I’ve been wanting to try a different aerial for 20m for some time. I currently either use a dipole in the loft, or a temporary SOTAbeams linked dipole on a 7m fibreglass mast. The magnetic loop antenna appeals because it’s compact, and if designed/built for efficiency, gives excellent performance.

They’re also expensive. One well-known UK dealer is selling the Ciro Mazzoni baby loop for £1299; another dealer sells the MFJ-1782X for £439; the Chameleon CHA F-Loop is available from a third dealer for £399. All are far, far outside my budget.

It’s not called amateur radio for nothing, and what I describe in this series of articles is truly amateur. I’m going collate my research sources, and try building a magnetic loop antenna (properly called a small transmitting loop antenna) using bits from my junk box, making good use of the things that I find, things that the everyday folk leave behind. I don’t have all the parts, but whatever I need to buy must not break the bank. Similarly, tools: I’ll need some, but they must be cheap 🙂

# Acknowledgements

Thank you to all my friends on Twitter: some amateurs, some not, who have made many helpful suggestions about this project, offered advice on construction, tools, hobbyist engineering, and much encouragement. Robin G7VKQ, Ian MW0IAN, Paul M0CNL, Michael G0POT, Suzy Richards, Leigh M5GWH, Steve G0LFP, Jenny G7CKF, Adam MU0WLV, David G7UVW.

Articles by Mario G8ODE, Steve AA5TB and Thomas KD6DED (links appear later in this post) were invaluable for this project.

# Requirements

Resonant on 20m, tunable from 14.050 MHz to 14.0956 MHz (CW to WSPR segments of the band, including JT modes, giving a centre frequency of 14.0728 MHz), initially for QRP 5W, but it would be nice to be able to “tune for maximum smoke” at my station limit of 100W. As the magnetic field strength of these loops is high, I won’t be standing anywhere near it at high power! 30M and 40M would also be nice to have. Cheap and easy to construct. Portable; on a stand that could go in my loft, or in the garden. Loft installation constrains the diameter of the loop to around 130cm; 1m for comfort. SO239 connector, for feeder, and probably for connecting different-sized loops. The stand should support the various size loops I’ll want to experiment with. Built with low ohmic losses to improve efficiency – although I don’t want to build a loop out of copper pipe at the moment, the design should not preclude that later. Stand and tuning system to accommodate manual tuning for now, with the addition of a motor-driven/reduction-gear/PWM controller at a later date. The moon, on a stick, cheaply.

# Design

For the basic loop design, I found A Simple RG213 Loop by Mario G8ODE, which provides excellent, detailed instructions on the design and construction of a loop made from RG213 cable (compatible with URM67). The design covers 30m-15m.

Off to an excellent start, as I have a reel of that, picked up cheaply from a rally several years ago! The Specifications of URM67 Cable are online, showing that the outer braid has a diameter of 8mm +/- 0.2mm. A solid loop made of copper pipe will undoubtedly be better, but I’ll try the stuff at hand first, as I have no pipe bending skills or tools.

The high Q of the magnetic loop means a very narrow bandwidth, hence the need to tune the loop with a variable capacitor which will have a very high voltage across its vanes – according to the G8ODE plan, greater than 2,300V at 30W RF.

In addition to the G8ODE plan, I’ve been looking at one of the first publications on magnetic loops, by Ted Hart W5QJR: Small, High Efficiency Loop Antennas from the June 1986 QST. Also of great help is Steve Yates, AA5TB’s “Small Transmitting Loop Antennas” page. Steve has an excellent MS Excel-based loop calculator.

