Magnetic Loop Antenna Mazzoni Stealth HE9DJB

Info: Widely used antenna from the manufacturer Mazzoni, Italy.
Conductor: Aluminium rectangular tube, outside 60mm x 20mm, wall 2 mm, painted gray.
Capacitor: Air capacitor, aluminum, painted gray
Environment: In the garden on the ground on patio slabs.
Thanks: Many thanks to Dieter HE9DJB for the opportunity to measure his antenna, for the active support, and for the interesting conversations.

Antenna Efficiency Overview

Band		40m	40m #2	30m	30m #2	20m	20m #2	17m	17m #2	15m	15m #2	12m	12m #2	10m	10m #2
Frequency f	MHz	7.103	7.101	10.127	10.129	14.168	14.168	18.116	18.113	21.235	21.213	24.911	24.957	28.850	28.814
Intrinsic bandwidth B_int	kHz	18.6	20.9	36.2	34.8	62.5	61.8	121.0	107.7	181.5	159.4	257.2	233.1	376.3	371.4
Source of B_int		S-Parameters
Loop diameter D	m	0.662
Conductor diameter d	m	0.051
Loop count n	1	1
Inductance L	H	1.10e-06
Capacitance C	pF	457	457	225	225	115	115	70.2	70.2	51.1	51.2	37.1	37	27.7	27.7
Unloaded Q₀	1	382	339	280	291	227	229	150	168	117	133	97	107	77	78
Damping resistance R_T	Ohm	0.128	0.145	0.250	0.241	0.432	0.427	0.836	0.744	1.254	1.101	1.777	1.610	2.599	2.566
Radiation resistance R_R	Ohm	0.00116	0.00116	0.00481	0.00482	0.0185	0.0185	0.0496	0.0495	0.0939	0.0935	0.178	0.180	0.323	0.321
Loss resistance R_Loss	Ohm	0.127	0.144	0.245	0.236	0.413	0.408	0.786	0.694	1.160	1.008	1.599	1.430	2.277	2.245
Power to antenna P_fwd	W	100	100	100	100	100	100	100	100	100	100	100	100	100	100
swr_min	1	1.07	1.36	1.03	1.15	1.08	1.08	1.14	1.20	1.29	1.34	1.40	1.49	1.65	1.91
eta_{SWR_ant}	%	99.9	97.7	100.0	99.5	99.9	99.8	99.6	99.2	98.4	97.9	97.2	96.1	94.0	90.3
Power antenna load P_load	W	100	98	100	100	100	100	100	99	98	98	97	96	94	90
Antenna efficiency η	%	0.904	0.785	1.92	1.99	4.28	4.33	5.90	6.60	7.37	8.31	9.77	10.7	11.7	11.3
Loop current I rms	A	27.88	25.99	19.99	20.33	15.21	15.30	10.91	11.55	8.86	9.43	7.40	7.73	6.01	5.93
Loop voltage U_loop rms	V	1368	1275	1399	1423	1489	1497	1366	1445	1300	1382	1273	1332	1198	1181
Magnetic dipole moment m	A m²	9.591	8.940	6.878	6.995	5.233	5.263	3.754	3.972	3.048	3.243	2.544	2.658	2.068	2.040
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Environment

Screenshot from datasheet.

The datasheet recommends to "leave a space of at least 2 meters all around" and to "place on a flat reflective surface and adjust the height of the rubber feet on the base".

The antenna stands freely on the meadow. A small pond begins 9 m away from the antenna.

Apart from the fence, there are no electromagnetic interference sources anywhere nearby.

The antenna stands on an area paved with garden slabs, 1.5 m by 1.5 m.
Directly beneath the antenna are garden slabs and gravel.
Compared with soil, this is likely a poorer conductor and rather dry.
A cable shaft made from a concrete pipe leads to a horizontal conduit that goes to the shed.

Reflective surface

The datasheet recommends a reflective surface under the antenna.
It also states that SWR can be optimized by changing the distance from the reflective surface.

