Transmitting Loops

Small magnetic transmitting loops (Alexloop, Chameleon F-Loop, Ciro Mazzoni), full-wave loops (delta, quad, hex beam), the high-Q narrow-bandwidth tradeoff and why magloops are the right answer for stealth HF

Section	Topic
1	About this volume
2	Geometry & theory — magnetic vs electric loop radiators
3	Small magnetic transmitting loops — the high-Q, narrow-bandwidth case
4	Full-wave loops — delta, quad, square
5	The hex beam — six element wire Yagi-on-a-spider
6	Feedpoint impedance and matching — capacitor tuning
7	Radiation pattern
8	Frequency response & bandwidth — the 1-2% reality
9	Best-case use
10	Worst-case use
11	Power handling — capacitor voltage, the 5 kV consideration
12	DIY build — a 1 m diameter HF magloop
13	Commercial buys
14	Companion gear
15	Common gotchas and myths
16	Resources

1. About this volume

The transmitting loop family is the strangest member of the wire-and-air cluster. Every other antenna in Vols 6-13 (dipoles, verticals, end-fed wires, Yagis, discones, LPDAs) is fundamentally a current-element radiator — the radiation comes from the antenna’s longitudinal current distribution. A loop antenna is fundamentally a magnetic-field radiator: when the loop is small relative to wavelength (≤ λ/10 circumference), the current in the loop is approximately uniform around its circumference and the radiation pattern is set by the loop’s magnetic dipole moment, not by a current-element distribution. This is a different category of antenna with different design rules, different bandwidth behavior, and a very different “what to expect” performance envelope.

This volume covers transmitting loops in two distinct families:

Small magnetic transmitting loops (§3): the high-Q narrow-bandwidth magnetic radiators — Alexloop, Chameleon F-Loop, Ciro Mazzoni, the MFJ-1786/1787/1788 series. The magnetic-radiator family. Tunable across HF by a variable capacitor. 1–2% per-band bandwidth. The stealth/portable HF specialty.
Full-wave loops (§4–§5): the electrically-large loops — delta, quad, square, hex beam. These behave as resonant current-element antennas (similar to a folded dipole bent into a closed shape) and have dipole-class bandwidth and pattern. The cubical quad and hex beam are the directional members of this family.

Receive-only loops (the noise-rejecting cousins of the small magnetic transmitting loop — K9AY, beverage, terminated flag/pennant/EWE, ferrite-rod, active receive loops like Wellbrook ALA1530) live in Vol 15. The dividing line is intent: this volume covers loops that radiate (transmit-capable); Vol 15 covers loops that pick signal from a specific direction (receive-only).

For the Hack Tools hub, the small magnetic loop is the stealth HF antenna — the right answer for HOA-restricted properties, apartment balconies, attic installations, hotel-room operation, and any case where a visible dipole or vertical isn’t possible. Its severe bandwidth limitation (1–2% per band, constant retuning across each QSY) is the cost; the compact form factor (1–2 m diameter for HF) and the deep ground-coupling-tolerance are the payoff.

2. Geometry & theory — magnetic vs electric loop radiators

2.1 Three loop-size regimes

A loop antenna’s behavior depends entirely on its circumference relative to wavelength. Three distinct regimes:

Loop circumference	Behavior	Radiation type	Typical Q
≤ λ/10	Small magnetic loop	Magnetic-field dominant	200–2000
λ/10 to λ	”Bad zone” — high loss, low gain	Mixed, lossy	varies
≈ λ (full-wave)	Resonant current-element loop	Electric-field dominant	50–80
> λ	Multi-wavelength loop	Multi-lobed pattern	30–50

The “bad zone” between λ/10 and λ is where loops are worst — too large for clean magnetic-dipole behavior, too small for resonant current-element behavior. The Q is moderate but the efficiency is mediocre and the pattern is messy. Practical loop antennas avoid this zone.

2.2 The small magnetic loop: high Q, narrow bandwidth

A loop ≤ λ/10 in circumference has approximately uniform current around its entire circumference (the phase delay around the loop is small compared to a full cycle). This uniform-current loop has the radiation pattern of a magnetic dipole perpendicular to the loop’s plane — a figure-8 in the plane of the loop with deep nulls along the loop’s axis.

The catch: a small magnetic loop has very high Q. The radiation resistance scales as (circumference/λ)⁴, which is tiny for sub-λ/10 loops. Typical numbers for a 1 m diameter (3.14 m circumference) loop:

Frequency	Circumference / λ	R_radiation	R_loss (copper)	Total R	Q
3.5 MHz (80 m)	0.037	0.06 Ω	0.03 Ω	0.09 Ω	~1500
7.0 MHz (40 m)	0.073	0.95 Ω	0.04 Ω	0.99 Ω	~800
14 MHz (20 m)	0.15	15 Ω	0.06 Ω	15.06 Ω	~300
21 MHz (15 m)	0.22	75 Ω	0.08 Ω	75.08 Ω	~200

Two observations: radiation resistance is tiny on lower bands (where most of the energy gets dissipated in conductor losses rather than radiated), and Q is very high (which translates directly to narrow bandwidth — bandwidth ≈ frequency / Q, so a 1500-Q loop on 80 m has ~2.3 kHz bandwidth at 2:1 SWR; you retune to QSY 5 kHz).

