EFHW Antenna Deep Research Report

01 / Fundamentals

EFHW Fundamentals & Impedance Characteristics

The EFHW (End-Fed Half-Wave) antenna is essentially a half-wave dipole fed at one end. Compared to center-fed dipoles, its core advantages are simple deployment — only one support point needed, feed point near the radio, and harmonic-based multi-band coverage.

// Impedance Characteristics

▸The end of a half-wave antenna presents extremely high impedance, typically 2000-3000 Ω (far above 50 Ω)
▸Requires an impedance transformer: 49:1 (7:1 turns ratio, e.g. 3:21) or 64:1 (8:1 turns ratio)
▸Actual impedance is affected by deployment height, environmental dielectric constant, ground conductivity, wire diameter, and other factors
▸Current distribution: maximum at the center of the half-wave wire, zero at both ends — identical to dipole physics
▸Feed point is a high-voltage, low-current node — 100W @ 2500Ω = 500V RMS / 700V peak

Key Physics Fact

The EFHW and dipole radiation patterns are nearly identical — both share the same current distribution. The EFHW is not a "magic antenna"; the only difference is the feed method and the resulting impedance matching challenge.

EFHW vs Dipole Comparison

Characteristic	EFHW (End-Fed Half-Wave)	Center-Fed Dipole
Feed Impedance	~2500 Ω, Needs 49:1 transformer	~50-75 Ω, Direct coax
Support Needed	Single support point	Center support + symmetric ends
Coax Length	Very short (feed near radio)	Usually longer
Multi-Band	Harmonic-based, no tuner	Needs tuner or multiple antennas
Transformer Loss	0.3-3dB (depends on config)	None
RFI Risk	CMC suppression needed	Naturally balanced, lower

02 / Transformer Design +3:21

49:1 vs 64:1 Transformer Design & Selection

Antenna Actual Impedance ÷ 50 = Ideal Transformation Ratio.Actual impedance is ~2000-3000 Ω, and 49:1 provides a universal compromise. But identical turns ratios do not guarantee identical performance — PA3HHO's measurements reveal a critical, long-overlooked variable: more primary turns dramatically improve low-band efficiency.

Turns Ratio Showdown: 2:14 vs 3:21

Characteristic	49:1 (Traditional 2:14)	49:1 (New 3:21)	64:1 (2:16)
80m Loss (PA3HHO)	2.3 dB (41%)	0.3 dB (8%)	N/A
40m Loss (PA3HHO)	1.9 dB (36%)	0.3 dB (8%)	N/A
Efficiency Comparison	Baseline	5× better on low bands	Similar to 2:14
Best Scenario	Obsoleted by new data	Best for 80-20m	Low-band only

PA3HHO Key Finding (2025)

The 3:21 winding is 5× more efficient on low bands than 2:14. The mechanism: more turns → higher inductive reactance → lower magnetizing current → dramatically lower core loss. On 80m, 3:21 loss is only 0.3dB (8%), while 2:14 reaches 2.3dB (41%). The trade-off is tighter winding space, which requires stacked cores to accommodate the extra turns.

Ferrite Core Selection

Research by RF.Guru and F5NPV reveals: no single core can maintain high efficiency across the entire 80m-10m range.

Core Material	μ Value	Best Bands	Recommended Use
Type 43 (FT240)	850	3.5-14 MHz	Most universal EFHW
2643251002	850	3.5-14 MHz	New-gen dual-aperture: 90-93% efficiency
Type 77 (FT240)	2000	<10 MHz	80m / 40m only
Type 52 (FT240)	250	14-30 MHz	20m / 10m high bands
Type 31 (FT240)	1500	1-20 MHz	CMC choke only

RF.Guru Warning

Type 43 efficiency on 10m drops as low as 55%; Type 52 on 80m reaches only ~50%. The so-called "wideband 80-10m EFHW transformer" is more marketing promise than reality.