The antenna was first measured without a reflective surface. Afterwards, a metal mesh was placed under the antenna and the measurements were repeated. Measurements with the mesh are marked with #2.

The mesh used has a pitch of 5 cm and galvanized steel wires with a diameter of 3.5 mm and the size is 1.5 m by 0.95 m.
The wires are conductively welded together.
Compared with the wavelength, the 5 cm spacing is negligibly small. In this frequency range, it is equivalent to a conductive sheet.

At higher frequencies, the SWR rises slightly above the typical values given in the datasheet.

However, the values are still uncritical overall.

Findings

For the measurement with the mesh, we did not tune the distance to the mesh. The SWR was good immediately, about as good as without the mesh.
The efficiency with and without the mesh does not differ significantly.

The reflective surface had no effect on efficiency, so the mesh is unnecessary. We removed it again after the test.

Measurement Info

The antenna was measured from the shack, which is about 40 m away from the antenna.
The antenna was tuned with the ATU (Automatic Tuner Unit from Mazzoni).

VNA measurements

The antenna S11 parameters were measured with the NanoVNA V2 Plus4.
The following values were derived from these measurements.
Details on the measurement method can be found here.

The calibration of the VNA was done at the antenna feed point: green line.
Cable: 40m low loss 10mm Aircom Premium, 5m LMR195.
The cable attenuation alpha and the cable delay tau in the following table should therefore be small.

File	model f0 MHz	model BSWR2_62 kHz	model alpha db	model_tau ns	SWR min	eta SWR
20260916_1546_no_mesh__7p1MHz_VALUES.py	7.103	18.6	0.000	3.62	1.07	0.999
20260916_1547_no_mesh__10p1MHz_VALUES.py	10.127	36.2	0.000	3.48	1.03	1.000
20260916_1549_no_mesh__14p2MHz_VALUES.py	14.168	62.5	0.001	3.11	1.08	0.999
20260916_1551_no_mesh__18p1MHz_VALUES.py	18.116	121.0	0.000	3.23	1.14	0.996
20260916_1553_no_mesh__21p2MHz_VALUES.py	21.235	181.5	0.002	3.20	1.29	0.984
20260916_1555_no_mesh__24p9MHz_VALUES.py	24.911	257.2	0.001	2.87	1.40	0.972
20260916_1556_no_mesh__28p9MHz_VALUES.py	28.850	376.3	0.000	2.99	1.65	0.940
20260916_1628_mesh_7p1MHz_VALUES.py	7.101	20.9	0.000	1.57	1.36	0.977
20260916_1629_mesh_10p1MHz_VALUES.py	10.129	34.8	0.000	3.43	1.15	0.995
20260916_1632_mesh_14p2MHz_VALUES.py	14.168	61.8	0.007	3.31	1.08	0.998
20260916_1634_mesh_18p1MHz_VALUES.py	18.113	107.7	0.000	3.31	1.20	0.992
20260916_1636_mesh_21p2MHz_VALUES.py	21.213	159.4	0.000	3.19	1.34	0.979
20260916_1639_mesh_25p0MHz_VALUES.py	24.957	233.1	0.000	3.18	1.49	0.961
20260916_1641_mesh_28p8MHz_VALUES.py	28.814	371.4	0.000	2.99	1.91	0.903

The following diagrams: red points = measured values; green line = fitted model.

