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These Capacitors Are A Cheap Gimmick | Hackaday

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If you search for an electrical engineering textbook, you may not find the term "gi head capacitor", but every old ham radio operator knows them. They come in handy when you need a small capacitor of unknown value. For example, if you are trying to balance the stray capacitance in the circuit, you may not know exactly the value you need, but you don't know much. That's when you want a capacitor with one head.

A taking-head capacitor is made by twisting two insulated wires together; the length and tightness of the twist determines the capacitance. Tighten or loosen the twisted wire, or trim off some wires to make it adjustable.

Due to the small capacitance involved, it is usually common in radio frequency equipment or high-speed logic, and the number of twists per inch is usually about 1 to 2 pF. The thicker the insulating layer, the smaller the capacitance, so magnet wires or other thin insulating layers are usually used. You can take this step further and reduce the spacing by stripping one wire (as long as it doesn't touch anything else).

Obviously, the insulating material must be sufficient to withstand the voltage on it, which is an important consideration in, for example, tube circuits. But beyond that, a-head capacitors are a tool that you can easily acquire design skills. Can we go further?

You may be wondering if this technology can be applied to PC boards. The answer is yes. Unless you use a very thin circuit board or a thin layer on a multilayer circuit board, a small amount of capacitance requires a lot of board space. Similarly, typical PCB materials will become damp or have other effects over time. In fact, unless you use a special board material and thickness, it is not very useful. Layout work has been done

, But we are not sure how this will translate to PCB layout. We have seen many other PC tracking components such as antennas, shunt resistors, inductors and transmission lines.

You can see that I made a bigger head that is larger than two inches. Then, I went to the laboratory to find something that could measure such a small capacitor. The component tester cannot. I have a good digital multimeter with a special plug-in for measuring capacitors and thermocouples, but it cannot reliably read anything below 25 pF. When I realized, I was thinking about building a circuit to test


[Jonathan] The capacitance meter was exactly what I needed, and I even threw it on the Arduino that was connected using the Arduino Create Web interface. It was easy. I actually used the newer "Mark II" code, but for the low values ​​I measured, it works the same. I calibrated it with 10 pF ceramics of garden variety. It may not be that accurate, but I really only want to see the changes more than the actual value, so I think this is enough.

A two-inch (called 6 cm) head reading is about 5.5 pF. This may not be completely accurate, but I expected about 4.5 pF, and the magnet wire insulation is very thin, so it is in the right place. Let us use this as a benchmark to measure change. Then, I cut off about 1.5 cm of the capacitor—about 25%—and the reading became 3.7 pF. Another centimeter reduces it to 2.6 pF.

Of course, the spacing of the manual winding is not very accurate, and my cutting or measurement is not very accurate, but it is only about 1 pF per centimeter. Obviously, your results will depend on the type of winding and wire used. [Harry Lisar]

Fold a single wire, fix it with pliers, and twist it. Then, when you are done, you can cut the loop.

It is easy to forget that any two conductors that are close to each other will have capacitance. Another common temporary capacitor is a piece of coaxial cable, connected at one end and disconnected at the other end. For example, RG-8 is approximately 30 pF per foot of cable. There is even one

For any given value. Of course, it depends on the type of coaxial cable, so remember to cut it short and trim it!

Next time you need a small adjustable capacitor (especially in a laboratory environment), don't forget the head. If you want to try a larger value, be sure to try a different kind of wire. We have seen

. In the case of the head, you might think very little, but when you are really looking

, You can also make these.

The problem with this technology is that it is inconsistent in frequency and can only work at lower frequencies. At higher frequencies, it is a radiating antenna.

It may work in Ham World, but it is not intended to be used in any product that requires multi-band support or has GPS. The generated harmonics will eventually radiate and cause various problems with your high-frequency radio

For the sake of clarity, on-board tracking technology has been used. The twisted pair technology is rough.

