Magnetic Loop Antenna Selfmade 1_9_heliax HB9ASB

Info: Mag loop made of Heliax 1-1/4 and variable vacuum capacitor.
Conductor: Heliax 1-1/4 corrugated coaxial cable. Approx. 36 mm outer diameter.
Capacitor: Variable vacuum capacitor 500 pF (Comet). For the 160 m band, two additional fixed-value vacuum capacitors are switched in.
Environment: Indoor, 3rd floor.
Thanks: Many thanks to Toni for sharing the measurement results and for the lively discussions on efficiency.

Antenna Efficiency Overview

Band 160m 80m 60m 40m 30m
Frequency f MHz 1.900 3.624 5.362 7.100 10.100
Intrinsic bandwidth Bint kHz 1.7 4.4 19.0 33.2 154.0
Source of Bint SWR = 2.62
Loop diameter D m 1.900
Conductor diameter d m 0.036
Loop count n 1 1
Inductance L H 4.83e-06
Capacitance C pF 1453 399 182 104 51.4
Unloaded Q0 1 1105 824 282 214 66
Damping resistance RT Ohm 0.0522 0.134 0.577 1.007 4.673
Radiation resistance RR Ohm 0.000405 0.00536 0.0258 0.0796 0.329
Loss resistance RLoss Ohm 0.0518 0.128 0.551 0.928 4.344
Power to antenna Pfwd W 200 200 200 200 200
swr_min_ant 1 1.00 1.00 1.00 1.00 1.00
etaSWR_ant % 100 100 100 100 100
Power antenna load Pload W 200 200 200 200 200
Antenna efficiency η % 0.775 4.02 4.47 7.90 7.05
Loop current I rms A 61.90 38.70 18.62 14.09 6.54
Loop voltage Uloop rms V 3569 4256 3030 3036 2005
Magnetic dipole moment m A m² 175.511 109.734 52.807 39.948 18.548
Link to calculator calculator calculator calculator calculator calculator

Environment

The antenna is located on the 3rd floor in a tall room beneath a sloped wooden roof with roof tiles.

Measurement Info

The SWR of the antenna was measured with a NanoVNA. The intrinsic bandwidth B_int is estimated to equal the B_SWR = 2.62 shown in the images below.

SWR measured with a NanoVNA at 475 kHz (630 m band). B_SWR_2.62: 1.18 kHz.
SWR measured with a NanoVNA at 1.9 MHz (160 m band). B_SWR_2.62: 1.72 kHz.
SWR measured with a NanoVNA at 3.624 MHz (80 m band). B_SWR_2.62: 4.4 kHz.
SWR measured with a NanoVNA at 5.362 MHz (60 m band). B_SWR_2.62: 19 kHz.
SWR measured with a NanoVNA at 7.1 MHz (40 m band). B_SWR_2.62: 33.2 kHz.
SWR measured with a NanoVNA at 10.1 MHz (30 m band). B_SWR_2.62: 154 kHz.

Final Remarks

Toni reports that the STL works well for NVIS contacts. Due to the surrounding mountains, low-angle radiation is limited.
Even on 160 m, SSB contacts as far as Paris or Berlin are not uncommon.
On 160 m, the SSB bandwidth is reduced to 1.5 kHz.
The STL performs similarly to an OCF dipole of 16 m length installed under the ridge of the chalet roof.

Many thanks to Toni for sharing the measurement results. It is a pleasure to be able to publish them here — even if Toni does not fully agree with the efficiency calculation approach used on this page.

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.