Magnetic Loop Antenna Mazzoni Baby HB9PWD

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: Mounted on a mast with a rotor 1.5m above the roof of a 5-storey building
Thanks: Many thanks to Stephan for allowing me to measure his antenna, for the pleasant conversations, and for showing me his workshop and shack.

Antenna Efficiency Overview

Band		40m	30m	20m	17m	15m	12m	10m
Frequency f	MHz	7.100	10.127	14.173	18.124	21.209	24.917	28.832
Intrinsic bandwidth B_int	kHz	5.9	9.4	34.7	31.3	182.6	99.9	271.4
Source of B_int		S-Parameters
Loop diameter D	m	0.940
Conductor diameter d	m	0.049
Loop count n	1	1
Inductance L	H	1.79e-06
Capacitance C	pF	281	138	70.6	43.2	31.5	22.8	17.1
Unloaded Q₀	1	1203	1076	409	579	116	249	106
Damping resistance R_T	Ohm	0.0662	0.106	0.389	0.351	2.051	1.122	3.047
Radiation resistance R_R	Ohm	0.00473	0.0196	0.0757	0.204	0.384	0.738	1.336
Loss resistance R_Loss	Ohm	0.0615	0.0860	0.313	0.147	1.666	0.384	1.711
Power to antenna P_fwd	W	450	450	450	450	450	450	450
swr_min_ant	1	1.17	1.28	1.50	1.13	3.12	1.34	3.16
eta_{SWR_ant}	%	99.4	98.5	96.0	99.6	73.5	97.9	73.0
Power antenna load P_load	W	447	443	432	448	331	441	328
Antenna efficiency η	%	7.10	18.3	18.7	57.8	13.8	64.4	32.0
Loop current I rms	A	82.17	64.77	33.33	35.73	12.70	19.82	10.38
Loop voltage U_loop rms	V	6550	7364	5303	7270	3024	5544	3361
Magnetic dipole moment m	A m²	57.027	44.948	23.128	24.795	8.814	13.754	7.206
Link to calculator		calculator	calculator	calculator	calculator	calculator	calculator	calculator

Environment

The antenna is located on the roof of a 5-storey building.

The antenna is mounted on a mast with a G450C rotor. The base of the loop is approximately 1.5 m above the roof ridge.
Next to the antenna there is a chimney with copper cladding.
A small Stampfl XOne active antenna is mounted below the loop.
Lower on the mast there is also a Technisat dish 85 cm for QO-100 and a Diamond X-30-N VHF/UHF antenna.

Highly simplified cross-section through the building.
The roof is tiled with clay tiles on a timber structure.
The attic contains almost no conductive objects.

The loop has been exposed to the weather for 3 years (2023...2026). Depending on the orientation of the antenna, the 10 m band is somewhat problematic (high SWR). It is suspected that the copper-clad chimney is causing interference.

Unfortunately the rotor was defective at the time of these measurements, so the antenna could not be rotated — it was fixed pointing west.

Measurement Info

The measurements were taken from the attic. There is a detachable connector between the cable and the antenna here, as well as a cable to the shack in the mast below.
A 5 m LMR195 cable runs from the VNA to a 3-way switch, CX-SW3N. The antenna can thus be switched either to the VNA or to the shack. The VNA calibration was performed at the antenna output of the switch.
From the switch, a roughly 10.4 m long Aircell 7 cable runs to the antenna. The cable length stated here was estimated based on the cable delay.

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 was done directly at the NanoVNA V2 Plus4: green line. The cable influence was measured and removed.
The cable attenuation alpha and the cable delay tau in the following table show the estimated cable values based on the NanoVNA V2 Plus4 measurement.

File	model f0 MHz	model Bint kHz	model alpha db	model_tau ns	SWR_min_vna	SWR_min_ant	eta SWR
20261027_1418_baby_hb9pwd_swr_1p2_7p1MHz_VALUES.py	7.100	5.9	0.156	38.71	1.16	1.17	0.994
20261027_1420_baby_hb9pwd_swr_1p4_10p1MHz_VALUES.py	10.127	9.4	0.205	41.44	1.27	1.28	0.985
20261027_1424_baby_hb9pwd_swr_1p4_14p2MHz_VALUES.py	14.173	34.7	0.277	42.28	1.46	1.50	0.960
20261027_1426_baby_hb9pwd_swr_1p4_18p1MHz_VALUES.py	18.124	31.3	0.277	42.04	1.12	1.13	0.996
20261027_1428_baby_hb9pwd_swr_1p7_21p2MHz_VALUES.py	21.209	182.6	0.272	41.50	2.87	3.12	0.735
20261027_1431_baby_hb9pwd_swr_1p5_24p9MHz_VALUES.py	24.917	99.9	0.388	41.98	1.30	1.34	0.979
20261027_1433_baby_hb9pwd_swr_2p6_28p8MHz_VALUES.py	28.832	271.4	0.428	41.90	2.78	3.16	0.730

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

Smith

SWR

Values

20261027_1418_baby_hb9pwd_swr_1p2_7p1MHz

model_f0	7.100	MHz
model_Bint	5.9	kHz
model_alpha	0.156	db
model_tau	38.71	ns
SWR_min_vna	1.16
SWR_min_ant	1.17
eta_SWR_ant	0.994

20261027_1420_baby_hb9pwd_swr_1p4_10p1MHz

model_f0	10.127	MHz
model_Bint	9.4	kHz
model_alpha	0.205	db
model_tau	41.44	ns
SWR_min_vna	1.27
SWR_min_ant	1.28
eta_SWR_ant	0.985

20261027_1424_baby_hb9pwd_swr_1p4_14p2MHz

model_f0	14.173	MHz
model_Bint	34.7	kHz
model_alpha	0.277	db
model_tau	42.28	ns
SWR_min_vna	1.46
SWR_min_ant	1.50
eta_SWR_ant	0.960

20261027_1426_baby_hb9pwd_swr_1p4_18p1MHz

model_f0	18.124	MHz
model_Bint	31.3	kHz
model_alpha	0.277	db
model_tau	42.04	ns
SWR_min_vna	1.12
SWR_min_ant	1.13
eta_SWR_ant	0.996

20261027_1428_baby_hb9pwd_swr_1p7_21p2MHz

model_f0	21.209	MHz
model_Bint	182.6	kHz
model_alpha	0.272	db
model_tau	41.50	ns
SWR_min_vna	2.87
SWR_min_ant	3.12
eta_SWR_ant	0.735

20261027_1431_baby_hb9pwd_swr_1p5_24p9MHz

model_f0	24.917	MHz
model_Bint	99.9	kHz
model_alpha	0.388	db
model_tau	41.98	ns
SWR_min_vna	1.30
SWR_min_ant	1.34
eta_SWR_ant	0.979

20261027_1433_baby_hb9pwd_swr_2p6_28p8MHz

model_f0	28.832	MHz
model_Bint	271.4	kHz
model_alpha	0.428	db
model_tau	41.90	ns
SWR_min_vna	2.78
SWR_min_ant	3.16
eta_SWR_ant	0.730

Final Remarks

A fairly inconspicuous antenna on a rooftop in the middle of Zurich.
The antenna has a fairly unobstructed position. The efficiency is presumably typical (but far below the values stated in the datasheet).
Many DX contacts have already been made with it.

Overview of all Antennas with filter/selection: compare page
Overview of all Antennas static: static compare page
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.