Magnetic Loops

1. Design Rules of Thumb
2. Calculators
3. Servo Controlled Loop
4. The Plastic Fantastic Magnetic Loop - Jim Tregellas VK5JST:
5. Building a Variable Capacitor
6. Papers from Adelaide Hills Amateur Radio Society Inc (AHARS)
7. VK5SFA's 160 Metre Transmitting Magnetic Loop Antenna Design By Steve Adler vk5sfa@wia.org.au
8. Gas Pipe
9. VK4ZPU's Mag Loop Capacitor
10. DL5DBM's 5KV 220pF Capacitor
11. Frank Dörenberg's Amateur Radio Pages
12. Leigh Turner VK5KLT - 7th July 2009
13. Auto Tuner

1. Antennas/My-Antennas/My-40m-MagLoop/My-40m-MagLoop.html

Design Rules of Thumb

Firstly, I want to understand the ground rules for building transmitting Magnetic Loop Antennas. Starting form relative ignorance I have been reading widely in an attempt to assemble a list if Rules of Thumb.

1. Magnetic Loops are naturally inductive so they are tuned to resonance by Capacitors
2. Magloops are DANGEROUS with voltage around 5000 volts
3. For highest efficiency, the conductor length for a small transmitting loop antenna should be greater than 1/8 wavelength
4. To avoid self-resonance, the conductor length for a small transmitting loop antenna should be less than 1/4 wavelength
5. Loops are best designed for a single band
6. Losses decrease inversely proportional to tube diameter. Double the diameter means half the loss. This is irrespective of loop size.
7. The coupling loop can overlay or under lap by about 5% of the outside the main loop.
8. Adjust the SWR by adjusting the overlap/underlap or by squashing the coupling loops circular shape.
9. For reasonable efficiency, the loop circumference should lie between 10-20% of the operating wavelength
10. The SWR bandwidth is about 15 kHz wide for any giver capacitance setting.
11. Tuning is very sensative to capacitance change. Eg. For 7 to 7.3 MHz a capacitance change of around just 8% is required or about 115 pF.
12. 300mm of 20mm copper in 25mm gas pipe gives about 200 pF.
13. When using gaspipe the aluminium layer must be removed for 25mm from end of loop to prevent flashover.
14. It is desirable to put the turning capicitor where the voltages are the highest at the top. This miniminises losses through ground capacitance leakage.

Calculators

Servo Controlled Loop

Following is the tutorial of a DIY magnetic loop antenna for the amateur radio band featuring a remote tuning system implemented on Arduino with a RC servo. :

The Plastic Fantastic Magnetic Loop - Jim Tregellas VK5JST:

Building a Variable Capacitor

Papers from Adelaide Hills Amateur Radio Society Inc (AHARS)

VK5SFA's 160 Metre Transmitting Magnetic Loop Antenna Design By Steve Adler vk5sfa@wia.org.au

This project is described in great detail be Steve Adler, VK4SFA and would make a great advanced stage project. It has a low RF take off angle and is efficient at 160 meters. At over 3 meters in diameter it is a challanging and potentially costly project.

Gas Pipe

The Plastic Fantasic Magntic Loop specifies Gastite™ as supplied by Bunnings. I soon found out that Bunnings no longer stocks this, at least locally, and that most plumbing suppliers have an almost identical product under their own brand name.

Gastite™ pipe is constructed from layers of aluminium and cross-linked polyethylene pipe and is impenetrable from gases/oxygen. The structure and design of the pipe compensates for snap-back forces and linear expansion caused by temperature changes. The system is simple, safe and pipe installation is fast. Pipe is easy to bend by hand, cut, chamfer, ream and crimp.

• Type of pipe: B1
• Pipe Colour: Yellow
• Material: PEX / AL / PEX
• Applications: Gas Supply
• Service Temperatures: -40C / +95C
• Max. Working Pressure: 10 Bar

I selected AUSPLEX gas pipe 25mm diameter and it came in 5 meter lenghts. I coiled it as large as possible but smaller enough to fit in the rear of the van.

The specification printed on the gas pipe.

I took along a offcut of 20mm copper pipe to test for size and it slipped inside the gas pipe with a snug fit.

The gas pipe cost me a little over $50.00 on February 2023.

VK4ZPU's Mag Loop Capacitor

VK4ZPU — 02/01/2023 9:22 PM

LDF4-50 coax is good for mag loops. By keeping the outer sheath on the heliax keeps its shine which is really good for minimizing losses when there are high ciculating currents and remembering that the currents are on the outside of the solid conductor due to skin effect.

I made up a loop, it was quite big and I made a frame from water pipe (see attached pic). I also made a tuning unit which used an air vane capacitor, a 12vDC motor which was highly geared. I still have these. See attached pic. The spray can in the pic is to give you perspective on sizes. It is really BIG.

Mounting the box on the frame was difficult but for a while it worked and one night I joined a DX net on 20m and got some good reports on 20m.

DL5DBM's 5KV 220pF Capacitor

Frank Dörenberg's Amateur Radio Pages

There is a wealth of information and references in these articles on MagLoops. For example, this diagram shows the voltage distribution around a MagLoop. The voltage are highest nearest the capacitor and it is therefore the most dangerous area.

Leigh Turner VK5KLT - 7th July 2009

Auto Tuner