Smith

SWR

Values

20260916_1546_no_mesh__7p1MHz

model_f0	7.103	MHz
model_BSWR2_62	18.6	kHz
model_alpha	0.000	db
model_tau	3.62	ns
SWR_min	1.07
eta_SWR_ant	0.999

20260916_1547_no_mesh__10p1MHz

model_f0	10.127	MHz
model_BSWR2_62	36.2	kHz
model_alpha	0.000	db
model_tau	3.48	ns
SWR_min	1.03
eta_SWR_ant	1.000

20260916_1549_no_mesh__14p2MHz

model_f0	14.168	MHz
model_BSWR2_62	62.5	kHz
model_alpha	0.001	db
model_tau	3.11	ns
SWR_min	1.08
eta_SWR_ant	0.999

20260916_1551_no_mesh__18p1MHz

model_f0	18.116	MHz
model_BSWR2_62	121.0	kHz
model_alpha	0.000	db
model_tau	3.23	ns
SWR_min	1.14
eta_SWR_ant	0.996

20260916_1553_no_mesh__21p2MHz

model_f0	21.235	MHz
model_BSWR2_62	181.5	kHz
model_alpha	0.002	db
model_tau	3.20	ns
SWR_min	1.29
eta_SWR_ant	0.984

20260916_1555_no_mesh__24p9MHz

model_f0	24.911	MHz
model_BSWR2_62	257.2	kHz
model_alpha	0.001	db
model_tau	2.87	ns
SWR_min	1.40
eta_SWR_ant	0.972

20260916_1556_no_mesh__28p9MHz

model_f0	28.850	MHz
model_BSWR2_62	376.3	kHz
model_alpha	0.000	db
model_tau	2.99	ns
SWR_min	1.65
eta_SWR_ant	0.940

20260916_1628_mesh_7p1MHz

model_f0	7.101	MHz
model_BSWR2_62	20.9	kHz
model_alpha	0.000	db
model_tau	1.57	ns
SWR_min	1.36
eta_SWR_ant	0.977

20260916_1629_mesh_10p1MHz

model_f0	10.129	MHz
model_BSWR2_62	34.8	kHz
model_alpha	0.000	db
model_tau	3.43	ns
SWR_min	1.15
eta_SWR_ant	0.995

20260916_1632_mesh_14p2MHz

model_f0	14.168	MHz
model_BSWR2_62	61.8	kHz
model_alpha	0.007	db
model_tau	3.31	ns
SWR_min	1.08
eta_SWR_ant	0.998

20260916_1634_mesh_18p1MHz

model_f0	18.113	MHz
model_BSWR2_62	107.7	kHz
model_alpha	0.000	db
model_tau	3.31	ns
SWR_min	1.20
eta_SWR_ant	0.992

20260916_1636_mesh_21p2MHz

model_f0	21.213	MHz
model_BSWR2_62	159.4	kHz
model_alpha	0.000	db
model_tau	3.19	ns
SWR_min	1.34
eta_SWR_ant	0.979

20260916_1639_mesh_25p0MHz

model_f0	24.957	MHz
model_BSWR2_62	233.1	kHz
model_alpha	0.000	db
model_tau	3.18	ns
SWR_min	1.49
eta_SWR_ant	0.961

20260916_1641_mesh_28p8MHz

model_f0	28.814	MHz
model_BSWR2_62	371.4	kHz
model_alpha	0.000	db
model_tau	2.99	ns
SWR_min	1.91
eta_SWR_ant	0.903

Inductance

The main loop inductance is an important parameter because it directly affects the antenna efficiency calculation.
The inductance can be estimated from geometry (L). In general, an additional measurement is used as a cross-check, especially for non-circular loops where the geometric estimate is more difficult.

Inductance cross-check measurement

The resonance frequency of the LC circuit depends on L and C. Additional known capacitors are connected in parallel with the existing capacitor, and the new resonance frequency is measured.