PA3HHO Recommendations Practical Guide

▸80/40m dedicated: 2×FT240-43, 3:21 winding (49:1), only 0.3dB (8%) loss
▸20/15/10m dedicated: 2×FT240-52, 3:21 winding, optimal high-band efficiency
▸Band-specific > all-band coverage — the most important design philosophy shift in this supplement
▸2×43 3:21 handles 400W PEP SSB; 3×52 (N4LQ design) reaches 1kW PEP

03 / Winding Methods

Winding Methods & W8JI Measurement Corrections

Traditional Twisted-Pair

First 2 turns of primary & secondary twisted together
Remaining turns coupled via core flux
Has crossover winding

Fuchs Split-Winding

Secondary wound alone on core
Primary wound alone between secondary
Needs 0.05λ counterpoise

Simple Parallel Winding (Recommended)

Primary & secondary wound in parallel
W8JI measured: better on HF
Distributed capacitance adds CM impedance

W8JI VNA Measurement Comparison Myth Debunked

Frequency	Split Winding	Parallel Winding	Winner
15.71 MHz	-33 dB	-47 dB	Parallel +14dB
37.1 MHz	-46 dB	-41 dB	Split +5dB
134.8 MHz	-19 dB	-4.6 dB	Split +14dB
157 MHz	-3 dB	-11 dB	Parallel +8dB

"Many 'improvements' we just blindly accept from reading articles... The split winding often just shifts peak performance points higher in frequency."

— W8JI (Tom Rauch)

Key Conclusion

• Split winding merely shifts peaks to higher frequencies, not a universal improvement
• On HF (<30MHz), simple parallel winding is actually better

04 / CMC In-Depth

Common-Mode Choke (CMC) In-Depth Analysis

100W+ High Power

Must Use

Without CMC, the coax shield becomes part of the antenna system → RFI into every appliance in the house

QRP (<10W)

Strongly Recommended

Problems are milder but still present — develop good habits to avoid chasing mysterious RF issues

CMC Placement

[Radio] ─── [Coax] ─── [CMC] ─── [49:1 Transformer] ─── [Antenna Wire]
                      ↑              ↑
               ≥3m from feed point      0.05λ counterpoise
  

Critical Warning

Never install CMC directly at the feed point, as it chokes the counterpoise and affects SWR. The ARRL Handbook recommends at least 10 feet (~3m) from the feed point.

Band	Ferrite	Form	Turns
80-20m	Mix 31	FT240-31	5-9 turns RG-8X
40-10m	Mix 43	FT240-43	6-10 turns RG-8X

RF.Guru VNA Warning NEW

Measuring coax shield common-mode impedance with a VNA does not prove choke effectiveness — shield current must be measured in the actual antenna installation. Bench VNA data cannot reflect real CMC operating conditions.

05 / Counterpoise

Counterpoise — Balance Ground

Common Misconception (PA9X)

"A half-wave EFHW doesn't need a counterpoise" — WRONG. No dedicated counterpoise → coax shield → radio chassis → power supply → every appliance ground → entire house radiates RF.

Length Formula

L = 0.05 × λ_max

40m→2m | 80m→4m | 160m→8m. Non-resonant length, sufficient to provide an RF return path without becoming a significant radiator.

AI6XG Minimalist Solution NEW

6ft RG316 coax = 0.05λ (40m). The coax segment between the transformer and CMC itself acts as the counterpoise — no extra wire needed.

06 / Multi-Band Mechanism

Multi-Band Harmonic Operation Mechanism

Band	Multiple	Match	Notes
40m	1× λ/2	Best	Fundamental resonance
20m	2× λ/2	Good	Even harmonic
15m	3× λ/2	Poor	Odd harmonic
10m	4× λ/2	Fair	Parasitic capacitance
30m/60m/17m/12m	-	Inoperable	Non-harmonic

WARC bands are inoperable. Chapter 12 trapped EFHW design solves the 30m problem.

