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Build an NVIS Dipole Antenna

Q: Why is 40m NVIS unreliable at night?

During the day, the D layer of the ionosphere absorbs lower HF frequencies but does not block 40m — the signal penetrates it and reflects off the F layer. At night, the D layer disappears, but the F layer's critical frequency rises. Above the MUF for near-vertical incidence, the signal passes through rather than reflecting. This unpredictability makes 80m the more reliable nighttime NVIS band.

Near Vertical Incidence Skywave (NVIS) propagation fills the coverage gap between ground wave (up to about 30 miles) and long-distance skywave skip — providing reliable HF communication from 50 to 600 miles. It is the propagation mode that makes amateur radio the backbone of emergency communications, and it is deliberately achieved by installing the antenna at low height to produce near-vertical radiation. This guide builds an optimized NVIS dipole: deliberately low, correctly positioned over ground, with the right current choke to keep the coax from becoming part of the antenna — exactly what is needed for emergency communications and regional nets on 40m and 80m.

50–600Mile coverage range

8–15 ftOptimal antenna height

24/7Day coverage on 40m

~$35Typical build cost

Understanding NVIS propagation

What NVIS Is and Why Low Antennas Work Better

NVIS propagation works by directing RF energy nearly straight upward — at elevation angles between 70° and 90° from horizontal. At these steep angles, the signal penetrates the ionosphere at a point almost directly overhead, reflects, and returns to earth within a few hundred miles of the transmitter. The result is a roughly circular coverage area around the station with a radius of 50 to 600 miles depending on frequency and ionospheric conditions.

The counterintuitive principle of NVIS antenna design: lower is better. A dipole close to the ground radiates more energy at high elevation angles because the ground reflection reinforces the upward radiation. The optimal NVIS antenna height is:

Optimal NVIS height formula: h = 0.1λ to 0.25λ above ground For 40m (7.150 MHz), λ = 137.4 ft: 0.1λ = 13.7 ft 0.25λ = 34.4 ft Optimal NVIS height: 8–20 ft For 80m (3.750 MHz), λ = 262.3 ft: 0.1λ = 26.2 ft 0.25λ = 65.6 ft Optimal NVIS height: 15–40 ft A 40m dipole at 10 feet height is a better NVIS antenna than the same dipole at 50 feet. A 40m dipole at 50 feet is a better DX antenna.

The NVIS dipole is not a compromise or a poor antenna — it is specifically optimized for a propagation mode that no high antenna can replicate effectively. When regional coverage matters more than DX, the NVIS dipole is the right tool.

NVIS Frequency Selection — 40m vs 80m

Frequency selection is critical for NVIS operation because NVIS only works when the operating frequency is below the ionosphere's Maximum Usable Frequency (MUF) for near-vertical angles. Above the MUF, the signal passes through the ionosphere rather than reflecting back. Below the MUF, NVIS works reliably.

NVIS frequency guidance: Daytime (sunrise to sunset): 40m (7.0–7.3 MHz): reliable NVIS — the F2 layer can support vertical-angle reflection during day 80m (3.5–4.0 MHz): usually absorbed by D layer during daylight — unreliable NVIS in daytime Nighttime (sunset to sunrise): 80m: excellent NVIS — D layer absent, reliable reflection at near-vertical angles 40m: often works but can skip over nearby areas as MUF rises — 80m preferred at night NVIS "dead zone" at high noon on active solar days: Both 40m and 80m may fail simultaneously for 1–2 hours if F2 MUF drops below 7 MHz — rare in high solar cycle years, more common at solar minimum Emergency comms planning: Primary NVIS band: 40m during the day Backup NVIS band: 80m after sunset Have antennas for both — do not rely on one band

For a dual-band NVIS capability: build a 40m dipole for daytime operation and an 80m dipole for evening/night operation, or build a fan dipole with both bands from one feedpoint.

