Lets Build High Voltage Butterfly Air Capacitor for Loop Antennas



Introduction: Lets Build High Voltage Butterfly Air Capacitor for Loop Antennas

About: I'm a licensed amateur radio operator based in Istanbul-Turkey

Hello Everyone,

Wanna build a magnetic loop antenna?

Magnetic loop antenna is a compact efficient antenna that is ideal for portable operation or limited spaces and can be improvised inexpensively.

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.

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.

If you don’t have a budget and if want to make a cheap loop antenna, you can make homebrew air capacitor yourself. Of course, although it does not have high capacitance values like the vacuum types, the last one I did was showing a value of 62-500 pF. Let’s see what is required if we want to do it ourselves.

Why Is a Butterfly Capacitor Better?

For high voltages and currents the use of Capacitors with wiper contacts is not recommended. That’s why they use capacitors in series. The pro for series’s capacitors is that the voltage rating is doubled. The anti is that the value of capacitance is divided by 2.
For the split stator capacitor the 2 capacitors are connected in series by the shaft (blue) and the red spots on the first drawing are losses. For the butterfly capacitor the 2 capacitors are directly connected in series by the rotors and gives less losses.

Step 1: ​Required Parts:

Laser cut butterfly style rotor and stator plates

Actually, these are the most important parts. Of course, the rotor and stator plates can be in different shapes, but I did the technical drawing as seen on first picture below. Because most of laser cutting shops do not cut plates from jpg images or hand drawing. They may ask for technical drawing in DXF or DWG file format. Instead you get get precise laser cut plates from following shops;

Threaded Rod (M5 type – 5mm)

The holes of the plates were set 5mm. Therefore, 1 meter of 5 mm (M5) metric 5 types of threaded studs should be used. Perhaps straight rod can be used, but fixing the plates can be a issue.

It is a cheap material that can be ordered from internet shops (see examples below) or from local hardware stores and DIY stores. If capacitor will stay in the outdoor environment a lot, you can choose a stainless steel one.

Where to buy


Aluminum Spacer, Unthreaded, M5 Screw Size x 10 mm OD x 5 mm ID x 6 mm Length

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 problems at high voltage.

For this, it is best to obtain regular sized aluminum type spacer parts. (M5 type 5-5.1 mm bore and 6mm wide)

Worst case you can use M5 5-6mm wide (~1/4″) nuts. In addition, 18-20 pcs M5 nuts are also required to attach the endplate panels.

Where to buy?

Planetary Reduction Drives

It would be good to fix it so that the middle shaft does not move back and forth, and more importantly it allows us to fine-tune it with a reduction ratio of 1: 6. However if you have difficulties to obtain this stuff like me, you can use alternative solutions to do so.

Where to buy?

I applied compression method from both sides of the panel. I used Openbuilds 5mm inner diameter clamps from Aliexpress. Maybe there are similar things that can be found in local stores.

Where to buy?

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. Then I designed 3D printable endplates with Fusion 360.

You can download STL files.

Step 2: How to Assembly

Step 1- First, let’s cut 4 pieces from our M5 threaded rod if you obtained 1 meter long or so, to install the stators. If you ask what size it will be, when I made 20 sets of plates, the 15cm were enough, it is 2cm more from the end. Also, let’s cut our middle rod a little longer by 5-6cm, because you need to insert a knob to turn it, you have to hold it by hand, or you may want to attach a motor.

Step 2- We will assemble the 4 pieces of threaded rods to the panels. If you are going to use 3D printed ones, install them by turning the rods while keeping the lower mounting side outside. If you get stuck, use pliers but do it with a rag, don’t hurt the teeths. Step 3- Next, let’s add an M5 nut to the bottom and install the edge rotator panels. Let’s not forget to add a spacer or M5 nut in between.

Then we will mount the rotor, the hub. We will mount it outside without attaching it to the panel beforehand, give some space at the bottom of the barrel (see picture above) and start by attaching the M5 nut, and when finished, add M5 nuts on the top and put them in a flat place and tighten all panels in order.

Step 3- Next, let’s add an M5 nut to the bottom and install the edge rotator panels. Let’s not forget to add a spacer or M5 nut in between.

Step 4- Slightly expand the hole of the bottom panel with a # 5 drill bit or something for easy insertion of the finished hub rotor assembly. Just expand and insert so that it rotates without forcing. Then fix the rotator plates to center the panels. Step 5- Before attaching the top panel, attach the planetery piece, (if you don’t have use the Openbuild compression clamps) to the hub rod. Also, install M5 nuts on the 4 corner rods for under panel tightening. To fit the rods you will need to slightly expand the 3d printed panel holes. Then install the panel and attach the M5 nut from the top and the Openbuilds bolted clamp in the middle and tighten it.

So how many plates you need to use for desired pf capacitance?
I suggest using following calculators.

With my butterfly design I use 80 mm plates and assempled as 8×9.8 cm, therefore I use 78.4 cm2 as capacitor plate area.

Thank you

TA1LSX, 73

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