// Typical SWR by band for 40m EFHW

07 / Real Performance PA3HHO Data

49:1 Transformer Real Performance & Loss Measurements

PA3HHO's systematic measurements reveal that configuration differences can cause up to 40% loss variation. This is currently the community's most comprehensive EFHW transformer efficiency comparison.

PA3HHO: Four-Configuration Measurement Comparison

Configuration	80m	40m	20m	Power
1×43 2:14	2.3dB (41%)	1.9dB (36%)	1.6dB (31%)	100W
2×43 2:14	0.8dB (17%)	0.7dB (15%)	0.6dB (13%)	200W
2×43 3:21	0.3dB (8%)	0.3dB (8%)	0.3dB (8%)	400W PEP
3×52 2:14	0.1dB (1%)	0.2dB (5%)	0.4dB (9%)	1kW PEP

The 3×52 Trade-off

3×FT240-52 achieves only 1% loss on 80m, but VSWR is elevated, requiring compensation components. Type 52 has narrow bandwidth and poor high-band matching. This is not a "perfect all-band solution."

"Low SWR does not equal high efficiency. An antenna at SWR 1.5:1 may be turning 30-50% of power into heat. A 100pF compensation capacitor improves SWR readings, not radiation efficiency."

— RF.Guru (ON6URE)

// PA3HHO Four-Configuration Loss Comparison (dB)

08 / Core Showdown NEW

Fair-Rite 2643251002 vs FT240-43

The 2643251002 dual-aperture core is a recent community highlight. SP9TKW and SurvivalComms reviews show efficiency reaching 90-93% (properly wound), 8-11% higher than single-core FT240-43.

Parameter	2643251002	FT240-43
Efficiency	90-93%	82-84% (single core)
Power	300W SSB	100W SSB (single core)
Inner Diameter	16.75mm (tight space)	35.6mm (ample space)
Turns Ratio	56:1 (7.5:1)	49:1 (7:1)
160-20m	Excellent	Fair
10m+	Steep drop-off	Fair
Efficiency Gain	+8-11%	Baseline

Core Trade-off

• 2643251002 is more efficient, but ID is only 16.75mm, making 3:21 winding nearly impossible
• FT240-43 has ample space for 3:21; stacking 2 cores brings efficiency (~92%) nearly equal
• SOTA Reflector measurements: only ±2% difference

Conclusion

The 2643251002 is an excellent core, but not a wholesale replacement for FT240-43. 2×FT240-43 3:21 efficiency essentially matches 2643251002. Choice depends on winding ability and target bands.

09 / Stacked Measurements NEW

PA3HHO Stacked Core Measurement Comparison

PA3HHO (Pleun) conducted systematic measurements of four transformer configurations. This is the community's most comprehensive EFHW transformer efficiency comparison dataset.

Finding 1: Stacking Reduces Loss

2 cores: loss cut in half. 1×43→2×43, 80m from 2.3dB→0.8dB. Doubled cross-section → halved flux density → exponentially reduced loss.

Finding 2: 3:21 Efficiency Leap

5× more efficient on low bands vs 2:14. More turns = higher inductive reactance = lower magnetizing current. The most underappreciated optimization.

Finding 3: Type 52 Triple Stack

Only 1% loss on 80m. But elevated VSWR and narrow bandwidth. N4LQ design reaches 1kW PEP.

Finding 4: Band-Specific Philosophy

43 for 80/40m, 52 for 20/15/10m. Band-specific optimization > all-band coverage.

// PA3HHO Efficiency vs Frequency (4 Configurations)

10 / Measurement Myth NEW

Back-to-Back Measurement Myth — RF.Guru Critique

RF.Guru (ON6URE) published an article in November 2025 that fundamentally exposed the flaws of "back-to-back" testing.

