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, 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 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 do it ourselves.

Required Parts:

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.

• KI6GD cpacitor calculator http://www.alexloop.com/capcalc.exe
• https://www.66pacific.com/calculators/capacitor-calculator.aspx

How to assembly?

Another quick video showing the butterfly capacitor 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.

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)

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

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 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 the Crimp Spade Connectors to the lower stator rods (see the yellow circles in the picture below).

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).

This is the result 🙂

Some finished example videos can be seen in the videos below

Some suggested magloop related sites;

• Magnetic Loop Antenna Project https://sites.google.com/site/rcarcs/magloop  
• 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

73, TA1LSX

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