2.3 Efficiency depends entirely on conductor loss

A small magnetic loop’s efficiency is η = R_rad / (R_rad + R_loss). The R_loss is dominated by skin-effect conductor resistance at HF. For 1″ (25 mm) copper tubing at 7 MHz, R_loss ≈ 0.04 Ω; for a thin #14 wire, R_loss ≈ 0.5 Ω. The 12× higher loss with the thin wire reduces efficiency from ~96% (with tubing) to ~65% (with wire).

The conductor diameter is the dominant design variable for small loops. The literature is unequivocal: use the fattest conductor you can afford, mechanically. 3/4″ to 1″ copper plumbing tubing is the canonical choice. Aluminum tubing works but with ~2× more loss than copper (skin-effect-limited conductivity). Hollow tubing performs identically to solid conductor (skin effect — only the outer ~10 μm carries RF current).

2.4 The full-wave loop: resonant current-element

A loop with circumference ≈ λ (or any integer multiple) is a resonant structure with strong radiation. The current distribution around the loop is sinusoidal with one full cycle per circumference — at any point on the loop, the current alternates in sign as you trace around.

For a square loop fed at one corner:

Feedpoint Z ≈ 100–150 Ω (depends on shape and feedpoint location)
Radiation pattern: figure-8 broadside to the loop plane (similar to a doublet but with slightly different gain — 1–2 dB more)
Polarization: horizontal (for a horizontal-plane loop) or vertical
Bandwidth: 3–5% (similar to a thick dipole — no extreme Q issue)

Full-wave loops include:

Square loop (4-sided): the canonical full-wave loop
Delta loop (3-sided triangle): often a single-element resonator, useful for stealth deployment with only 3 mast points
Quad loop (square with mostly equal sides, often slightly rectangular): one element of a “cubical quad” array — see §4 below

2.5 The size scale problem at HF

A full-wave 80 m loop is 80 m of wire in a closed shape — much bigger than a typical residential lot. Practical implications:

Band	Wavelength	Full-wave loop circumference
160 m	160 m	160 m (impractical for residential)
80 m	80 m	80 m (most residential lots can’t fit this)
40 m	40 m	40 m (challenging but possible)
20 m	20 m	20 m (fits on most lots — common SOTA antenna)
17 m	17 m	17 m
15 m	14 m	14 m
12 m	12 m	12 m
10 m	10 m	10 m (very practical — 2.5 m × 2.5 m square loop)
6 m	6 m	6 m (compact)

For 20 m and above, full-wave loops are practical. For 40 m and below, the small magnetic loop is usually the more practical choice — it fits in 1–2 m of diameter regardless of operating band.

3. Small magnetic transmitting loops — the high-Q, narrow-bandwidth case

3.1 The standard small-loop geometry

A small magnetic transmitting loop consists of:

Main loop: 1–2 m diameter of large-diameter copper tubing (or sometimes aluminum), forming a closed circle with a small gap
Tuning capacitor across the gap: variable, typically 30–300 pF range
Coupling: a small Faraday-shielded loop (1/5 the main loop’s diameter), or a gamma match, transferring power from the 50 Ω coax to the main loop

   Side view of a small magnetic loop:
   
                       ●  variable capacitor (10-300 pF, vacuum or air)
                       │
                       │
                  ╱─────────╲
                ╱             ╲
              ╱                 ╲
            ╱                     ╲
           │  main loop             │
           │  1/2 - 1 m diameter    │
           │  1" copper tubing      │
            ╲                     ╱
              ╲                 ╱
                ╲             ╱
                  ╲─────────╱
                       │
                       │  small coupling loop
                  ╱─────────╲
                 │           │  1/5 main-loop diameter
                  ╲─────────╱
                       │
                       │
                  50 Ω coax to rig

The main loop is the radiating element; the coupling loop transfers power; the capacitor sets the resonant frequency.

3.2 Resonance tuning by the variable capacitor

The loop + capacitor form a series-LC resonant circuit. The loop’s inductance is set by its geometry (typically 1–5 μH for HF loops); the capacitor sets the resonance:

f_res (MHz) = 159 / sqrt(L_μH × C_pF)

For a 1 m diameter loop (L ≈ 3 μH):

At 80 m (3.65 MHz): C = 633 pF
At 40 m (7.15 MHz): C = 165 pF
At 20 m (14.175 MHz): C = 42 pF
At 10 m (28.5 MHz): C = 10 pF

A 30–300 pF variable capacitor covers 40 m to 12 m on a 1 m loop. Lower bands (80 m, 160 m) require either a larger capacitor (rare) or a smaller loop (the trade-off being less radiation resistance).

3.3 The variable capacitor types

Three families of variable capacitors are commonly used:

Type	Voltage rating	Capacitance	Cost	Notes
Vacuum variable (Jennings, Comet)	5–15 kV	10–1500 pF	$400–1500 used	The high-end choice. Smooth, lossless, sealed.
Butterfly air variable	1–5 kV	30–500 pF	$50–200	Mid-range. Visible plates. Common in commercial loops.
Plate air variable (“low-loss”)	0.5–2 kV	30–500 pF	$30–100	Budget. The cheap surplus option.