The Dead Zone — What NVIS Does Not Cover

Every NVIS antenna has a "dead zone" — a ring-shaped area at a specific distance where contacts are difficult or impossible. The dead zone occurs at distances beyond the ground-wave range (roughly 30 miles) but inside the nearest skip distance of the longer-range skywave path (roughly 200–600 miles depending on frequency and ionosphere).

At 100W on 40m NVIS during the day, contacts from 50 to 400 miles are reliable
Contacts from 5 to 50 miles may also be possible via ground wave, depending on terrain
Contacts at 400–800 miles may be in the skip zone during high solar activity
On 80m NVIS at night, the coverage extends out to 600+ miles reliably
For contacts beyond 600 miles, NVIS does not work — you need a higher antenna for conventional skywave DX
NVIS and DX are mutually exclusive from the same antenna — you must choose which mode to optimize for, or have separate antennas for each

Ground Effects on NVIS — Why Soil Matters

At NVIS heights (8–20 feet), the antenna wire is very close to the ground relative to the wavelength. The ground's electrical properties — conductivity and dielectric constant — directly affect the antenna's impedance, resonant frequency, and radiation pattern. This has important practical consequences:

Moist soil (after rain): better reflector → more efficient NVIS radiation → lower feedpoint impedance → resonance shifts slightly lower in frequency. Best performance conditions.
Dry sandy soil: poor reflector → some NVIS radiation absorbed rather than reflected → higher feedpoint impedance → resonance shifts slightly higher. Worst NVIS performance conditions.
Over water or wet clay: excellent reflector — NVIS performance approaches theoretical maximum. Ideal for emergency deployment near rivers, lakes, or moist lowlands.
Over concrete or asphalt: poor for NVIS — the hardscape prevents ground-wave currents and reduces reflection. Avoid deploying NVIS over paved areas.
Practical implication: a NVIS dipole deployed over a wet grassy field significantly outperforms the same dipole at the same height over dry rocky ground. Soil selection matters for emergency deployment.

Optimal NVIS height by band and frequency

Band	Frequency	Wavelength	Optimal height (0.1–0.25λ)	Practical target	Coverage range	Best time of day
40m	7.000–7.300 MHz	134–141 ft	13–35 ft	8–15 ft	50–400 miles	Daytime; marginal at night
60m*	5.330–5.405 MHz	181–185 ft	18–46 ft	12–20 ft	100–500 miles	Day and evening; excellent NVIS
80m	3.500–4.000 MHz	246–280 ft	25–70 ft	15–30 ft	100–600 miles	Evening and night
160m	1.800–2.000 MHz	492–543 ft	49–136 ft	30–60 ft	200–800 miles	Night only; limited NVIS utility

Interactive calculator — NVIS dipole

Nvis Dipole Calculator

This design has published dimensions for more than one band. The default shown below is the first/most common one on the page -- change the frequency and recalculate for the other bands.

Design frequency (MHz)

*60m (5 MHz) is the acknowledged best NVIS frequency band — it works both day and night and covers the most useful emergency communications range. US operators have limited 60m privileges (specific channels only). FEMA and ARRL emergency plans use 60m as the primary NVIS band when available.

Wire lengths for NVIS dipoles

40m NVIS Dipole — Daytime Emergency Band

A 40m NVIS dipole is the standard daytime regional communications antenna for ARES, RACES, and most state emergency communications plans. Cut the dipole for the primary emergency communications frequency in your area — check with your ARES coordinator for the local standard frequency.

40m NVIS dipole lengths: Standard formula: 468 / f(MHz) Each leg = 234 / f(MHz) For 7.200 MHz (common ARES net): Total = 468 / 7.2 = 65.0 ft Each leg = 32.5 ft → cut 33.5 ft (3% long) For 7.285 MHz (some state ARES nets): Each leg = 234 / 7.285 = 32.1 ft → cut 33.1 ft For 7.050 MHz (CW/digital NVIS): Each leg = 234 / 7.05 = 33.2 ft → cut 34.2 ft Height adjustment to wire length: At 8–15 ft height, ground loading shifts resonance lower by 50–150 kHz compared to the free-space formula. Cut legs 2–3% longer than calculated and trim to resonance in situ.