Capacitor switching concept — Schematic of the switched capacitors

f_NIX	14.176310	MHz	Resonance frequency with no additional capacitors connected.
f_OFF	14.077398	MHz	Capacitors and switches are physically connected at the antenna capacitor. A small parasitic capacitance from wiring and switches lowers the resonance frequency.
f₁₀₀	9.808864	MHz	Resonance frequency with an additional 100 pF capacitor switched in.
f₅₆₀	5.106150	MHz	Resonance frequency with an additional 560 pF capacitor switched in.
C₁₀₀	100.0	pF	Additional capacitance used for the 100 pF branch.
C₅₆₀	579.0	pF	Additional capacitance used for the 560 pF branch.
L	1.099e-06	H	Calculated from geometry of the main loop.
L₁₀₀	1.355e-06	H	Derived from the resonance frequencies f_OFF and f₁₀₀ deviation +23% vs L
L₅₆₀	1.457e-06	H	Derived from the resonance frequencies f_OFF and f₅₆₀ deviation +33% vs L
C_NIX	1.312e-12	As/V	Derived from using L₁₀₀, f_OFF, and f_NIX estimated parasitic capacitance of switches and wiring; expected value 1 ... 5 pF

The maximum deviation between L and the capacitor-based L_1x values is +33%. The deviation is quite large. The geometry of the Stealth is unusual. There are places, for example the spring at the bottom, where the conductor cross-section is relatively small. This could explain the inductance deviation.
The uncertainty in the inductance also leads to uncertainty in the efficiency. The efficiencies calculated on this page must therefore be treated with appropriate caution.
L is used for the calculations of the antenna efficiency.

Cross-check H-field

The H-field can be calculated under free-space conditions. In practice, however, the building contains numerous conductive objects that distort the field. To quantify the extent of this distortion, the H-field was measured and compared with the theoretical predictions.

The H-field is measured with a small measurement loop. The measuring setup is described in https://arxiv.org/abs/2607.10828.

Top view of the garden. Measurement points A and B are each 10 m away from the antenna.
The preferred transmission direction is downward (arrow).

f = 7.023 MHz

tx_power_w	100.0
f_Hz	7023000
attenuation_cables_connectors_total_dbm	0.85 dB
tx_after_cable_w	82.1
I_main_loop_A	23.6
magnetic dipole moment m (Am2)	8.1

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	10.0	0.0	1.7	0.0022	0.0047	2.138
B	0.0	10.0	2.1	0.0012	0.0095	7.995

f = 14.055 MHz

tx_power_w	10.0
f_Hz	14055000
attenuation_cables_connectors_total_dbm	1.09 dB
tx_after_cable_w	7.8
I_main_loop_A	4.2
magnetic dipole moment m (Am2)	1.5

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	10.0	0.0	1.7	0.0007	0.0026	3.684
B	0.0	10.0	2.1	0.0009	0.0036	3.851

f = 28.050 MHz

tx_power_w	10.0
f_Hz	28050000
attenuation_cables_connectors_total_dbm	1.42 dB
tx_after_cable_w	7.2
I_main_loop_A	1.6
magnetic dipole moment m (Am2)	0.5

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	10.0	0.0	1.7	0.0006	0.0023	4.182
B	0.0	10.0	2.1	0.0015	0.0033	2.281

The measured field does not correspond to the expected field under free-space conditions.
The direction of the field lines was in some cases completely different from what would be expected based on the geometry.

There are no known iron structures in the immediate vicinity. There is no plausible explanation for the large deviation.

Final Remarks

The antenna had failed after 2 years in the weather and had to be repaired first.
The contact in the stainless-steel joint over the moving part was so poor that the ATU could no longer tune the antenna.
The aluminum loop was under stress against the side plastic plates. All plastic parts were warped.
It is worth taking precautions against corrosion. For details, see www.positron.ch/rf/2026_mazzoni_stealth

The antenna has a reasonable SWR across the full frequency range.

The measured bandwidths do not match the bandwidths specified in the datasheet.

The calculated efficiency is not outstanding. Given the size of the build and its proximity to the ground, it is acceptable.
I strongly suspect that a larger distance from the ground would increase the efficiency significantly.

The antenna is difficult for laypeople to recognize as an antenna and lives up to its name.
It is the dream of every antenna to be allowed to stand in such a beautiful environment.

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2026 Peter Märki (HB9ISP). This project is created in my free time and has no commercial background. Provided without warranty of any kind. Feedback is welcome.