Vacuum variables are the right choice for HF transmitting loops at any power above QRP. The 5–10 kV that develops across the capacitor at resonance under 100 W will arc air-variable capacitors with insufficient plate spacing; vacuum-variable capacitors are sealed (or vacuum-equivalent) and don’t have this failure mode. Commercial high-end loops (Ciro Mazzoni, Magnum) use vacuum variables exclusively.

Butterfly air variables (the Jackson Brothers / SAE Variable family) are the mid-tier choice and work fine at 100 W with adequate plate spacing. Below 50 W they’re indistinguishable from vacuum variables in practice.

Cheap surplus plate-variable capacitors should be avoided for transmitting loops — they arc, the plates corrode, and the tuning is coarse. They’re fine for receive-only loops or for QRP experimental builds.

3.4 The tuning sensitivity reality

The most-cited gotcha about magnetic loops: the tuning is extraordinarily sensitive. A capacitor that moves 1 pF shifts the resonance by 20–40 kHz at 7 MHz. This means:

Manual tuning by hand-rotating the capacitor shaft is impractical for fine-tuning
Motorized tuning (stepper motor + ACME screw) is standard for commercial loops
Remote operation is mandatory (the operator can’t reach the antenna at every QSY)
Auto-tuning controllers exist that monitor SWR and adjust the cap (CG Antenna’s CG-3000 controller, MFJ-1786 with built-in motor)

The community practice: design the loop with motor-driven tuning from inside the shack via a coax-fed motor controller. This eliminates the “go outside to retune the antenna” annoyance and makes the loop’s narrow bandwidth merely tedious rather than impossible.

3.5 The narrow-bandwidth reality

A magnetic loop’s bandwidth is set by its Q:

BW (Hz) ≈ f_res / Q

For an 80 m loop with Q ≈ 1500: BW ≈ 3.65 MHz / 1500 = 2.4 kHz (2:1 SWR).

This is a single 2.4 kHz “window” of operation; QSY by 5 kHz takes you off the loop’s tuned frequency and requires retuning the capacitor. The community is sanguine about this — they call it “the loop tax” — but it’s a fundamental property of the loop and there’s no engineering escape. The Chu-Harrington limit (small antenna size constrains bandwidth × efficiency) ensures small loops can’t have wide bandwidth at high efficiency.

Band	Q (1 m copper loop)	Bandwidth (2:1)
80 m	1500	2.4 kHz
40 m	800	8.8 kHz
20 m	300	47 kHz
15 m	200	105 kHz
10 m	150	190 kHz

The narrow-bandwidth issue is most acute on the lower bands (where the loop is electrically smallest). On 10 m the bandwidth is wide enough to be operationally tolerable; on 80 m it’s a constant retune.

4. Full-wave loops — delta, quad, square

4.1 The geometric family

Full-wave loops are wires of total length ≈ λ shaped into a closed planar geometry. The shape variations:

Shape	Geometry	Feed point	Notes
Square loop	4 equal sides	One corner or one side	The simplest full-wave loop
Delta loop	3-sided triangle	One corner or middle of one side	The 3-mast version; common for stealth
Diamond loop	Square rotated 45°	One corner	Pattern is similar to square
Circular loop	Continuous circle	Any point	Rare — hard to construct
Rectangular loop	Aspect ratio ≠ 1	Center of one short side	Pattern depends on aspect ratio

The choice of geometry is usually dictated by mechanical convenience: how many support points are available, what shape fits the lot, what dimensions are practical. Pattern differences between square and delta are < 1 dB at the same circumference.

4.2 Feedpoint impedance and matching

Full-wave loops present feedpoint impedances of 100–150 Ω, depending on shape and feed-location:

Shape	Feed location	Z (Ω)
Square	Corner	100–120
Square	Middle of one side	50–80
Delta	Apex (top)	100–130
Delta	One bottom corner	80–120
Delta	Middle of one bottom leg	50–80

For a corner-fed square loop, the 100–120 Ω impedance maps to 50 Ω coax through a 2:1 transformer (or a 4:1 BALUN that overshoots slightly). For a center-of-side fed loop, the impedance is closer to 75 Ω and matches a 75 Ω/50 Ω 1.5:1 transformer or even a direct 50 Ω feed with mild mismatch.

The standard configuration is corner-fed via a 4:1 current BALUN — the BALUN handles the 100–120 Ω feedpoint impedance and converts to 50 Ω at the coax. SWR < 1.5:1 is typical.

4.3 Radiation pattern and gain

Full-wave loops have a broadside radiation pattern similar to a dipole, but with slight gain enhancement:

Free-space gain: 3.0 dBi (vs 2.15 dBi for a dipole) — about 0.85 dB more gain
Over real ground: similar to a dipole at the same height (gain peaks at 5–7 dBi at h ≈ λ/2)
Polarization: depends on feedpoint location — corner-fed gives diagonal polarization, side-fed gives polarization along the feed axis

The 0.85 dB gain advantage over a dipole is real but modest. The full-wave loop’s main practical advantage isn’t gain; it’s noise rejection — the closed loop has lower local-noise pickup than an open-wire dipole because the loop’s geometry partially cancels out near-field electric noise sources.