80m NVIS Dipole — Evening and Night Coverage

An 80m NVIS dipole provides reliable regional coverage after sunset and is the standard nighttime emergency communications antenna. The optimal NVIS height for 80m is 15–30 feet — achievable with modest supports while still producing excellent near-vertical radiation.

80m NVIS dipole lengths: For 3.940 MHz (common ARES phone): Total = 468 / 3.94 = 118.8 ft Each leg = 59.4 ft → cut 61.2 ft For 3.985 MHz (national calling): Each leg = 234 / 3.985 = 58.7 ft → cut 60.5 ft For 3.750 MHz (mid-band): Each leg = 234 / 3.75 = 62.4 ft → cut 64.3 ft For 3.500 MHz (CW/digital NVIS): Each leg = 234 / 3.50 = 66.9 ft → cut 68.9 ft Ground loading note: At 15–20 ft height, ground shifts 80m resonance lower by 80–200 kHz — more than on 40m. Expect significant trimming from initial cut length.

Materials list — dual-band 40m and 80m NVIS system

Complete materials for a deployable 40m and 80m NVIS dipole system

📏#14 AWG stranded CCS wire, 75 ftFor the 40m NVIS dipole — two legs at 33.5 ft each

📏#14 AWG stranded CCS wire, 135 ftFor the 80m NVIS dipole — two legs at 61.2 ft each

🔩Dipole center feedpoint insulators, 2 piecesOne for each band — commercial SO-239 type

🔘FT-240-31 toroid cores, 2 piecesOne current choke per antenna — essential at NVIS heights

🔌RG-8X coax, 50–100 ftFrom feedpoint to operating position — length to suit deployment

🪝Egg insulators, 4 pieces (2 per antenna)For the wire ends — keeps ends clear of supports

🪢UV-resistant paracord, 100 ftFor center and end supports — paracord for deployment portability

🏕️Telescoping mast or military surplus fiberglass mast sectionsFor the center support — target 10–15 ft for 40m, 20 ft for 80m

📡NanoVNAFor resonance verification in the deployed position — essential for NVIS

🛠️Self-amalgamating tape, 1 rollFeedpoint weatherproofing

🪛Soldering iron (field-deployable) and rosin core solderFor emergency field assembly and connection repair

🎒Carry bag for complete systemKeep all components together for rapid deployment

Finished NVIS dipole antenna deployed at low height on a short center mast with legs sloping down to ground-level anchors

Complete build — step by step

Building the NVIS Dipole System

This guide builds both a 40m and an 80m NVIS dipole — the complete dual-band system for emergency communications coverage day and night. Build both antennas at a home location, tune them at the correct NVIS height, and store them ready for rapid deployment.

Identify the Target Frequencies

Before cutting any wire, confirm the exact frequencies you are building for. Contact your local ARES Emergency Coordinator (EC) to obtain the current primary and alternate frequencies for your county or section. These change periodically and the correct frequency depends on your geographic area and state ARES plan.

Common US emergency communications frequency assignments:

40m: 7.200–7.285 MHz depending on region and band plan
80m: 3.940–3.985 MHz for phone; 3.500–3.525 MHz for CW/digital
60m (if licensed): 5.330, 5.346, 5.357, 5.371, or 5.403 MHz (US channel plan)

Tip: Build for the center of the expected operating frequency range, not a specific single frequency. An NVIS dipole tuned to 7.200 MHz covers 7.150–7.250 MHz at SWR below 2:1, giving useful bandwidth across the entire ARES phone section. If the net operates anywhere in that range, the dipole works without retuning.

Cut Both Wire Sets

Cut the 40m wire set: two legs at 33.5 feet each (for 7.200 MHz target). Cut the 80m wire set: two legs at 61.2 feet each (for 3.940 MHz target). Label both legs of each pair with colored tape at the feedpoint ends — use different colors for 40m and 80m wire sets so they cannot be confused during field deployment.

