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High Voltage DIY Butterfly Capacitor For Magnetic Loop Antennas

TA1LSX Butterfly Capacitor

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, or solid copper or aluminum tubing or ribbon. These magloops also have a very narrow bandwidth, requiring 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 are used.  To maintain the lowest series circuit resistance, the connections are preferably soldered and a split-stator or “butterfly” capacitor is preferred.  

Disclaimer: If not all precautions are taken, Magnetic Loops can be dangerous.

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

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

However, those type 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 gap between the plates is very narrow. It will work if you are going to use it on QRP radios. However, when you increase the output power, the voltage transmitted to the antenna increases, so you will hear voltage jumps between plates. This is not good.

So, isn’t there a solution for high wattage outputs? Of course there is. If your budget is suitable, there are vacuum type capacitors up to 15.000 volts. You can buy these types of capacitors in internet stores, ebay etc. Most of them are mostly Russian made and very successful. Some of them have high capacitance values in the range of 20-1000 pF and support 20,000 volts. However, they are expensive. There are cheaper ones in the lower capacitance value range at the price level of 150-200 USD.

If you don’t have the budget and want to make a cheap loop antenna, you can make a homebrew air condansator yourself. Let’s see what is required if we want to build our own capacitor.

Required Parts:

Butterfly Capacitor Part List

1- Rotor and stator plates, i.e. butterfly plates

Actually, these are the most important parts. Of course, the rotor and stator plates can be in different shapes, but butterfly type is the most famous one. I did the technical drawing of butterfly plates as seen on first picture below. Because laser cutting shops do not cut metals from jpg images or hand drawing. They require a drawing in DXF or DWG file format.

   Butterfly Capacitor Plates

If you want to buy laser cut plates or complete kits you can get it from following stores (Aluminum, stainless steel) or drop an email.

2- Threaded Rod (M5 type - 5mm)

The holes of the plates are set to 5 mm. Therefore, 5 mm (M5) metric 5 types of threaded studs should be used.

5mm Gijon

Available from online hardware stores or from local hardware stores and DIY stores. If the capacitor will stay in the outdoor environment too long you can get stainless steel one. Brass studs may be preferred, but they are more expensive than stainless. This is a matter of budget.

3- Spacer (or Nuts) for plate spacing

Aluminum Spacer, Unthreaded, M5 Screw Size x 10 mm OD x 5 mm ID

We will use it to provide space between the plates. You will need 5 pcs for each set of plates. It is recommended that the space between the stator plates that we will lay against each other should be 5mm or idealy 6mm gap in order to avoid arcing at high voltage.

It is best to obtain regular sized aluminum type spacer parts. (M5 type 5-5.1 mm bore and 5mm or 6mm wide)

Alternatively you can use M5 4-6mm wide (~1/4″) nuts. Plus 18-20 pcs M5 nuts are also required to attach the endplate panels. You can also use M5 flanged nuts for endplates.

spacer or

4. Openbuild Style Lockcollars - ID:5mm

It is recommended to apply a method of clamping and fixing the center rod from either side of the front end plate. This part does good job if you don’t have planetery recuction drive.

openbuilds lock

If you can’t find this part even though it is more troublesome you can use M5 nuts and washer combination.

5- End Plates

When I make my first capacitor, I made the end plates by cutting the acrylic plexiglass, but it was a little difficult to match the stud holes. If you click on below picture, my initial build video will open.

When I decide to build another one, and I designed endplate in Fusion 360 that can be printed with 3D printer. I uploaded the STL files for free for download from Thingiverse or Cults3d

For those who would like to adjust lower/higher pf values I’ve added v2 design with adjustable stator stud holes. You can now adjust the distance of opposing stators.

 3d printing endplate 3d printed endplate 3d printed endplate

You can buy printed endplates from Etsy Shop: https://www.etsy.com/your/shops/SnowistCorner/

6- M5 Nuts and Washers (or flanged nuts)

m5nut-washer  flanged-nut

You will need 30-35 pcs M5 size nuts and washers. I suggest getting some flanged nuts instead. They all are cheap and easy to find.

How many metal plate sets you will need?

Actually it depends on combination of your loop antenna specification and desired HF band. I suggest using https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx calculator to determine capacitance range need and refer to following web sites and table for the required sets.

Capacitance pF Suggested Max.
PLATE SET (1mm) 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
Disclaimer: Measurements using the 6 mm spacer are actual measurements and made by me. Other measurements were calculated using the KI6GD Capacitor Calculator using various plate spacings. The actual capacitance range of the finished capacitor can vary depending on your build and the accuracy of your capacitance meter.
Plate spacing formula = (Spacer length – plate thickness)/2

For QRP operations you can use butterfly capacitor in parallel wiring mode to get x4 times capacitance. (Short the both stators and wire the rotor and stator)

How to assembly?

Step 1- Cut 4 pieces from M5 threaded rod for stators. For 20 set plates, 15cm is enough. However cut the a little longer around 4-5cm for the middle rod, because you will need to insert a knob to turn it, or you may want to attach a motor.

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

Then place the stator plates and spacers respectively as shown in the picture below (use half of the stator sets) and finalize that using the flanged nuts on top. Finally, tighten the nuts on both sides with the help of pliers.

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

Butterfly Stator Assembly

Cap build Cap Building

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

Butterfly Capacitor Assembly Step-3

Step 4- Insert a regular M5 nut on top of the one with flanged on the center bar. See following picture.

Step3a

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

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

Step3b

Step 5- Place the other endplate.

Insert nut+washer to the stator rods and tighten all of them from both sides. Insert the Crimp Spade Connectors to the lower stator rods (see the yellow circles in the picture below).

Step 5a

Insert other lock collar to the center rod and you can move the center rod back and forth a little to align the rotor plates to the stators. Then lock the lock collars from both side using the allen key (see the red circle below).

Step 5b

This is the result 🙂

Butterfly Capacitor

Some finished example videos can be seen in the videos below

YouTube videos of hams making beautiful capacitors using TA1LSX butterfly capacitor plates or kits

Some suggested magloop related sites;

If you have some suggestions to improve this, I would appreciate it if you let me know. You can write in the comment section below or drop an e-mail [email protected].

73, TA1LSX

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