4.4 The cubical quad — multi-element loop array

A cubical quad is a Yagi-Uda array where each element is a full-wave loop instead of a half-wave dipole. The director-reflector-driven element configuration is the same as a Yagi; the difference is the element shape.

The quad’s claim over a Yagi:

~0.5–1 dB more gain per element (the loop is a slightly better radiator)
Wider bandwidth (the loop has lower Q than a dipole)
Better F/B on dual-band designs

The cost:

More mechanical complexity (loop spreaders, wire tensioning)
Higher wind load
Larger physical envelope (the loops are 3D structures, not flat boom)

Cubical quads were popular in the 1970s–1990s for amateur HF DX. Modern LFA Yagis (Vol 11 §6) achieve similar performance with less mechanical complexity and have largely replaced cubical quads in commercial use.

5. The hex beam — six element wire Yagi-on-a-spider

5.1 The hex beam concept

The hex beam (Mike Traffie K1WHS, 1992; later commercialized by SteppIR, K6XX, MFJ, and others) is a compact directional HF antenna built as a Yagi-Uda but with wire driven elements + parasitic reflector elements arranged around a fiberglass spider frame. The geometry is:

6 fiberglass spreaders forming a “spider” shape (radial from a central hub)
Wire elements running along the spreaders, alternating driven and reflector configurations
5–6 bands (typically 20/17/15/12/10 m) on a single spreader assembly
Compact: ~22 ft (6.7 m) turning diameter — fits on most lots
Light weight: ~12 kg total

   Top view of a hex beam (simplified, showing one band's elements):
   
                        ●─────────●
                       ╱           ╲
                     ╱  driven       ╲
                   ╱   element         ╲
                 ╱                       ╲
                ●  ←  hub                 ●
                 ╲                       ╱
                   ╲   reflector       ╱
                     ╲  element       ╱
                       ╲           ╱
                        ●─────────●
                            
                        (6 spreaders forming hex)

The hex beam stacks multiple bands’ elements on the same fiberglass structure — typically 5 separate driven elements and 5 separate reflector elements (one pair per band) all attached to the same 6-spreader spider.

5.2 Hex beam performance per band

Band	Gain (dBi)	F/B (dB)	HPBW
20 m	5–6	18–22	60°
17 m	5–6	18–22	60°
15 m	5–6	18–22	60°
12 m	5–6	18–22	60°
10 m	5–6	18–22	60°

Per-band gain of 5–6 dBi is comparable to a 2-element Yagi (~5.5 dBi) but in a more compact mechanical envelope. F/B of 20 dB is respectable.

The hex beam is the low-budget multi-band Yagi: gain that rivals a 2-element commercial Yagi at a fraction of the cost and a smaller turning radius. For an amateur who wants directional HF on multiple bands but can’t afford a full-size Yagi tower install, the hex beam is the standard answer.

5.3 Hex beam commercial market

K6XX Optibeam KU-3F: 6-band hex beam, ~$1500
SteppIR DB6: motorized hex beam, ~$3500
MFJ-1840: budget hex beam clone, ~$700
MyAntennas H6F: 6-band hex beam, ~$1100
K4KIO Hex Beam: K4KIO is the de-facto reference design, ~$1200

The K4KIO design is the canonical reference for DIY builders; the K6XX is the upper-tier commercial product.

6. Feedpoint impedance and matching — capacitor tuning

6.1 Small magnetic loops

A small magnetic loop’s feedpoint impedance is dominated by the coupling loop ratio:

Coupling loop / main loop diameter	Z_feedpoint at resonance
1/8	~12 Ω
1/5	~50 Ω
1/4	~80 Ω
1/3	~150 Ω

The 1/5 ratio is the standard “natural 50 Ω match” — the coupling loop’s geometry sets the impedance ratio without an explicit transformer.

Alternative matching schemes:

Gamma match: a series cap + tap on the main loop (rare for transmitting loops; common for receive loops)
Direct-connection at a specific point: works if the impedance happens to land at 50 Ω

The variable capacitor is the resonance tuning; the coupling loop is the impedance matching. These are two separate adjustments.

6.2 Full-wave loops

A full-wave loop’s natural 100–150 Ω feedpoint is matched with a 4:1 current BALUN to 50 Ω coax. The BALUN itself is the same Mix-31 toroidal design used for OCFD dipoles (Vol 7 §3); the build instructions transfer directly.

6.3 Hex beam

Each band of a hex beam has its own driven element with its own feed point. The hex beam uses a feed network at the hub that combines all bands’ feedpoints into a single 50 Ω coax output. The feed network is band-specific; SWR is typically < 1.5:1 across all 5 bands.

7. Radiation pattern

7.1 Small magnetic loop pattern

A small magnetic loop has a figure-8 pattern in the plane of the loop, with deep nulls perpendicular to the loop plane (i.e. axial to the loop’s normal vector).