Install egg insulators at the far end of each wire leg using the wrap-and-solder method. Attach 18 inches of paracord to each insulator for end support. Label the paracord ends with the same color code as the wire legs they are attached to.

Tip: For an emergency communications kit, cut the wires slightly shorter than the calculated length and include a pair of wire extension sections in the kit bag. It is much easier to add wire in the field than to trim it, and having extension pieces allows one set of dipoles to serve multiple frequencies by adjusting length.

Build the Current Chokes

Wind 8 turns of RG-8X coax through each FT-240-31 toroid. The current choke is even more critical for NVIS than for a high dipole because the low-height NVIS antenna is in close proximity to the coax feedline — without a choke, the coax outer shield becomes a highly coupled radiating element that destroys the NVIS radiation pattern and introduces common-mode noise.

A poorly choked NVIS antenna often shows SWR that changes dramatically when the operator moves or when the coax is repositioned — a clear sign that the coax is radiating. In an emergency communications scenario where RF exposure near people and equipment is unavoidable, common-mode current on the coax also means RF exposure at the operating position. The current choke eliminates this.

NVIS and common-mode current: At 10-foot antenna height, the coax feedline runs very close to the ground — sometimes directly on the ground. Coax lying on or near wet ground with common-mode current flowing will have significant RF leakage that can interfere with nearby electronics, cause RF in the shack, and distort the NVIS radiation pattern. Always use a current choke on any NVIS antenna.

Assemble the Feedpoints

Assemble both feedpoints using the standard dipole center method: strip and loop wire ends, connect to the SO-239 dipole center screw terminals, apply No-Ox-Id, and tighten. Connect the current choke's coax to the SO-239. Weatherproof each feedpoint assembly with self-amalgamating tape.

Label each complete feedpoint assembly clearly: "40m NVIS" and "80m NVIS" with the target frequency and wire leg lengths written on the weatherproofing tape or a zip-tied label. In a field deployment under stress, clear labeling prevents the common error of connecting the wrong antenna to the wrong feedline.

Set Up the Initial NVIS Installation for Tuning

This step is the key difference between a standard dipole build and an NVIS dipole build: you must tune the antenna at the NVIS height, not at a standard height. Choose a flat, open grassy area at your home location. Set up a center mast at exactly the height you plan to use for deployment:

40m NVIS: center support at 10 feet height (inverted-V configuration) with legs sloping down to anchors at 2–3 feet height at their far ends
80m NVIS: center support at 18–20 feet height with legs sloping to 3–5 feet at ends

Run both wire legs horizontally or as a flat inverted-V from the center support. For the flattest possible NVIS pattern (maximum overhead radiation), keep both wire legs as level as possible — a nearly flat dipole at low height is the most effective NVIS configuration.

Why flat is better than inverted-V for NVIS: An inverted-V configuration concentrates the feedpoint at the highest point but drops the wire ends lower. At NVIS heights, this means the wire ends may be too close to the ground and the average wire height is lower than for a flat dipole at the same center height. If possible, use two separate low supports for a flat NVIS dipole rather than a single higher center mast with an inverted-V.

Measure Resonance at NVIS Height

Connect the NanoVNA at the operating-position end of the coax. Sweep the target band. Because the antenna is close to the ground, expect the resonant frequency to be 100–250 kHz lower than the free-space formula predicts:

Expected ground loading shift at NVIS heights: 40m dipole at 10 ft height: Formula predicts resonance at: 7.200 MHz Actual resonance likely near: 7.000–7.100 MHz Shift: -100 to -200 kHz 80m dipole at 18 ft height: Formula predicts resonance at: 3.940 MHz Actual resonance likely near: 3.750–3.850 MHz Shift: -100 to -200 kHz This shift is NOT a problem — it means the wire is longer than needed, and trimming will bring it to the target. Do NOT trim without first measuring at NVIS height.

