Magnetic Loop Antenna Mazzoni Midi HB0SM

Info: Widely used antenna from the manufacturer Mazzoni, Italy.
Conductor: Aluminium tube, 2 mm wall thickness, bare untreated surface.
Capacitor: Variable air capacitor; the plate stack at the top of the antenna is telescoped in and out.
Environment: Indoor, 3rd floor below the roof.
Thanks: Many thanks to Stefan for the support and for allowing me to publish these measurement values and supporting documents.

Antenna Efficiency Overview

Band		80m	60m	40m	30m	20m
Frequency f	MHz	3.650	5.362	7.097	10.116	14.156
Intrinsic bandwidth B_int	kHz	6.2	18.2	47.2	57.1	182.1
Source of B_int		S-Parameters
Loop diameter D	m	1.920
Conductor diameter d	m	0.073
Loop count n	1	1
Inductance L	H	4.04e-06
Capacitance C	pF	471	218	124	61.3	31.3
Unloaded Q₀	1	590	295	150	177	78
Damping resistance R_T	Ohm	0.157	0.462	1.197	1.449	4.623
Radiation resistance R_R	Ohm	0.00576	0.0269	0.0829	0.346	1.352
Loss resistance R_Loss	Ohm	0.151	0.435	1.114	1.103	3.271
Power to antenna P_fwd	W	100	100	100	100	100
swr_min	1	1.70	2.61	3.93	2.33	4.04
eta_{SWR_ant}	%	93.3	80.1	64.7	84.1	63.6
Power antenna load P_load	W	93	80	65	84	64
Antenna efficiency η	%	3.42	4.66	4.48	20.1	18.6
Loop current I rms	A	24.38	13.17	7.35	7.62	3.71
Loop voltage U_loop rms	V	2259	1792	1324	1956	1333
Magnetic dipole moment m	A m²	70.600	38.124	21.282	22.052	10.742
Link to calculator		calculator	calculator	calculator	calculator	calculator

Build Details

Mazzoni Midi Loop with integrated Baby Loop — Midi and Baby Loop mounted interleaved at a 90° rotation on a rotor.

This is a special setup: inside a Mazzoni Midi Loop, a Mazzoni Baby Loop is mounted with a 90° vertical rotation. This arrangement minimizes mutual coupling.

The antennas are installed on the 3rd floor, in an attic room with a high ceiling.

When operating the Midi, the Baby is fixed to 10 m. In addition, the Baby antenna port is switched to a dummy load via a relay antenna switch.
When operating the Baby, the Midi is fixed to 80 m. In addition, the Midi antenna port is switched to a dummy load via a relay antenna switch.

The antennas can be rotated using a rotor.
There are sensitive bands where the antenna can only be tuned by the ATU in a specific orientation.

Rotor controller and Mazzoni ATU units — Top: rotor controller. Bottom: one Mazzoni ATU 2.0 for each antenna.

A lot of experimentation was done with the relative antenna positions and with their placement inside the room. The gamma match of the Midi Loop was modified (increased coupling) to optimize SWR.

Modified gamma match of the Midi Loop to improve SWR.

Environment

Room position of the Baby Loop — Antenna positions inside the installation room, rotated so that the Baby Loop is visible.

Room position of the Midi Loop — Antenna positions inside the installation room, rotated so that the Midi Loop is visible.

Floor plan. Green marking: antenna position. Unit: mm

Soft iron sheets, 1 mm thick, are placed on the floor. They shield the floor below.

Measurement Info

The antenna measurement setup is very practical. I sit at a table about 9 m away from the antenna.
A 10 m long LMR195 cable runs from the antenna to NanoVNA V2 Plus4. It is just barely long enough.
After tuning with the ATU, I walk to the antenna and plug in my cable. Back and forth, again and again.