For a loop mounted vertically (with its plane vertical):

Two main lobes pointing horizontally, along the loop plane
Two deep nulls pointing vertically (up and down)
Vertical polarization (the radiated E-field is vertical because the current is horizontal)
Pattern azimuth: figure-8

For a loop mounted horizontally (with its plane horizontal):

Main lobe pointing straight up (zenith)
Null at the horizon
Horizontal polarization
Useful for NVIS (near-zenith) HF receive; less useful for transmit

The vertical-loop configuration is the standard for transmit applications because the low-angle radiation is what reaches DX.

   Vertical magloop pattern (top view):
   
              ←─ N ────→
                ↓
                  ↓
                ●  ← loop axis points North-South
                ↑      (loop plane is East-West)
                ↑
              ←─ S ────→
              
   Pattern: figure-8 with main lobes East and West
            (along the loop plane), deep nulls
            North and South (perpendicular to loop)

7.2 Full-wave loop pattern

A full-wave loop’s pattern depends on orientation and feed location:

Square loop, side-fed: broadside pattern similar to a dipole, polarization along the fed side
Square loop, corner-fed: diagonal polarization, slight pattern asymmetry
Delta loop, apex-fed: vertical polarization, omnidirectional-ish (cardioid in azimuth)
Delta loop, bottom-corner-fed: horizontal polarization, slight directionality

For amateur use, the most common configuration is a square loop mounted vertically, fed at one corner, which gives a pattern similar to a horizontal dipole but with slightly higher gain (~3 dBi vs 2.15 dBi free-space) and slightly better SNR on receive.

7.3 Hex beam pattern

A hex beam has a directional cardioid-like pattern with:

HPBW: ~60°
F/B: ~18–22 dB
Gain: 5–6 dBi per band
Pattern similar to a 2-element Yagi

The hex beam is aimed with a rotator (same as a Yagi). Its broader HPBW (60° vs a 9-element Yagi’s 35°) means less precision aiming is needed — useful for casual operation where the operator doesn’t want to micro-adjust the rotator.

8. Frequency response & bandwidth — the 1-2% reality

8.1 Small magnetic loop bandwidth

The defining property of a small magnetic loop: 1–2% bandwidth at 2:1 SWR. This applies to each band on a tunable loop; the operator retunes the capacitor to QSY across the band.

Band	Center frequency	2:1 SWR bandwidth (typical 1m copper loop)
80 m	3.65 MHz	~2.4 kHz
60 m	5.358 MHz	~5.5 kHz
40 m	7.150 MHz	~9 kHz
30 m	10.125 MHz	~17 kHz
20 m	14.175 MHz	~47 kHz
17 m	18.118 MHz	~75 kHz
15 m	21.225 MHz	~105 kHz
12 m	24.940 MHz	~150 kHz
10 m	28.500 MHz	~190 kHz

The 80 m / 40 m / 30 m bandwidths are narrower than the SSB channel spacing of typical operating practice — meaning every QSY requires a retune. On 20 m and higher, the bandwidth is usable for a multi-QSO session without retuning.

8.2 Full-wave loop and hex beam bandwidth

Full-wave loops have typical resonant-antenna bandwidth — 3–5% per band for a clean #14 wire loop, up to 8–10% for a fat-conductor loop.

Hex beams have ~4–5% per-band bandwidth across all 5 bands (5-band hex beam).

These are operationally comfortable — the antenna covers a full SSB phone segment without retuning.

9. Best-case use

The transmitting loop family wins when:

Stealth HF — attic, balcony, hidden in a closet: the small magnetic loop is the HOA-restricted HF antenna. The MFJ-1788 series and the Alexloop made small-loop HF accessible at $700–2500. A 1 m loop tucked into an attic can work 40 m DX from inside a residential structure.
Field-portable / backpack HF: the Alexloop Walkham is 1.5 kg in a backpack — the lightest “real HF antenna” for SOTA / POTA operations. The narrow bandwidth is annoying but the weight win is real.
Apartment / hotel-room operation: a 1 m magloop fits on a windowsill or a portable mast inside the room. The operator can do HF DX from any building’s interior with an antenna invisible from outside.
High-RFI urban environments: the magnetic-field-dominant pattern of the small magloop rejects vertically-polarized noise from local sources (switching power supplies, plasma TVs) by 6–10 dB relative to a vertical antenna.
Multi-band wire-loop installations: a full-wave square loop covers multiple bands (with retuning) on a single feedline. The corner-fed square loop is the canonical wire multi-band antenna.
Compact directional multi-band HF: the hex beam covers 5 bands of DX-capable directional HF on a 22 ft turning radius — the most compact multi-band directional HF antenna available.
Stealth directional: the wire elements of a delta or quad loop are less visible than aluminum Yagi elements, which can matter for HOA-restricted properties where vertical structures are forbidden but “wire decorations” may be tolerated.