Record the actual resonant frequency and minimum SWR before any trimming. This baseline measurement is essential for calculating the correct trim amount.

Trim Both Dipoles to Target Frequency at NVIS Height

Trim each dipole to resonance at its target emergency communications frequency while maintaining the NVIS installation height. Trim both legs equally in small increments, re-measuring after each trim. At NVIS heights the trim rate is similar to a standard dipole:

NVIS trim rates (at low installation height): 40m at 10 ft height: 1 inch per leg = ~12–15 kHz shift (similar to high-dipole trim rate) 80m at 18 ft height: 2 inches per leg = ~8–10 kHz shift (similar to standard 80m trim rate) Key difference from standard tuning: After trimming, verify the SWR minimum is truly at the target frequency and below 1.5:1. Ground loading makes the SWR curve slightly asymmetric — the dip is less symmetric than at high installation heights.

Seasonal re-tuning: An NVIS dipole's resonant frequency shifts more with season than a standard high dipole. As soil moisture changes from summer to winter, the resonant frequency can move 100–200 kHz. After initial tuning, mark the resonant frequency on the feedpoint label and re-verify at the start of each season. Carry the NanoVNA with the emergency kit for field re-verification.

Verify the NVIS Radiation Pattern — Practical Check

After tuning, perform a practical NVIS effectiveness check. Transmit a brief signal on 40m (daytime) or 80m (evening) and check the Reverse Beacon Network or call on a local repeater-linked HF net to get signal reports from stations in the NVIS coverage zone (50–400 miles). Compare these reports to signal reports from the same location using a high antenna:

Regional stations (50–300 miles): the NVIS dipole should give stronger or similar signal reports compared to a high dipole — the near-vertical radiation reaches regional stations more efficiently
DX stations (1000+ miles): the NVIS dipole will be weaker than a high dipole — this is expected and confirms the antenna is working as designed
A signal that is stronger to regional stations and weaker to DX stations confirms the NVIS radiation pattern is correct

If the NVIS dipole performs equally well for DX and regional contacts, the antenna is likely too high — lower the center support and re-verify.

Prepare the Emergency Deployment Kit

Pack both NVIS dipoles as a complete deployable kit. Each antenna should be wound onto its own spool or folded into its own bag. Include everything needed to deploy both antennas from scratch in an unfamiliar location:

Both feedpoint assemblies with chokes pre-attached
Both wire sets, wound and labeled
50–100 ft of coax on a reel
Center support mast sections (collapsible or sectional for transport)
Paracord (pre-cut to deployment lengths) for end supports
Ground stakes, 6 pieces (for center and end anchors)
NanoVNA for field resonance verification
Wire cutters and pliers
Spare wire for splicing or emergency extension

Tip: Time yourself deploying the 40m NVIS dipole from the packed kit to first transmission. A well-practiced deployment should take under 15 minutes. Run a "drill" deployment at home monthly to maintain the skill and check that all kit components are present and functional.

Document and Store the System

Create a deployment checklist for the NVIS kit — laminate it and attach it to the outside of the carry bag. The checklist should include: target frequencies, wire leg lengths, deployment heights, coax length, and the expected SWR at target frequency. This allows any trained operator to deploy the system correctly even without the original builder present.

Document the tuned wire lengths for both dipoles, the resonant frequency and SWR at each antenna's NVIS height, the date tuned, and the soil conditions during tuning (wet/dry). Store the kit in a location accessible during power outages and emergency activations — not in a locked storage room that requires electricity to open.

Tip: Register your emergency antenna kit with your local ARES group's equipment inventory. Many ARES sections maintain a database of equipment and antenna resources that can be called on during major activations — having your NVIS system registered means it can be deployed efficiently where needed, not just at your home QTH.

