High Voltage DIY Variable 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, 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.  

Our subject here is not to show how to make a magnetic loop antenna, but how to make a high-voltage air variable capacitor, that is required to use it with high output power transceivers (Lets say with over 10 watt and higher)

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 is there no solution for high watt outputs? There are of course. If your budget is suitable, there are vacuum type capacitors, for up to 15,000 volts. You can find these types of capacitors in minternet market mostly Russian made and they are very successful. Some of them have high capacitance values in the range of 20-1000 pF and capable of 20.000 volts. However they are expensive. There are cheaper ones with less capacitance value range, in the 150-200 USD price range.

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.

Required parts:

1- Laser cut 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 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.

If you are wondering about how many plate sets you need, let me give you an example. I have achieved a capacitance of 380pF with 15 sets and 500pF with 20 sets.

2- 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 problem.

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 type.

3- Spacer

Aluminum Spacer, Unthreaded, M5 Screw Size x 10 mm OD x 5.1-5.3 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 gap 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 5.1-5.3mm hole 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 tighten the panels.

 or

4- 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.

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

5- Side Panels

For the first capacitor I made, I made the panels by cutting the acrylic plexiglass, but it was a little difficult to match the bolt holes. If you click on it, a video will open.

That’s why the second time I did it, I used my own 3D design panels that can be 3D printed. I uploaded the STL file free for download from Thingiverse or Cults3d

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 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.

The finished version can be seen in this video

Capacitance measurement (62-498 pF) with 20 sets – 6mm plate spacing

Capacitance measurement (9-100pF) with 5 sets – 6mm plate spacing

73, TA1LSX

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