By plugging the figures into Steve’s spreadsheet (Design Frequency 14.073 MHz, Loop Diameter 3.28′ (1m, for deployment in the loft), Conductor Diameter 0.315″ (8mm), no loss resistance (I doubt I’ll achieve that), RF Power 5W), I see that the loop would have an efficiency of 43.69%, which is – to use a technical term – pants. The loop is 14.743% of the design frequency, so is a little larger than a “small loop”. The bandwidth (-3dB points) is 22.5 kHz which is fine. A tuning capacitor of 41.684pF is required, and the loop has a circumference of 3.141m. The following power ratings, capacitor voltages and minimum plate spacings were calculated:

• 5W,  921.124V, 0.312mm
• 10W, 1302.665V, 0.441mm
• 20W, 1842.247V, 0.624mm
• 50W, 2912.848V, 0.986mm
• 75W, 3567.496V, 1.208mm
• 100W, 4119.390V, 1.395mm

For deployment in the garden, I can build a bigger loop. By doubling the diameter of the loop to 6.562′ (2m), efficiency increases to 86.109%. Much better. Loop circumference is 6.283m. The tuning capacitor is 18.256pF. The ratings at the extremes are:

• 5W, 738.152V, 0.250mm
• 100W, 3301.117V, 1.118mm

So, 1.4mm vane separation will do.

To achieve 90% efficiency requires a 2.265m diameter/7.115m circumference loop; 95% needs a 2.905m diameter; 99% requires 5.036m diameter. But that’s hardly a small transmitting loop.

# The Tuning Capacitor

I think that the tuning capacitor is the riskiest part of the project, as good quality variable capacitors suitable for transmitting – with a large gap between the vanes – are not impossible to find, but they are quite expensive. A Jackson Brothers 150pF TX5 capable of 6KV is available on eBay for £170.67. Undoubtedly this is a magnificently engineered component that will give no problems, and comes from a historic radio component manufacturer – see a history of Jackson Brothers and earlier Mainline Electronics articles for background.

## Research

So I’m considering building my own transmitting variable capacitor, wishing I’d done more workshop/metalwork at school in the late ’80s. I’ve considered a few approaches, but the cheapest and easiest approach seems to be to that taken by Alex PY1AHD, who designed and sells the Alexloop – see how to build a Homebrew Butterfly Capacitor. Basically cut out the stator/rotor vanes from thin aluminium sheet using straight aviation tin snips/hacksaw/Dremel metal cutting disks, sand the edges to prevent sharp points that could lead to arcing, and build the frame out of ceramic tile or perspex/plexiglass with readily-available nuts/bolts/threaded rod/washers/spacers.

An interesting article by Thomas C. Stevens, KD6DED called A Homemade High Power Tuning Capacitor from the June ’83 issue of QST gives many construction hints, essential formulae, and data on plate separation vs peak voltage (which don’t match values determined from the spreadsheet; the article gives higher separations than the spreadsheet). This is for a single-stator design. It states “The maximum capacitance of one section of a capacitor (two stator plates plus one rotor plate) is equal to the capacitance of this section at full mesh plus the minimum capacitance, which is about 10% of the maximum.” i.e. calculate the capacitance using the formula shown below, then 10% of that will be the minimum capacitance, and 110% will be the maximum capacitance.

Another useful article is Tech Notes – Variable Capacitors by Philip VK5SRP, which gives useful descriptions of the differences between the various forms, butterfly and split stator. Also see Homebrew High Voltage (5KV) plate capacitors by DL5DBM (single-stator), and the River City Amateur Radio Communications Society Magnetic Loop Project.

One homebrewer offering a kit of parts for transmitting butterfly capacitors is Vasile VA6POP, whose QRZ.com page shows off the kits on offer, with many pictures and videos. I’m tempted to ask him if he’s still offering the kits; how much they might cost, and how much it would cost to import one to the UK from Canada!

There’s an excellent video from Bob VE3UK at YouTube, showing his commercial assembly of a Variable Capacitor Butterfly Style Plates Great for a Home Brew Magnetic Loop.

The butterfly capacitor seems to be the best choice, since RF flows from one stator to the other without passing through rotor wiper contacts; the rotor is “cold”.

A very informative article on construction, illustrating the difference between split stator and butterfly capacitors, including details of calculating the capacitance, and the number of vanes required for a butterfly capacitor is Magnetic Loop Antennas by Tony ON4CEQ.

I am basing my stator/rotor design on that provided by Tony. I don’t agree with his calculation of the plate area, however. His design is shown on pages 11 and 12 of the above PDF.