Field deployment and emergency operations

Rapid Field Deployment Technique

In a genuine emergency deployment, speed matters. A practiced NVIS deployment sequence minimizes setup time:

Site selection (2 minutes): choose flat, open ground away from metal structures, power lines, and buildings. Avoid low areas that collect water (proximity to water increases ground loading shift from expected resonance). Orient the dipole so its length runs perpendicular to the primary direction of communications — broadside to the target region for maximum NVIS effectiveness.
Center mast (3 minutes): assemble center mast sections and drive the base stake. Guy with 3 paracord lines to stakes at 120° spacing. Do not proceed to the wire until the mast is secure — a falling mast damages the antenna and wastes time.
Wire deployment (5 minutes): attach feedpoint to mast top, run each wire leg straight out from the mast, secure wire ends to stakes at appropriate height.
Coax and radio (3 minutes): run coax from feedpoint to operating position, connect radio, quick SWR check on built-in meter.
First transmission (2 minutes): announce on the net frequency, check in with net control, begin operations.
Target total time: under 15 minutes from vehicle stop to first transmission.

NVIS for Specific Emergency Scenarios

Different emergency scenarios create different NVIS antenna deployment constraints:

Shelter-in-place (home station): use the permanently installed NVIS dipole at home at the correct NVIS height — 10 feet for 40m, 18–20 feet for 80m. Do not raise the antenna to normal height thinking this improves performance — it changes the mode from NVIS to conventional skywave and may create coverage gaps for nearby county EOCs.
Deployed to EOC: deploy at the EOC site using the kit. EOC parking lots and fields are usually available. Choose a location away from the building's metal structure — metal buildings severely detune nearby antennas at NVIS heights.
Field hospital or shelter site: low-height NVIS deployment at the perimeter of the site, away from crowds. The 10-foot deployment height keeps the wire above people traffic while maintaining NVIS pattern. Rope-off the antenna perimeter for safety.
Vehicle-mounted operations: use a mag-mount center mast on the vehicle roof and run both wire legs outward as a flat dipole at vehicle roof height (typically 5–6 feet). At this height, ground loading is extreme but the antenna still provides NVIS coverage — shorter range than an optimized NVIS installation but functional.
Mountainous terrain: the ionosphere is the same regardless of terrain. However, mountains between stations can block ground wave and create unusual NVIS propagation paths. Test on-air with target stations from any specific mountain deployment site.

Dual-Band Switching for 24-Hour Coverage

A complete emergency communications station needs both 40m (daytime) and 80m (night) NVIS coverage. Options for managing the band transition:

Two separate dipoles, one active at a time: simplest approach — disconnect 40m and connect 80m as needed. Requires two coax runs or a switching arrangement at the antenna.
Fan dipole (40m + 80m): a fan dipole with 40m and 80m leg pairs from one feedpoint provides both bands from one feedpoint. Both bands can be used without switching. See the fan dipole build guide for details.
Single 80m dipole at 20 feet: an 80m dipole at 20 feet works for both bands — it is at NVIS height for 80m (0.08λ) and at low-DX height for 40m. On 40m, a tuner is needed (non-resonant) but the antenna still provides some NVIS-type regional coverage on 40m. Not optimal but a one-antenna solution for operators who can deploy only one antenna.
Transition time: switch from 40m to 80m NVIS approximately 1 hour before sunset. Local ionospheric conditions vary — monitor the operating frequency for signal quality changes and switch early if 40m signals become unstable.

Fan dipole build guide →

Measuring NVIS Coverage with Low-Cost Tools

Verifying that an NVIS antenna is actually producing near-vertical radiation — rather than just being a low dipole — can be done with inexpensive tools:

WSPR beacon: run WSPR on 40m for several hours. The WSPR map shows which stations are receiving your signal. A successful NVIS deployment shows many spots from 50–400 miles and few or no spots from over 1000 miles. A dipole that is too high shows distant spots and misses nearby stations.
Reverse Beacon Network: send a brief CW signal and check the RBN. Regional spots from 50–300 miles at good SNR confirm NVIS operation. Few distant spots confirms low DX radiation — exactly what NVIS should produce.
Local net comparison: report your signal strength on the local ARES net and compare with previous high-antenna reports. NVIS typically improves reports from nearby county net control stations while reducing DX-station signal reports.
NanoVNA impedance comparison: measure feedpoint impedance at NVIS height and compare to published values for low dipoles. A feedpoint resistance significantly different from 73Ω suggests the ground interaction is unusual for the specific site — useful diagnostic information.