10. Worst-case use

VHF/UHF: small loops at VHF are too small (< 1 cm circumference) — use a J-pole, Slim Jim, or quarter-wave whip instead. Loops on VHF are a misallocation.
Wideband digital modes spanning a wide range: PSK31 across multiple bands without retuning is impossible with a small magloop. The operator either retunes constantly or accepts terrible SWR.
Multi-band rapid-QSY operation: the small magloop’s per-band retune time (5–15 seconds with motor drive) is acceptable for casual operation but painful for contesting or rapid-QSY DXing.
High-power AM/SSB transmit at 1.5 kW+: the voltage across the capacitor at full legal limit on 80 m can hit 15–20 kV — beyond all but the largest vacuum-variable capacitors. Use a dipole or full-wave loop instead.
Field operations in fog / dew: the high-Q magloop’s capacitor is sensitive to humidity-induced losses. Operating in damp weather degrades the loop’s efficiency and shifts the resonance.
Backyard receive-only listening: a beverage or K9AY (Vol 15) is much better for receive-only use; transmitting loops aren’t the SNR-optimized choice.
HF DX from a typical residential setup: a horizontal dipole at 30 ft outperforms a vertical magloop for most HF DX. Use the magloop when the dipole isn’t possible; use the dipole when it is.

11. Power handling — capacitor voltage, the 5 kV consideration

11.1 The voltage problem

A small magnetic loop’s tuning capacitor sees high voltage at resonance because the loop’s high Q means high circulating current, and the capacitor’s reactance times the current produces high voltage:

V_cap = I_loop × X_cap = (sqrt(P / R_rad)) × (1 / (2π × f × C))

For a 1 m loop on 80 m at 100 W:

I_loop ≈ 33 A
X_cap = 1/(2π × 3.65 MHz × 633 pF) = 69 Ω
V_cap ≈ 33 × 69 ≈ 2.3 kV

At 1.5 kW: V_cap ≈ 9 kV. At 5 kW: V_cap ≈ 16 kV.

These voltages are across the small gap between the capacitor’s plates. Air-variable capacitors with 1–2 mm plate spacing arc above ~3 kV. Vacuum-variable capacitors with 10+ mm internal spacing handle 10+ kV.

11.2 The arc failure mode

When the capacitor arcs, several bad things happen:

The arc plasma carbonizes the capacitor’s insulators, permanently degrading its performance
The SWR jumps to infinity (the arc shorts the LC circuit)
The amplifier’s protection circuit may trip and back off output, or it may fry the amplifier
The audible “ping” of the arc carries 50+ m and identifies the failed antenna immediately

Air-variable capacitor failures are typical on amateur transmit loops without proper voltage de-rating. The fix: oversize the capacitor for the worst-case voltage at the lowest frequency at the maximum power.

11.3 Capacitor sizing table

For a typical 1 m magloop:

Operating power	Required cap voltage at 80 m	Suitable capacitor
5 W (QRP)	~500 V	Any plate-variable or butterfly
25 W	~1.2 kV	Butterfly air-variable with 2 mm spacing
100 W	~2.3 kV	Butterfly air-variable with 4 mm spacing, or vacuum-variable
500 W	~5.2 kV	Vacuum-variable (any reasonable size)
1500 W	~9 kV	Vacuum-variable (10 kV-rated)

For amateur HF operation up to 100 W, a 5 kV-rated butterfly air-variable is the budget choice; a vacuum-variable is the conservative choice. For amplifier operation (500W+), vacuum-variable is mandatory.

12. DIY build — a 1 m diameter HF magloop

This is a serious-amateur-grade small magnetic transmitting loop covering 40 m through 10 m. About 8 hours of work plus tuning. Total parts cost ~$250 USD (with vacuum-variable capacitor) or ~$80 USD (with butterfly air-variable).

12.1 Geometry

Main loop diameter: 1.0 m (3.14 m circumference)
Main loop conductor: 3/4″ (19 mm) copper plumbing tubing
Coupling loop diameter: 200 mm (1/5 of main loop)
Coupling loop conductor: #14 enamel wire
Capacitor: vacuum-variable, 10–500 pF, 10 kV (Jennings UCSL-500-7.5 or equivalent)
Coverage: 40 m to 10 m (one capacitor position per band)
Coverage with motor: continuous 7–30 MHz

12.2 Bill of materials

Part	Specification	Source	Mid-2026 price
Copper tubing	3/4″ Type L copper plumbing pipe, 3.5 m (1 standard length)	Local home center	$35
Vacuum-variable capacitor (10–500 pF, 10 kV)	Jennings UCSL-500-7.5 or Comet EBHC-500-10 (used market)	eBay surplus	$200
Alternatively, butterfly air-variable (30–300 pF, 5 kV)	Surplus military or commercial	eBay surplus	$40
Coupling loop wire	#14 enamel magnet wire, 1 m	DigiKey	$2
Stepper motor	NEMA-17 + ACME-screw mechanism for cap tuning	Amazon (3D printer parts)	$25
Motor controller	Arduino + stepper driver	local	$20
Coax pigtail	RG-8X, 1 m, with SMA	Times Microwave	$10
Mounting hardware	PVC tee + mast clamp	Local hardware	$15
Capacitor enclosure (weatherproof)	Hammond 1591-XX with feedthrough bushings	DigiKey	$15
Total (vacuum-variable)			~$320
Total (air-variable)			~$160

12.3 Step-by-step construction

Form the main loop. Bend the 3.5 m copper tubing into a 1.0 m diameter circle with a 5 cm gap at the top. The bend can be done with a tube bender or by carefully forming over a 1 m diameter drum. The two ends of the tubing point upward into the gap where the capacitor will mount.