The area of the vane (when fully meshed) is the quarter circle that forms each half of the rotor, minus a quarter of the square part about the central rotor axle.

So, for an 8cm tall rectangle, the radius of the circle will be 40mm. Given a 12mm2 central square part, I calculate the area of the vane to be:

$A = \frac{\pi 40^2 }{4} - \frac{12^2}{4} = 1220.64mm^2 = 12.21cm^2$

(Tony achieves a figure of 11.7cm2, but does not illustrate his calculation.)

The formula for calculating the capacitance of a parallel plate capacitor is:

$C = \frac{(0.0885 KA)}{t} (n-1)$

Where:

• C is the capacitance in picoFarads
• K is the dielectric constant (air = 1.0)
• A is the area of the plates, in cm2
• t is the thickness of the dielectric in cm
• n is the number of plates

Given the 8cm tall rectangle, 2 plates (a stator, a rotor), and requiring a vane separation of 1.4mm, this gives:

$C = \frac{0.0885 \times 12.21}{0.14} = 7.718pF$

With two stators and one rotor:

$C = \frac{0.0885 \times 12.21}{0.14} 2 = 15.437pF$

With three stators and two rotors:

$C = \frac{0.0885 \times 12.21}{0.14} 4 = 30.874pF$

And four stators and three rotors:

$C = \frac{0.0885 \times 12.21}{0.14} 6 = 46.311pF$

Which is around the value I need for the small loop (41.694pF).

That’s fine for a normal air variable capacitor, but what about the butterfly nature of the capacitor? It has a stator on each side! Agostino 1SF072’s article on loops and capacitors explains:

“A butterfly capacitor is electrically like a series circuit of two capacitors, and therefore it’s composed of two identical sections having twice the capacitance each. In a butterfly capacitor, the overall capacitance becomes half and so does the charging voltage in each section. That’s the reason butterfly capacitors can stand over twice the voltage compared to other types of variable capacitors.”

So, given what we know about capacitors in series, I’ll need twice the capacitance in the above calculation, as the end result is effectively halved.

So, choosing six stators and six rotors – essentially cut from six rectangles:

$C = \frac{0.0885 \times 12.21}{0.14} 12 = 92.62pF$

And divide by 2 to achieve 46.311pF.

I thought of moulding end plates out of ceramic, but that’s hard and I have no equipment. Perspex would be easier, but for that cheap-as-chips alternative, there’s an Ikea Kitchen Chopping Board for £1. I bought one of those last year for making dipole centre pieces and end insulators; still got plenty left.

Metal parts from the frame are all from Toolstation’s Nuts and Washers range, or also from Precision Technology Supplies, using all M5 parts. Threaded rod is available in 1m lengths, and the nuts and washers are all available at low prices in bags of 100. Cost of all the metal parts is about £10. Nice.

I considered using M5 spacers and ball races to reduce friction on the rotor, but have abandoned that idea. M5 spacers are hard to find, so I’m making do multiple washers as KD6DED did. I did find these on eBay, £2.95 for a pack of 5. That page also gives a useful table showing approx external dimensions for different threads. I’m using M5, which has an external diameter of 8mm. I need two such spacers: to pass through ball races in the end plates to ensure easy rotation of the rotor. A set of 10 ball races with internal diameter of 8mm is available via Amazon for £4.40. Bob VE3UK says no ball races are needed.

An initial sketch of the basic layout looks like this (click image to view full screen):

I’ll need some sheet aluminium from which I’ll cut the stator and rotor vanes; my capacitor design is not yet finished, so I don’t know how much I’ll need, but my tinsnips will cut a maximum thickness of 1.8mm, and I don’t want blisters – so a 500mm x 500mm x 0.9mm sheet of 1050A Aluminium Sheet from Aluminium Warehouse costs £3.55 (ex VAT and P&P).

Next: draw up the design for the stator/rotor plates, determine exactly how much aluminium I need, and and how many washers/threaded rods/nuts are required, order everything – and work out how to effectively drill 5mm holes with my Dremel 🙂