High-Voltage DIY Butterfly Capacitor For Magnetic Loop Antennas

Magnetic loop antenna is a compact efficient antenna that is ideal for portable operation or limited spaces and can be improvised inexpensively. A typical magnetic loop antenna looks like:

Typically, magnetic loop antennas can be built from coaxial cable, hardline, solid copper or aluminum tubing or ribbon. These magloops also have a very narrow bandwidth, necessitating the use of a variable capacitor for tuning to the operating frequency. As voltages on the order of thousands of volts develop across the capacitor, air variables or vacuum variable capacitors should be used. To maintain the lowest series circuit resistance, the connections should preferably be soldered and a split–stator or “butterfly“ capacitor is preferred.

Our topic here is not to show how to make a magnetic loop antenna, but to demonstrate how to make a high–voltage butterfly–style air capacitor, which is necessary for use with high–power transceivers.

In amateur radio groups, you have seen a lot of discussions about these types of antennas and even low–power QRP devices with distances of thousands of kilometers. With enthusiasm just like me, you can find a few meters of aluminum or copper bars locally, bend them, and attach a variable capacitor similar to the one below, which can be found in old radios, and start using it.

However, those types of variable capacitors taken out from radios generally do a good job of listening, but when you transmit from the transceiver, they can withstand a power of 1–8 watts depending on their quality. Because the gap between the plates is very narrow, it will work if you use it on QRP radios. However, when you increase the output power, the voltage transmitted to the antenna increases, resulting in voltage jumps between the plates, which is not desirable.

Is there a solution for high wattage outputs? Of course there is. If your budget allows, there are vacuum type capacitors up to 15,000 volts that can be purchased from online stores or eBay. Most of them are Russian–made and very successful. Some of them have high capacitance values ranging from 20–1000 pF and can support up to 20,000 volts. However, they are expensive. There are cheaper ones with lower capacitance values that cost around 150–200 USD.

If you don‘t have the budget and want to make a cheap loop antenna, you can make a homemade air capacitor yourself. Let‘s see what is required if we want to build our own capacitor.

Required Parts:

How many metal plate sets you will need?

It actually depends on the combination of your loop antenna specifications and the desired HF band. I suggest VK3CPU Magnetic Loop Calculator to determine the capacitance range.

• KI6GD capacitor calculator (exe windows application) http://www.alexloop.com/capcalc.exe
• https://www.66pacific.com/calculators/capacitor-calculator.aspx

			Capacitance pF		Suggested Max.
PLATE SET (1mm thickness)	Spacer	Plate Spacing	min	max	Power	Voltage
5 SET	3mm/0.12″	1mm/0.04″		58	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		39	100 watt	4,300
	5mm/0.20″	2mm/0.08″		29	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	1	23	200+ watt	7,400
10 SET	3mm/0.12″	1mm/0.04″		131	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		87	100 watt	4,300
	5mm/0.20″	2mm/0.08″		66	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	5	53	200+ watt	7,400
15 SET	3mm/0.12″	1mm/0.04″		204	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		136	100 watt	4,300
	5mm/0.20″	2mm/0.08″		102	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	9	79	200+ watt	7,400
20 SET	3mm/0.12″	1mm/0.04″		277	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		185	100 watt	4,300
	5mm/0.20″	2mm/0.08″		139	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	12	110	200+ watt	7,400
25 SET	3mm/0.12″	1mm/0.04″		350	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		233	100 watt	4,300
	5mm/0.20″	2mm/0.08″		175	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	15	136	200+ watt	7,400
30 SET	3mm/0.12″	1mm/0.04″		423	50 watt	3,000
	4mm/0.16″	1.5mm/0.06″		282	100 watt	4,300
	5mm/0.20″	2mm/0.08″		212	200 watt	5,900
	6mm/0.24″	2.5mm/0.10″	19	163	200+ watt	7,400

How to assembly?

Step 1- Cut four pieces of M5 threaded rod for the stators. For the twenty set plates, 15 cm is enough; however, cut the middle rod a little longer, around 4–5 cm, as you will need to insert a knob to turn it or attach a motor.

Step 2-Take two stator rods and insert flanged nuts (or a regular nut and washer) leaving a 2 cm gap from the end of the rod.

Place the stator plates and spacers as shown in the picture below (using half of the stator sets), and secure them with the flanged nuts on top. Finally, tighten the nuts on both sides with pliers.

Repeat the same for the other side of the stator plates, the center rod, and the rotors.

Step 3- Take one of the end plates and insert the stator rods, positioning the lower mounting part outside as seen in the picture below. To fix the stator rods with the end plate, use washers and nuts together, or you can use flanged nuts; it doesn’t matter.

Step 4- Insert a regular M5 nut next to the flanged nut on the center bar. See the following picture.

Then carefully insert the center rod into the center hole of the endplate and insert a flanged nut, but keep it loose. The rotor and stator plates should align with that position, but we will fine–tune that while assembling the other side.

For the other side; Insert nuts+washers to the stator rods and align them. Insert a lock collar to the center rod and keep it loose for now.

Step 5- Place the other endplate.

Insert nut+washer to the stator rods and tighten all of them from both sides.

Insert another lock collar onto the center rod and you can move the center rod back and forth slightly to align the rotor plates with the stators. Then lock the lock collars from both sides using the Allen key (see the red circle below).

This is the result. 🙂

Some examples of finished capacitors the shared with us:

YouTube videos of ham radio operators making beautiful capacitors using TA1LSX butterfly capacitor plates or kits

• M0MSN Magnetic Loop Capacitor Build  https://youtu.be/dXrfOVt8xzU
• M0MSN Mike’s Youtube Channel – The “Capacitor” (Mike shares a lot of cool projects as well as nice loop antennas, I suggest you follow him)

Magnetic Loop Controllers and Motor Controller Systems

• Greg KF5N Magnetic Loop Controller Project (Fork from J.Pardum’s version) v2
• Jack Purdum‘s Magnetic Loop Controller Project https://groups.io/g/SoftwareControlledHamRadio
• Dave‘s (G7IYK) MagLoop Tuner Project https://www.instructables.com/Magnetic-Loop-Antenna-Automated-Tuner/
• EA7HVO‘s Magnetic Loop Controller (for 4 Antennas) https://www.instructables.com/Magnetic-Loop-Controller-for-4-Antennas/
• Loftur‘s (TF3LJ / VE2LJX) Automatically tune a Magnetic Loop Antenna Project
  • • Another Howto Guide (ON5IA)
    • Example video1
• Cheap Stepper Motor Driver Controller (Aliexpress)
• WiFi Controllers
  • WiFi Stepper Motor Controller by rehamradio.com  /  Video
  • Control a Stepper Motor remotely using WiFi to tune a Magnetic Loop Antenna
  • WiFi Stepper Project (DIY)

Some suggested magloop related sites;

• https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
• https://ki5aif.wordpress.com/2019/07/26/capacitor-selection-and-design-for-magloop-antennas/
• https://pa0fri.home.xs4all.nl/
• https://k1fm.us/2014/06/k1fm-mini-magnetic-loop/
• https://www.qsl.net/kp4md/magloophf.htm

Magloop & Capacitance Calculators

• VK3CPU Magnetic Loop Calculator
• 66pacific Capacitance Calculator
• 66pacific Small Transmitting Loop Antenna Calculator
• Alexloop Capacitor Calculator 

73, TA1LSX