Popular Method (WRONG)

[Radio 50Ω] → [Transformer A] → [Transformer B] → [Dummy Load 50Ω]
"100W in, 94W out = 3W loss per transformer"
"0.13dB ≈ nearly lossless!" ← This is a placebo
  

Why It's Wrong

Back-to-Back Ideal Conditions

• Perfect 50Ω at both ends
• Symmetric structure cancels CM
• Purely resistive, zero reactance

EFHW Real-World Conditions

• Load 2-5kΩ + reactance
• Reactive current circulates as heat
• SWR 3:1-5:1
• Coax shield = third conductor

"Back-to-back testing only proves the transformer survives in easy-mode. 500W without smoke ≠ 500W efficient radiation."

— ON6URE, RF.Guru (2025-11)

Correct Evaluation Method

• Use 2.4kΩ complex-impedance load (not purely resistive)
• RF current probe to measure coax shield common-mode current
• Temperature rise test at actual power + duty cycle

11 / VNA Methodology NEW

Correct VNA Measurement Method — AI6XG 3-Step Separation

AI6XG (Dan) proposed a rigorous methodology to decompose transformer loss into three independent components. This is key to distinguishing "bad transformer" from "antenna mismatch".

// AI6XG 3-Step Separation Method

Step 1: Mismatch Loss MM

MM = -10·log(1 - |S₁₁|²)

Reflection loss from impedance mismatch. Energy is reflected back to the transmitter, not turned into heat.

Step 2: Load Loss LL

LL = |S₂₁|² / (1 - |S₁₁|²)

Loss in the load resistor itself. Must first measure load complex impedance via Series-Through!

Step 3: Transformer Loss TL

TL = S₂₁ - MM - LL

The real transformer loss. This reflects the actual performance of the core, winding, and turns ratio.

Common Mistake

Assuming load is an ideal resistor → efficiency calculated >100% on high bands (absurd). Without measuring actual load complex impedance, MM and LL cannot be correctly separated, making transformer loss calculation meaningless.

AI6XG Key Findings

• Primary capacitance does not affect transformer efficiency — only affects mismatch loss MM
• Larger core = lower transformer loss. FT240 > FT140 > FT50
• NanoVNA-F + OSLIT calibration + fixture = repeatable amateur measurements

12 / Trapped EFHW NEW

Trapped EFHW Design — Multi-Band, No Tuner Needed

AI6XG, K6ARK, and W2AEW have each developed trapped EFHW designs, solving one of EFHW's biggest pain points: no 30m WARC band coverage.

AI6XG 20m/30m/40m Trapped Design

[Transformer]──[9.4m]──[20m Trap]──[1.18m]──[30m Trap]──[2.06m]

20m: only 9.4m | 30m: 9.4+1.18m | 40m: all

Miniature Trap Parameters

Trap	Capacitor	Coil	Core	Resonant Freq
20m	15pF NP0	39T	T50-2	13.9 MHz
30m	33pF NP0	37T	T50-2	10.0 MHz

Measured SWR

1.2

20m

1.6

30m

1.2

40m

Design Comparison

Design	Band Change	30m	Efficiency	Best For
Trapped	Auto	Yes	Higher	SOTA/POTA
Linked	Manual	Optional	Highest	No rush to change bands
Pure Harmonic	Auto	None	Moderate	Simplest

If you need 30m and don't want an ATU, the trapped EFHW is currently the most elegant solution. K6ARK open-source design, total cost <$15.

13 / Dual-Mode Matching NEW

A+C Dual-Mode Matching System — A New Direction for Low + High Bands

A traditional single 49:1 ferrite transformer fails at both 80m (insufficient inductance) and 10m (capacitive-dominant, efficiency collapses to 55%). The A+C approach decomposes the problem: ferrite wideband transformer for low bands, powdered-iron resonant coupler for high bands, each playing to its strengths.