Solder the gap connections. At each end of the tubing, solder a brass adapter (3/4″ to 1/4-20 threaded stud) that the capacitor will mount onto. The capacitor connects across the gap, completing the LC circuit.

Mount the capacitor. The vacuum-variable cap mounts on a small bracket that bridges the gap. The cap’s shaft extends through the bracket; the motor drives the shaft via a coupling. Weatherproof the cap in a Hammond enclosure with feedthrough bushings for the capacitor leads.

Build the coupling loop. A small loop of #14 enamel wire, 200 mm in diameter (1/5 of main loop), positioned inside the main loop near the bottom (180° from the capacitor). The coupling loop’s two ends solder to the inner and outer of the coax pigtail.

Mount the motor. A NEMA-17 stepper motor + ACME-screw mechanism connects to the capacitor’s shaft via a flexible coupling. The motor mounts in the same Hammond enclosure as the capacitor, with the motor’s signal cable exiting through a separate bushing.

Wire the motor controller. An Arduino + stepper-driver board accepts commands from a serial connection (or a small dedicated controller). For amateur use, a simple “up/down” pair of buttons on the controller suffices; for serious use, an SWR-monitoring auto-tune circuit can adjust the cap automatically as the operator changes frequency.

Mount on a non-metallic mast. A 1.5 m PVC mast or fiberglass pole supports the loop vertically. Metal masts couple to the loop’s magnetic field and degrade performance — non-metallic is mandatory.

Test with NanoVNA. Connect the coax pigtail to the NanoVNA. Sweep 7–30 MHz. At various capacitor positions, look for narrow SWR minima — each at a different resonance frequency, scanning across the bands as the cap is adjusted.

12.4 Tuning verification

A successful 1 m magloop build shows:

40 m band: cap setting ~165 pF, SWR < 1.5:1 at 7.15 MHz, 2:1 BW ~9 kHz
30 m band: cap setting ~80 pF, SWR < 1.5:1 at 10.12 MHz, 2:1 BW ~17 kHz
20 m band: cap setting ~42 pF, SWR < 1.5:1 at 14.175 MHz, 2:1 BW ~47 kHz
17 m band: cap setting ~26 pF, SWR < 1.5:1 at 18.10 MHz, 2:1 BW ~75 kHz
15 m band: cap setting ~19 pF, SWR < 1.5:1 at 21.225 MHz, 2:1 BW ~105 kHz
12 m band: cap setting ~14 pF, SWR < 1.5:1 at 24.93 MHz, 2:1 BW ~150 kHz
10 m band: cap setting ~10 pF, SWR < 1.5:1 at 28.5 MHz, 2:1 BW ~190 kHz

If SWR is high (>2:1) at the resonant point, the coupling loop is the wrong size — adjust by increasing or decreasing its diameter (1/5 of main is a starting point).

13. Commercial buys

Sorted by tier (USD, mid-2026):

Tier	Model	Bands	Price	Notes
Budget	Alexloop Walkham	40–10 m	$400	The portable backpack magloop — 1.5 kg, the SOTA-friendly choice
Budget	CHA TIA-Mini Loop	40–10 m	$250	Chameleon’s compact loop
Budget	MFJ-1786	10–30 MHz	$650	The entry-level commercial magloop with manual tuning
Mid	MFJ-1786 with motor drive	10–30 MHz	$1100	Adds the motor for remote tuning
Mid	MFJ-1787 (larger)	7–30 MHz	$900	Larger loop covers 7 MHz
Mid	MFJ-1788 (even larger)	3.5–30 MHz	$1200	Lowest-band capable (80 m)
Mid	Chameleon LOOP-30	7–30 MHz	$850	Chameleon’s mid-tier
Mid	Ciro Mazzoni Baby Loop	3.5–14 MHz	$1500	Italian high-quality
Mid	NavyDavy Loop	10–30 MHz	$700	Small US maker
Premium	AlexLoop ANTAL	7–30 MHz + motor	$2000	The premium portable magloop
Premium	Ciro Mazzoni Stealth Junior	7–30 MHz + motor	$2500	Ciro Mazzoni’s stealth-optimized
Premium	Comrod CTL series	1.6–30 MHz	$3000+	Military/commercial-grade
Premium	Magnum / Vector remote-tuning kits	1.8–30 MHz	$3500+	High-power capable

Hex beam commercial market:

Tier	Model	Bands	Price	Notes
Budget	MFJ-1840	6-band (20–10 m + WARC)	$700	Budget hex beam clone
Mid	MyAntennas H6F	6-band	$1100	Mid-tier hex beam
Mid	K6XX Optibeam KU-3F	6-band	$1500	High-quality, NEC-optimized
Mid	K4KIO Hex Beam	5-band	$1200	The reference hex beam design
Premium	SteppIR DB6	5-band motorized	$3500	Motorized hex beam
Premium	InnovAntennas Hex Beam Plus	5-band	$1800	UK manufacturer premium

Full-wave loop / cubical quad commercial:

