceptance thresholds (indicative)

• Link margin ≥ 15 dB under worst-case weather for PTP backhaul.
• Throughput on target MCS for sustained test window.
• Packet loss <0.1%, rtt/jitter within sla.
• Return loss of the assembled feedline consistent with spec.

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10) Selection Playbooks you can copy-paste

10.1 Feedline by run length (1–6 GHz)

Run Length	Typical Site	Cable Pick	Connector Family	Notes
≤5 m	Rooftop PTP	LMR-200/240	SMA/N	Compact, easy routes
5–30 m	Tower mid-height	LMR-400	N-type	Sweet spot for loss vs cost
>30 m	Tall tower	1/2″ coax/heliax	7/16 DIN or 4.3-10	Consider moving ODU up the mast

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11) Mini-case: over-water LOS backhaul at 5 GHz

• Symptoms: Daytime throughput dips; evening improves.
• Likely culprit: Multipath (sea surface reflection) + refractive ducts.
• Fixes: Increase height asymmetrically, tighten beam with larger dish, add fade margin, adjust polarization, consider moving off the worst-affected channel.
• Feedline: Keep mast-top runs short and low-loss (e.g., LMR-400 jumpers) with fully sealed N-type terminations.

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12) Your pre-deployment worksheet (fill before you buy)

Distance & band: __ km at __ GHz
Throughput/SLA: __ Mbps, availability target __%
Heights (A/B): __ m / __ m (room to raise? Y/N)
Terrain risks: Water / Ridge / Urban canyon / None
Fresnel clearance: ≥60% across path? Y/N
Feedline plan: Type __, length __ m, loss __ dB @ band
Regulatory: EIRP cap __ dBm; DFS/coexistence constraints? Y/N
Acceptance: Margin ≥15 dB; torque log; photos; baseline KPIs

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13) Frequently Asked Questions (FAQ)

Q1. What percentage of the first Fresnel zone must be clear?
For stable, year-round service, target ≥60% clearance. Mission-critical links often aim for ≥80%.

Q2. Why do we see poor performance over water even with geometric LOS?
Specular reflections and thermal ducts create multipath fading. Narrower beams (dishes), height tweaks, polarization changes, and extra margin mitigate it.

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14) Welcome Your Inquiry

Get a free LOS planning pack in 24 hours.
Email us your band(s), A↔B coordinates (or addresses), target throughput/availability, and any tower constraints. We’ll return:

• A link budget with FSPL, Fresnel/horizon checks, and fade margin
• A tailored antenna + feedline BOM (with buy-now SKUs)
• A one-page acceptance checklist (torque, sealing, KPIs)

Contact: sales@bafitop.com | +86-15817341810