// System Architecture

Parameter	Unit A (Low Bands)	Unit C (High Bands)
Core	2×FT240-43 Stacked	T200-2 Powdered Iron (Type 2)
Material	Mn-Zn Mix 43, μ=850	Carbonyl Iron Powder, μ=10
Topology	49:1 Wideband Transformer (3T:21T)	AA5TB Parallel LC Coupler (3T:19T)
Band Coverage	80m, 40m	30m, 20m, 17m, 15m, 12m, 10m
Bandwidth	Wideband, no tuning	Narrowband (per-band variable capacitor tuning)
Efficiency Target	~92% (0.3dB)	90-95% (0.2-0.5dB)
Antenna Wire	20m (same wire, DPDT switched)
Counterpoise	4m (shared)

Why Ferrite and Powdered Iron Must Be Separate

The physical properties of ferrite Type 43 and powdered iron Type 2 determine their respective optimal frequency ranges:

Property	Ferrite (Type 43)	Powdered Iron (Type 2)	Advantage
μ (Permeability)	850	10	Low μ = low core loss
Resistivity	1-10 Ω·m	10&sup5;-10&sup6; Ω·m (Granular insulation)	T200-2: extremely low HF eddy currents
30MHz Loss	Significant	Very Low	T200-2 wins
Temp Stability	Fair	Excellent (95ppm/°C)	T200-2 wins
Saturation Flux	~0.3T	~0.8-1.0T	T200-2: extremely hard to saturate
100W Heating	Noticeable	Almost no heating	T200-2 wins
Bandwidth	Wideband	Narrowband (needs tuning)	FT240-43 wins

Key Engineering Challenges

Quantitative analysis shows the A+C approach is theoretically sound (band-specific core materials), but the current prototype has 8 engineering challenges to resolve:

#	Issue	Severity	Proposed Fix
1	Non-resonant band impedance mismatch — 20m wire at 20m/10m is integer wavelength multiples, end impedance ~50-300Ω not ~2500Ω	🔴 Fundamental	Add switchable turns ratio to Unit C
2	Variable capacitor voltage rating inadequate — LC resonant boost effect yields V_peak of 1900-6300V	🔴 Safety	Vacuum variable capacitor (3-15kV)
3	Counterpoise incompatibility — 4m at 10m = 0.38λ, becomes a radiator	🔴 Severe	Switch to 0.5m counterpoise for Unit C
4	Variable capacitor outdoor reliability — condensation/corrosion/contact oxidation, 6-24 month lifespan	🟡 Reliability	Vacuum cap + desiccant
5	DPDT high-voltage switching — 707V peak exceeds 250VAC rating by 2×	🟡 Moderate	Move switching to 50Ω side
6	Over-design — DMEGC Ni-Zn single core achieves 90%+ results at 1/5 cost	🟡 Design	Evaluate single-core approach first
7	Inter-unit parasitic coupling — idle unit absorbs ~2W	🟢 Minor	Short idle unit primary
8	T200-2 core saturation	🟢 Non-Issue	39 gauss vs 800 gauss

Comparison with Simpler Solutions

Design	80m	40m	20m	10m	Complexity	Cost	Reliability
Single FT240-43 2T:14T	Poor	Good	Good	Fair	Very Low	¥50	★★★★★
DMEGC Ni-Zn μ=1000	Good	Good	Good	Good	Low	¥80	★★★★★
A+C Dual-Mode	Good	Good	Excellent*	Excellent*	Very High	¥400-600	★★

* High-band efficiency advantage is ~0.2-0.4 dB, imperceptible in FT8 weak-signal modes. The real FT8 bottleneck is RX noise floor.

One-line summary: The A+C approach represents an important exploration direction — acknowledging the physical limits of single ferrite transformers and using band-specific multi-core designs to break through bottlenecks. The current prototype needs engineering iteration (HV switching, capacitor reliability, impedance matching range), but the direction is sound and worth continued exploration.

// Full analysis in EFHW knowledge base: efhw-knowledge/references/efhw_ac_dual_mode.md

14 / L-Network

L-Network Alternative Matching Solutions

VA3KOT compared 49:1 vs L-Network. A 49:1 autotransformer with 100pF capacitor is topologically an L-Network.

