High Voltage Butterfly Capacitor for Loop Antennas - TA2WK (TA1LSX)

by TA1LSX

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Intro: High Voltage Butterfly Capacitor for Loop Antennas - TA2WK (TA1LSX)

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: 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.

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.

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 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.

You can check whole assembly process from attached PDF file with pictures.

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

TA2WK (old TA1LSX), 73

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