Magnetic Loop Antenna Selfmade foil_pvc_0_71 F4WDO

Info: Mag loop made of copper foil glued to PVC pipe, with an air-variable capacitor.
Conductor: Copper foil 0.1 mm glued to a 45 mm PVC pipe. Joints were soldered with soft solder.
Capacitor: Air-variable capacitor, 4 mm spacing, 10-175 pF, unknown type or brand.
Environment: In the attic, in a wooden structure with brick walls, 1.86 m above floor.
Thanks: Many thanks to Richard F4WDO for the measurements, drawings, and emails.

Antenna Efficiency Overview

Band		20m	15m	10m
Frequency f	MHz	14.074	21.127	28.093
Intrinsic bandwidth B_int	kHz	41.3	83.6	189.2
Source of B_int		S-Parameters
Loop diameter D	m	0.729
Conductor diameter d	m	0.045
Loop count n	1	1
Inductance L	H	1.31e-06
Capacitance C	pF	97.6	43.3	24.5
Unloaded Q₀	1	341	253	149
Damping resistance R_T	Ohm	0.340	0.689	1.558
Radiation resistance R_R	Ohm	0.0266	0.136	0.429
Loss resistance R_Loss	Ohm	0.314	0.553	1.129
Power to antenna P_fwd	W	100	100	100
swr_min	1	1.86	1.58	1.05
eta_{SWR_ant}	%	90.9	94.9	99.9
Power antenna load P_load	W	91	95	100
Antenna efficiency η	%	7.10	18.7	27.5
Loop current I rms	A	16.35	11.74	8.01
Loop voltage U_loop rms	V	1895	2042	1853
Magnetic dipole moment m	A m²	6.827	4.903	3.345
Link to calculator		calculator	calculator	calculator

Build Details

The basic loop shape is an octagon with a width of 0.71 m from tube center to tube center.

PVC tubes are glued together. A 0.1 mm copper foil was applied to the tube and the joints were soldered with soft solder.
The two black cables are the connections to the rotary capacitor, which is mounted on the mast.
A neon glow lamp is visible near the upper center of the image. At the shown location, where the capacitor is mounted, the voltage is highest. The lamp is only capacitively coupled through the air and glows while transmitting.

Applying the copper foil was a fiddly job. The (AliExpress) 0.1 mm copper foil was surprisingly springy. The thickness was chosen based on skin-depth considerations.

The rotary capacitor with stepper-motor drive is mounted on the mast. A blue plastic spring element is used to compensate for alignment errors and provide electrical isolation.
The capacitor axis is at the midpoint potential of the capacitor. The voltage between the axis and the stepper motor is therefore not very high.

A nice rotary capacitor. Brand and type are unknown.
The stator packets at the upper left and lower right were connected to the loop. The axis with the rotating vanes runs from left to right. The axis is at rotor potential.
It is a 2-gang capacitor, 20–350 pF per gang, used in series as a 10–175 pF capacitor.

One half of the capacitor
Plate thickness	d_p	0.8	mm
Clearance	d_c	1.6	mm
Plate spacing	d_s = 2 d_c + d_p	4	mm
Stator plates	n_s	15
Rotor plates	n_r	15

Total air gap for both capacitors in series: 3.2 mm.
Rule of thumb for dielectric strength: 3 kVp/mm.
U_p = 9.6 kVp.
U_rms = U_p / √2 = 6.8 kV.

“The coupling is achieved with a small coupling loop at the top. The coupling loop is made from 3 mm diameter stranded copper mains cable (outer diameter 5 mm). Exact dimensions are difficult to specify because it has an awkward shape, but it is roughly 22 x 10 cm.
Ideally, the coupling loop would be placed at the bottom, but in practice this is not easily possible. It is not known whether routing the coax through the loop diameter has any negative effects.
There are 26 #43 ferrite toroids over the feed to the coupling loop as a choke, covered with heat-shrink tubing.”

Environment

The antenna is mounted on a parasol stand in the attic, in a wooden structure with brick walls, 1.86 m above the ground.

Measurement Info

The measurements were taken from the shack. The VNA was calibrated directly at the VNA. There was 15 m of LLF240 coaxial cable between the VNA and the antenna.
The equipment used was the 'original small NanoVNA' with NanoVNA-Saver v0.73.
Richard performed these measurements following my instructions. The sweep range was from 0.5 MHz to 30 MHz logarithmically, with 200 segments and 20,000 measurement points. This worked well, and my evaluation procedure worked perfectly.

VNA measurements

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

The calibration was done directly at the 'original small NanoVNA': 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 'original small NanoVNA' measurement.

File	model f0 MHz	model BSWR2_62 kHz	model alpha db	datasheet alpha db	model_tau ns	datasheet tau ns	SWR min	eta SWR
F4WDO_magl_10m_VALUES.py	28.093	189.2	0.705	0.601	64.91	60.00	1.05	0.999
F4WDO_magl_15m_VALUES.py	21.127	83.6	0.617	0.547	65.36	60.00	1.58	0.933
F4WDO_magl_20m_VALUES.py	14.074	41.3	0.525	0.481	66.29	60.00	1.86	0.885

The measured damping coefficient aplha and delay tau agree very well with the values estimated from the datasheet.

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

Smith

SWR

Values

F4WDO_magl_10m

model_f0	28.093	MHz
model_BSWR2_62	189.2	kHz
model_alpha	0.705	db
alpha datasheet	0.601	db
model_tau	64.91	ns
tau datasheet	60.00	ns
SWR_min	1.05
eta_SWR_ant	0.999

F4WDO_magl_15m

model_f0	21.127	MHz
model_BSWR2_62	83.6	kHz
model_alpha	0.617	db
alpha datasheet	0.547	db
model_tau	65.36	ns
tau datasheet	60.00	ns
SWR_min	1.58
eta_SWR_ant	0.933

F4WDO_magl_20m

model_f0	14.074	MHz
model_BSWR2_62	41.3	kHz
model_alpha	0.525	db
alpha datasheet	0.481	db
model_tau	66.29	ns
tau datasheet	60.00	ns
SWR_min	1.86
eta_SWR_ant	0.885

Final Remarks

This is a nice home-built antenna.
The large conductor diameter results in remarkable efficiency. It is great to see projects like this.

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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.