Comparison	49:1 Transformer	L-Network
Bandwidth	Wideband multi-band	Single-band
Efficiency	Has core loss	Higher (no core)
Portable	Ready to use	Needs on-site tuning

15 / Radiation Patterns

Radiation Patterns & NEC Modeling

N6CC conducted in-depth NEC modeling analysis. The EFHW is the "last resort" for 80/60/40/30m regional coverage.

• 80m sloper 45°: 4 nulls in DX elevation, -25dB rear, -9dB sides
• Higher harmonics: deeper nulls, more lobes. 50ft: 8dB front null. 20ft: 10dB front null
• Inverted L: ground/sky wave interference → hot-spot dead zones → self-induced QSB
• NVIS at 6m height: 80/40m omnidirectional ±1dB, usable for 300km regional

"High gain, wide bandwidth, small size — pick two. The EFHW in practice delivers none of the three."

— N6CC (Tim)

16 / Portable · Safety · Conclusions

PortableDeployment, SafetySafety &Key Conclusion

2026 Portable Products Updated

Product	Bands	Power	Features
Spooltenna Ultra	40/20/15/10m	100W SSB	162g open-source
AI6XG Trapped	40/30/20m	5W QRP	No-tuner SOTA
K6ARK Mini	40/20/15/10m	QRP	Transformer built into BNC connector
Chameleon LEFS	40/20/15/10m	25W SSB	113g

High-Voltage Safety

• 100W @ 2500Ω = 500V RMS / 700V peak
• Use 1kV+ rated insulation wire
• 30 MHz / 100W: safe distance ≥ 2.26m
• FT8 high duty cycle: derate power 50% or use larger cores

Quick Deployment Tips

▸Sloper: feed point 1m, far end 6-8m, 5 minutes
▸AI6XG Single Feedline: 6ft RG316 coax = 0.05λ counterpoise
▸K6ARK Ultralight: transformer built into BNC connector

Key Conclusion (Supplement Edition)

01 3:21 > 2:14

Same 49:1 turns ratio, 3-turn primary is 5× more efficient on low bands. The most underappreciated optimization.

02 Band-Specific Design

Avoid pursuing full 80-10m coverage. Use 43 for 80/40m, 52 for 20/15/10m.

03 Optimal Balanced Solution

2×FT240-43 3:21, only 0.3dB loss across all bands, 400W PEP.

04 Measurement & Bottom Line

Back-to-back testing is a placebo. CMC + 0.05λ counterpoise are non-negotiable. Low SWR ≠ high efficiency.

05 Trapped Design

Trapped EFHW solves the 30m problem. AI6XG/K6ARK designs are open-source, cost <$15.

06 VNA 3-Step Method

AI6XG: measure load complex impedance first, then separate MM/LL/TL. Assuming ideal resistor → efficiency >100% error.

17 / System Architecture NEW

EFHW Fuchs ATU V3.0 — Premium Architecture Board

The premium board splits the system into two real chains: the RF/DC physical path and the measurement/control loop. The ATR1000 sits explicitly behind the radio and before the Bias-T, at the shack-side reference plane; the remote ATU is only an actuator, and tuning quality is observed indirectly through the near-end SWR meter.

Topology accuracy

The ATR1000 is drawn as a near-end coupler on the RF main path — behind the radio, before the Bias-T — and is no longer mistakenly placed as a side sensor on the outdoor ATU.

Clear reference plane

Every SWR optimization is based on the radio-side reference plane. The remote ATU's effect reflects back through the long coax to this plane, where MRRC reads it.

Auditable control path

The RF physical path and the WebSocket control path are drawn as separate layers, exposing every latency / failure point: meter read, MRRC, WiFi, ESP32, servo.

No safety overclaim

The diagram deliberately keeps the "not directly detectable" boundary: capacitor arcing, tank heating, real servo position, current and water ingress cannot be substituted by ATR1000 sensing.