"Plastic Fantastic" Magnetic Loop- Linear actuator drive

"Plastic Fantastic" Magnetic Loop- Linear actuator drive

Introduction

I have constructed a linear actuator drive version of VK5JST's trombone capacitor tuned "Plastic Fantastic" magnetic loop antenna for 20 m. The linear actuator drive considerably simplifies the construction and waterproofing.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.

Tuning the antenna while operating a SDR TRX is possible as the noise is visibly higher in a spectrum scope waterfall; Win4IcomSuite on IC-7300. The noise on the S meter rises from S2 to S6.
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The design

The antenna uses the underground plastic gas pipe now widely available, including Bunnings. The pipe has a PTFE inner, a layer of aluminium then an outer of yellow plastic. With the PTFE inner, 19  mm copper pipe can be used to create a trombone capacitor. See Tregellas 2017 for details of the design.

I have used a 200 mm linear actuator for tuning rather than the geared motor screw drive of the original. These cost little more than the geared motor arrangement, but simplify construction and waterproofing. The cost is about $47 for the 200 mm 12 V actuator.

The linear actuator is positioned so as not to push the trombone capacitor out; they have internal limit switches. However, the actuator will contract too far and needs an external limit switch at maximum capacitance.

https://www.ebay.com.au/itm/12V-750N-100-200mm-Linear-Actuator-Electric-Motor-Opener-Heavy-Duty-Lifting-AU/332524444416?epid=15013316556&hash=item4d6c005b00:m:mSjXeORzrUMEAZG23JbL-pw

The design is further simplified using a plastic chopping board as the main  mount, together with lots of zip ties.

The completed antenna

Detail of the linear actuator drive for the trombone tuning capacitor, mounted on a chopping board.

The coupling loop, which largely determines SWR.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed. I used a PWM speed controller from Jaycar at $35. They are available on eBay from about $5 and up.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.

Performance

The antenna produces a sharp dip in SWR across the 20 m band with the linear actuator working well.

Tuning the antenna while operating a SDR TRX is possible as the raised noise is visibly higher in a spectrum scope waterfall. I used my IC-7300 with the new Win4IcomSuite.

Unfortunately 20m has been very quiet during the day, just an odd bit of CW.

Conclusion

The "plastic fantastic" magnetic loop antenna works well with a linear actuator, provided its speed is reduced by a PWM speed controller. As a proof of concept, usable RX performance can be obtained, with some room for optimisation. It is a pity that 20m is so quiet at the moment to allow further on-air testing.

References

Jim Tregellas VK5JST, "The Plastic Fantastic: a Magnetic Loop costing around $54 for 40 metres" Amateur Radio (Australia) Sept 2017 pp 14-17.
http://www.ahars.com.au/uploads/1/3/9/8/13982788/plasticloop1.pdf

VK5KLT paper: http://www.ahars.com.au/uploads/1/3/9/8/13982788/article-antenna-mag-loop-2.pdf

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