Frequently asked questions

Can I use my normal HF antenna for NVIS by just lowering it?

Yes — an existing dipole or inverted-V lowered to NVIS height works as an NVIS antenna. This is actually the most practical approach for many operators: have a high-antenna position for normal DX and a low-antenna position for NVIS, and switch between them as needed. If you have an inverted-V with adjustable center height, lower the apex to 10–15 feet for 40m NVIS operation. The main caveat is that the resonant frequency shifts significantly at NVIS height due to ground loading — always verify SWR with the NanoVNA when lowering an antenna for NVIS use, and retune if necessary.

Why is 40m NVIS unreliable at night?

During the day, the D layer of the ionosphere (which absorbs lower HF frequencies) is present but does not block 40m — the 40m signal penetrates it and reflects off the F layer. At night, the D layer disappears, but the F layer's critical frequency rises. Above the MUF for near-vertical incidence, the signal passes through rather than reflecting — the coverage skip distance increases dramatically. On some nights, 40m NVIS works fine; on others, 40m signals from 100 miles away skip completely over nearby stations. This unpredictability makes 80m the more reliable nighttime NVIS band — 80m's lower frequency almost always reflects at near-vertical angles when the D layer is absent at night.

Does the NVIS dipole orientation matter?

For pure NVIS operation, wire orientation matters somewhat but less than for DX. A horizontally polarized dipole at NVIS height produces a radiation pattern that is strongest broadside to the wire and somewhat weaker off the ends — the same figure-8 pattern as any dipole, but at near-vertical angles. For regional emergency communications where signals arrive from all directions, orient the dipole so the wire runs in the direction that minimizes the null — perpendicular to the primary direction of communications. If communicating primarily to the north, orient the wire east-west so the broadside (strongest) radiation faces north and south.

What is the maximum reliable range of an NVIS antenna?

The practical maximum reliable NVIS range is approximately 400 miles on 40m during the day and 600 miles on 80m at night. Beyond these ranges, the signal's path through the ionosphere transitions from near-vertical reflection to longer-skip-angle propagation — the NVIS mode ends and conventional skywave skip begins. At ranges beyond the NVIS maximum, contacts may be possible when propagation conditions are ideal but are not reliable for emergency communications planning. State emergency communications plans typically target NVIS for intrastate communication (100–400 miles) and use different propagation modes for coordination with more distant resources.

Can I do NVIS with a vertical antenna?

Technically yes, but poorly. Vertical antennas by definition radiate primarily toward the horizon (low elevation angles) — the opposite of what NVIS requires. A short vertical loaded to resonance at very low height has a somewhat elevated radiation angle, but it never approaches the near-vertical pattern of a low horizontal dipole. If a vertical is the only antenna available, it will provide some NVIS-type regional communication but much less effectively than a low dipole. For dedicated emergency communications planning, a low horizontal dipole is strongly preferred over any vertical antenna for NVIS operation.

How does 60m compare to 40m and 80m for NVIS?

The 60m band (5.3–5.4 MHz) is widely considered the optimal NVIS frequency — it works reliably both day and night, provides consistent 100–600 mile coverage, and is less affected by D-layer absorption than 80m during daytime while being more reliable than 40m at night. Many emergency communications coordinators and FEMA resources specifically designate 60m as the primary NVIS band because of this around-the-clock reliability. US operators have limited 60m privileges (specific channel frequencies, maximum power 100W PEP on most channels) but within those constraints, a 60m NVIS dipole cut for one of the 5 US 60m channels is an excellent emergency communications antenna for any operator who has access to a 60m-capable radio.