Magnetic Loop Antenna Selfmade 3P HB9BPO

Info: Mag loop made of copper tubing with an air variable capacitor.
Conductor: Copper tube OD 12 mm, ID 10 mm, bare surface.
Capacitor: Air variable capacitor, self-made, CNC-milled from aluminum sheet.
Environment: In the garden, on a Styrofoam box on a wooden table. Center loop 1.40 m above ground.
Thanks: Many thanks to Peter HB9BPO for the support, the fun conversations, and the catering.

Antenna Efficiency Overview

Band		20m	17m	15m	12m	10m
Frequency f	MHz	14.072	18.116	21.253	24.947	28.604
Intrinsic bandwidth B_int	kHz	99.1	102.7	123.6	165.6	201.9
Source of B_int		S-Parameters
Loop diameter D	m	0.762
Conductor diameter d	m	0.012
Loop count n	1	1
Inductance L	H	2.02e-06
Capacitance C	pF	63.2	38.1	27.7	20.1	15.3
Unloaded Q₀	1	142	176	172	151	142
Damping resistance R_T	Ohm	1.261	1.306	1.572	2.107	2.568
Radiation resistance R_R	Ohm	0.0316	0.0872	0.166	0.316	0.550
Loss resistance R_Loss	Ohm	1.229	1.219	1.406	1.790	2.018
Power to antenna P_fwd	W	10	10	10	10	10
swr_min	1	1.39	1.04	1.03	1.08	1.11
eta_{SWR_ant}	%	97.4	100.0	100.0	99.9	99.7
Power antenna load P_load	W	10	10	10	10	10
Antenna efficiency η	%	2.44	6.67	10.5	15.0	21.4
Loop current I rms	A	2.78	2.77	2.52	2.18	1.97
Loop voltage U_loop rms	V	497	637	682	691	717
Magnetic dipole moment m	A m²	1.266	1.260	1.149	0.992	0.898
Link to calculator		calculator	calculator	calculator	calculator	calculator

Build Details

Homemade magnetic loop made from copper tube. A common-mode choke is clearly visible on the vertical cable.

At the top is a coupling loop made from brass tubing (loop diameter: 170 mm, tube diameter: 6 mm).
The coupling loop is fixed only with adhesive tape. The SWR was very good with exactly this setting, and the position was kept unchanged for all measurements.

Air-variable tuning capacitor with hand knob and stepper motor drive (black stepper motor in the rear).

Detail view of the loop construction and mounting hardware.

Homemade tuning capacitor, CNC-milled on a self-built milling machine from aluminum sheet.

For lower bands, additional capacitors can be switched in.
The switch was open for all measurements shown here, so it was not used.
The connection to the copper tube is made using screwed ring lugs and copper wires.

With the variable capacitor, the frequency can be tuned from 13.41 MHz up to 29.78 MHz.
The capacitor has external dimensions of approximately 40 mm x 40 mm x 220 mm.

Environment

The antenna stands on a wooden table in the garden. A styrofoam box was used to increase the ground clearance a bit more.

The loop center is 1.40 m above ground.
The nearest object in the surroundings is a clothesline post at a distance of about 2.5 m (visible on the left in the picture).
The grass is dry.

Measurement Info

The antenna measurement setup is very practical. We sit at a table about 5 m away from the antenna.
A 10 m long LMR195 cable runs from the antenna to NanoVNA V2 Plus4.
Here we can also switch over to IC-7300 MK2 as transmitter.
For tuning, we measure the resonance with the VNA, and the capacitor is adjusted with a stepper motor.

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
20260827_1344_antenne_14p1MHz_VALUES.py	14.072	99.1	0.000	6.24	1.39	0.974
20260827_1409_antenne_28p6MHz_VALUES.py	28.604	201.9	0.000	4.96	1.11	0.997
20260827_1419_antenne_24p9MHz_VALUES.py	24.947	165.6	0.000	5.22	1.08	0.999
20260827_1426_antenne_21p3MHz_VALUES.py	21.253	123.6	0.000	5.73	1.03	1.000
20260827_1435_antenne_18p1MHz_VALUES.py	18.116	102.7	0.007	6.01	1.04	1.000

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

Smith

SWR

Values

20260827_1344_antenne_14p1MHz

model_f0	14.072	MHz
model_BSWR2_62	99.1	kHz
model_alpha	0.000	db
model_tau	6.24	ns
SWR_min	1.39
eta_SWR_ant	0.974

20260827_1409_antenne_28p6MHz

model_f0	28.604	MHz
model_BSWR2_62	201.9	kHz
model_alpha	0.000	db
model_tau	4.96	ns
SWR_min	1.11
eta_SWR_ant	0.997

20260827_1419_antenne_24p9MHz

model_f0	24.947	MHz
model_BSWR2_62	165.6	kHz
model_alpha	0.000	db
model_tau	5.22	ns
SWR_min	1.08
eta_SWR_ant	0.999