Tier	Model	Bands	Price	Notes
Mid	Cushcraft A3S-WB (3-band cubical quad)	20/15/10 m	$700	The classic 2-element cubical quad
Mid	DX Engineering Wire Quad	80–10 m wire	$200–400	DIY-kit wire quad
Premium	Force 12 Magnum Quad	20–10 m	$2500	High-quality cubical quad
Premium	KMA Antennas KMA-2	5-band 2-el quad	$2000	Heavy-duty quad

What to avoid:

Extruded-aluminum “loops” with cheap air-variable capacitors — they arc at 50 W and the capacitor is the failure point. Watch for the “$30 amateur magloop” listings on Amazon; they’re toys.
“All-band magloops 1.5–30 MHz” claiming continuous coverage — possible with a large cap range, but check the lowest frequency’s coverage (the loop has to be physically large for 1.5 MHz operation, which contradicts “compact” claims).
Hex beam clones without published feed-network details — the feed network is the IP that distinguishes good hex beams; cheap clones cut corners here.

14. Companion gear

Motor controller for remote tuning (often integrated in commercial units; for DIY, an Arduino + stepper driver + buttons + small LCD display)
SWR-monitoring auto-tune (auto-tune controllers monitor SWR and adjust the cap automatically — CG Antenna CG-3000 is the canonical reference unit, ~$300)
Non-metallic mast or tripod for vertical magloop mounting (PVC, fiberglass — metal masts couple to the magnetic field and degrade performance)
Coax pigtail with current choke (Mix-43 string-of-beads, 8–10 beads) just outside the loop’s enclosure
Counterweight or guying for stand-alone tripod-mounted magloops (the loop catches wind despite its small footprint)
Lightning protection (Vol 20 §5) — required for permanent outdoor installations
Vacuum-variable capacitor if upgrading from butterfly — Jennings, Comet, or similar 5–15 kV units from surplus channels
Stepper-motor driver + microcontroller for controlling the capacitor remotely

15. Common gotchas and myths

“Magloops are bad at radiating” — false. Properly built and grounded, a 1 m copper-tube magloop is 80% efficient on 40 m. The 1–2% bandwidth is the cost, not low efficiency. The myth comes from comparing magloops built with thin wire (poor efficiency due to skin loss) to commercial tubing-based loops.
“Magloops work better than dipoles in the noise” — true only on receive. The magloop’s pattern rejects vertically-polarized noise from local sources by 6–10 dB. On transmit, a horizontal dipole at 30 ft outperforms a vertical magloop for most HF DX paths.
“Hex beam is ‘just’ a 5-band antenna” — it’s a 5-band directional antenna with 4–6 dBi gain per band. Closer to a small Yagi than to a dipole. The compact size hides the per-band performance.
“Vacuum-variable capacitors are unobtainium” — they’re available on the surplus market for $100–300. Jennings, Comet, and similar surplus from broadcast / military / commercial equipment regularly appears on eBay.
“I’ll just use a cheap air-variable cap” — true for QRP (5 W). Above 25 W, cheap air-variables arc and fail. The capacitor is the most-stressed component in the magloop; oversize it.
“A small loop is less visible than a vertical” — sometimes true, but a 1 m diameter loop is still 1 m of structure that can be seen. The “stealth” claim usually means “not as obviously antenna-like as a vertical.”
“My loop’s SWR drifts during operation” — true, and it’s the loop’s biggest annoyance. The capacitor’s plate spacing changes slightly with temperature, and the loop’s electrical length changes slightly with humidity. Auto-tune controllers can compensate; manual tuning means re-tuning every few QSY.
“Full-wave loops are quieter than dipoles” — partly true. The closed-loop geometry partially cancels near-field electric noise, giving ~3 dB lower local-noise pickup. The effect is real but modest.
“Cubical quads are obsolete” — modern LFA Yagis match cubical quad performance with less mechanical complexity, but cubical quads still work fine and have a niche in stealth installations (wire elements less visible than aluminum).
“Hex beam needs a tower” — the standard hex beam mounts on a 6–10 m mast (much less than a Yagi tower). For amateur use, a hex beam at 8 m height is competitive with a 3-element Yagi at 12 m on a tower.
“I can tune the magloop by hand at the antenna” — for casual operation, yes. For serious operation, remote tuning is mandatory. A motor + Arduino + remote controller costs $40 and makes the antenna usable.

16. Resources

ARRL Antenna Book Ch. 11 (loop antennas) — canonical reference.
AA5TB magnetic loop calculator — online tool for designing small magloops by dimensions and frequency.
Ciro Mazzoni / Alexloop technical documentation — published commercial-product design notes.
N1MM small-loop design papers — community-published optimization notes.
K4KIO hex beam construction guide — the reference hex-beam DIY documentation.
ON4UN, Low-Band DXing — covers full-wave loops for low-band DX.
L. B. Cebik (W4RNL) papers on magnetic loops and full-wave loops — antenneX library.
Hex Beam Yahoo Group archives — community forum with build experiences.
CG Antenna CG-3000 user manual — auto-tune controller documentation.
Jennings vacuum-variable capacitor datasheets — published specifications for the canonical caps.

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