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.
Common-mode choke at the antenna: positron.ch/rf/choke_simple
Used cables: 80 cm RG400 (including the choke) and 10 m 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
20260820_1824_midi_swr_1p6_330grad_4MHz_VALUES.py	3.650	6.2	0.000	-1.87	1.70	0.933
20260820_1828_midi_swr_2p1_330grad_5MHz_VALUES.py	5.362	18.2	0.000	4.85	2.61	0.801
20260820_1831_midi_swr_2p5_330grad_7MHz_VALUES.py	7.097	47.2	0.000	4.71	3.93	0.647
20260820_1837_midi_swr_2p5_300grad_10MHz_VALUES.py	10.116	57.1	0.000	3.95	2.33	0.841
20260820_1842_midi_swr_2p5_315grad_14MHz_VALUES.py	14.156	182.1	0.065	4.55	4.04	0.625

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

Smith

SWR

Values

20260820_1824_midi_swr_1p6_330grad_4MHz

model_f0	3.650	MHz
model_BSWR2_62	6.2	kHz
model_alpha	0.000	db
model_tau	-1.87	ns
SWR_min	1.70
eta_SWR_ant	0.933

20260820_1828_midi_swr_2p1_330grad_5MHz

model_f0	5.362	MHz
model_BSWR2_62	18.2	kHz
model_alpha	0.000	db
model_tau	4.85	ns
SWR_min	2.61
eta_SWR_ant	0.801

20260820_1831_midi_swr_2p5_330grad_7MHz

model_f0	7.097	MHz
model_BSWR2_62	47.2	kHz
model_alpha	0.000	db
model_tau	4.71	ns
SWR_min	3.93
eta_SWR_ant	0.647

20260820_1837_midi_swr_2p5_300grad_10MHz

model_f0	10.116	MHz
model_BSWR2_62	57.1	kHz
model_alpha	0.000	db
model_tau	3.95	ns
SWR_min	2.33
eta_SWR_ant	0.841

20260820_1842_midi_swr_2p5_315grad_14MHz

model_f0	14.156	MHz
model_BSWR2_62	182.1	kHz
model_alpha	0.065	db
model_tau	4.55	ns
SWR_min	4.04
eta_SWR_ant	0.625

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

Inductivity picture: 20260820_164701101_2_induktivitaet_midi.jpg — The switched capacitor is visible in the lower part of the picture.
The connections were temporarily taped to the capacitor plates with yellow tape.
With two switches the two capacitors can be switched in.

f_NIX	14.158855	MHz	Resonance frequency with no additional capacitors connected.
f_OFF	13.794990	MHz	Capacitors and switches are physically connected at the antenna capacitor. A small parasitic capacitance from wiring and switches lowers the resonance frequency.
f₁₀₀	6.812003	MHz	Resonance frequency with an additional 100 pF capacitor switched in.
f₅₆₀	3.170368	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	4.04e-06	H	Calculated from geometry of the main loop.
L₁₀₀	4.128e-06	H	Derived from the resonance frequencies f_OFF and f₁₀₀ deviation +2% vs L
L₅₆₀	4.123e-06	H	Derived from the resonance frequencies f_OFF and f₅₆₀ deviation +2% vs L
C_NIX	1.636e-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 +2%. This is considered a small deviation and is accepted. 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.

Floor plan with measurement point A marked. Unit: mm

f = 3.740 MHz

tx_power_w	100.0
f_Hz	3740000
attenuation_cables_connectors_total_dbm	0.44 dB
tx_after_cable_w	90.4
I_main_loop_A	23.2
magnetic dipole moment m (Am2)	67.1

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	-6.7	-6.1	-0.5	0.0129	0.0691	5.362

The measured field does not correspond to the expected field under free-space conditions.
The measured and calculated fields are well below the safety limit.

I can think of the following possible reasons:

The soft-iron sheets under the antenna may distort the field.
Measurement point A is quite close to the transmitter. The antenna cable has no common-mode choke, which may increase the measured field at point A.

I would have liked to perform additional H-field measurements to investigate the large factor further. Unfortunately, the travel distance to the installation site is long and the effort is too high, so I leave this measurement as it is.

Final Remarks

I find the idea of mounting these two antennas interlocked and rotated by 90 degrees brilliant.

This design saves a lot of space. In addition, both antennas can be rotated with a single rotor.
If the two antennas were mounted on separate rotors, it would be quite difficult to keep them oriented so that the mutual coupling always remains minimal. With both antennas on one rotor, this is ensured automatically. The efficiency for indoor operation is impressive.

The rotor direction has a strong influence on antenna performance. The antenna couples strongly to the building, which is typical for indoor installations.
If the SWR tuning is too poor, tuning with the ATU does not work.
The losses caused by poor matching are unfortunate. For this type of application, it would be beneficial to have adjustable coupling and a more tolerant ATU.

A great deal of effort, money, and dedication has gone into this solution. I can recommend it as a design worth replicating.

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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.