20260827_1426_antenne_21p3MHz

model_f0	21.253	MHz
model_BSWR2_62	123.6	kHz
model_alpha	0.000	db
model_tau	5.73	ns
SWR_min	1.03
eta_SWR_ant	1.000

20260827_1435_antenne_18p1MHz

model_f0	18.116	MHz
model_BSWR2_62	102.7	kHz
model_alpha	0.007	db
model_tau	6.01	ns
SWR_min	1.04
eta_SWR_ant	1.000

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: 20260827_115219934_cap_switch.jpg — The switched capacitor is visible in the picture above.
The connections were temporarily taped to the main loop with yellow tape.
With two switches (only one is visible in the picture because they are exactly behind each other), the two capacitors can be switched in.

f_NIX	18.112769	MHz	Resonance frequency with no additional capacitors connected.
f_OFF	17.296894	MHz	Capacitors and switches are physically connected at the antenna capacitor. A small parasitic capacitance from wiring and switches lowers the resonance frequency.
f₁₀₀	9.369003	MHz	Resonance frequency with an additional 100 pF capacitor switched in.
f₅₆₀	4.450705	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	2.024e-06	H	Calculated from geometry of the main loop.
L₁₀₀	2.039e-06	H	Derived from the resonance frequencies f_OFF and f₁₀₀ deviation +1% vs L
L₅₆₀	2.062e-06	H	Derived from the resonance frequencies f_OFF and f₅₆₀ deviation +2% vs L
C_NIX	3.656e-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.

A was used as the transmitter: 10 W, FM.

HB9ISP Peter taking measurements.
The measurement loop is at the same height as the antenna and above the measuring tape lying on the ground.

Even small deviations in position cause large changes in the measured value, because points A and C are very close to the antenna and the field therefore changes rapidly.
At positions A and B, the field lines run exactly as expected from the theoretical free-space field-line pattern. At position C, the field lines are tilted slightly upward (orientation of the measurement loop at maximum measured value).

f = 14.072 MHz

tx_power_w	10.0
f_Hz	14072000
attenuation_cables_connectors_total_dbm	0.76 dB
tx_after_cable_w	8.4
I_main_loop_A	2.5
magnetic dipole moment m (Am2)	1.2

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	0.0	2.0	0.0	0.0113	0.0089	0.790
B	0.0	7.0	0.0	0.0010	0.0031	3.019
C	-2.0	0.0	0.0	0.0237	0.0246	1.039

f = 18.116 MHz

tx_power_w	10.0
f_Hz	18116000
attenuation_cables_connectors_total_dbm	0.83 dB
tx_after_cable_w	8.3
I_main_loop_A	2.5
magnetic dipole moment m (Am2)	1.1

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	0.0	2.0	0.0	0.0104	0.0120	1.145
B	0.0	7.0	0.0	0.0018	0.0033	1.851
C	-2.0	0.0	0.0	0.0267	0.0177	0.663

f = 21.253 MHz

tx_power_w	10.0
f_Hz	21253000
attenuation_cables_connectors_total_dbm	0.89 dB
tx_after_cable_w	8.2
I_main_loop_A	2.3
magnetic dipole moment m (Am2)	1.0

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	0.0	2.0	0.0	0.0095	0.0177	1.862
B	0.0	7.0	0.0	0.0022	0.0031	1.398
C	-2.0	0.0	0.0	0.0271	0.0173	0.639

f = 24.947 MHz

tx_power_w	10.0
f_Hz	24947000
attenuation_cables_connectors_total_dbm	0.95 dB
tx_after_cable_w	8.0
I_main_loop_A	2.0
magnetic dipole moment m (Am2)	0.9

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	0.0	2.0	0.0	0.0093	0.0180	1.937
B	0.0	7.0	0.0	0.0027	0.0039	1.459
C	-2.0	0.0	0.0	0.0256	0.0155	0.605

f = 28.604 MHz

tx_power_w	10.0
f_Hz	28604000
attenuation_cables_connectors_total_dbm	1.00 dB
tx_after_cable_w	7.9
I_main_loop_A	1.8
magnetic dipole moment m (Am2)	0.8

	X	Y	Z	expected	measured	factor
	m	m	m	A/m	A/m
A	0.0	2.0	0.0	0.0102	0.0132	1.303
B	0.0	7.0	0.0	0.0032	0.0058	1.816
C	-2.0	0.0	0.0	0.0249	0.0205	0.825

The measured field matches the free-space prediction reasonably well.
The largest deviations occur at point B.

Final Remarks

This homebrew magnetic loop is a very nice DIY build.

It is lightweight, simple, and easy to transport.
The self-made capacitor with CNC-milled plates looks excellent and works flawlessly.
The coupling loop was already nearly perfectly matched from the start and was kept unchanged for all measurements.
The efficiency is not outstanding, but for the small loop diameter and simple construction it is appropriate.
There are probably many similar homebrew antennas around the world.

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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.