It depends on your definition of higher frequency. Hams has been used for many years at VHF frequencies. Generally, if the frequency increases, the capacity will decrease and the wire length will also decrease. I think that in many filter applications, this distortion is self-shielding. Don't get me wrong, I won't put it in a production tool (although it is done). And, yes, passing high frequencies these days, it may not be because the wire has become 1/2 wavelength. But this is not the whole world.

Google VHF header capacitors, you will see some examples.


Do you think certain geometric shapes, certain shapes and shapes of winding wires can help you? Capacitance must only be affected by general conditions, board size, compactness and dielectric.

What if you take a certain length of twisted pair and wind it up? Connect the two ends of each wire together. The same amount of capacitive filler, but it will be very different compared to the antenna. I hardly understand RF. What about spirals? It's basically a way to make the antenna work on itself, and the head cap maintains a pleasant capacitive state.

> "At higher frequencies, it is a radiating antenna."

But since the "ground" plane is along the antenna, will the effective length of the antenna be short?

For example, take the dipole antenna and flip the other leg up in the same way. Isn't the radiation too big?

As the angle of the V antenna decreases to zero, the SWR rises to infinity. The SWR measures the matching degree between the source impedance and the antenna impedance. Ideally, it should be 1.0-the higher is, the less power is transferred to the antenna and therefore the less radiation.

This is not only useful at low frequencies. An RF engineer personally told me about this concept, he used to be a telemetry principle design engineer used in the Apollo project. I am the prototype technology of the microwave laboratory, working on the modulator of the large TACAN transmitter used for NATO base. When asked the RF engineer about the use of "gi head" caps, he added that such items have even been used in Apollo telemetry radios. Since then, I have encountered them in several types of RF test equipment. By definition, the resulting value is a few picofarads at best, so their usefulness is more effective at higher frequencies than at lower frequencies. Their emission characteristics are those of any twisted pair. Commonly used techniques to reduce the impact of emissions and other sources.

agree. When I was working on high-frequency equipment, I found that the same concept was actually designed in the PCB. I think this is a defect. I saw two magnet wires twisted together but cut at the top. Later I found that it was not. This was intentional. There is actually a schematic symbol, I think it is a twisted pair symbol, it has been a long time since I worked with them and saw it used.

How much inductance does such a long wire have?

I have never seen a capacitor with a capacitor head as long as the picture. They are short enough to stand up by themselves. They are not to simulate the value of a capacitor you don't have, but when you need a smaller value, it may be partly because you want to change it.

There is a well-known Hallicrafters shortwave receiver, the receiver's IF has a capacitor on the head, and the feedback from the plate to the grid gets some regeneration. Only a small value is needed, and two wires are cheaper than a trimmer.

Another place I have seen them is to feed the oscillator into another stage. You need a smaller value, and the wire makes it variable.

The change is just to bring a wire close to the input of the stage, but it may be physically unstable.


No, I may never use that long time. I just want something easy to measure and show the effect of shortening it.

Oh wow, that's it!

I have seen these very short twisted pairs on the old junk printed circuit board in the parts bin. I always thought that something was cut off at the other end of the line before it was scrapped. I never realized this was true.

Sadly, I have never bought these boards because they usually don't have what I want.

Neat article, thank you, just like the high-frequency system wiring, but also like passive components, I think it makes sense. I wanted to make a ten-year LCR box into a ten-year resistance box. I was thinking at the time that I must invest in standards and I must read more books. Among them, it is possible to perfect cheap LCR meters and use variable components through some kind of pre-calibration check. This would be another way of cheap material resources.

In both cases, you need to ensure that the electrical size of the capacitor is also small (less than 1/10 of the wavelength). At high frequencies, you will start to see the effects of the transmission line.

Transmission line stubs can be used to simulate capacitive and inductive components at high frequencies. Unfortunately, they are usually narrowband.

The ideas and commonalities of Kirchhoff’s law are unfortunate, unfortunately, because there are also comments elsewhere (a strange impression of capacitance, inductance and reactance in the entire EMF/EMS range), only high Impedance at frequency:

There is also some inductance, so don't expect to measure an a-head capacitor and replace it with standard components and get the same result.

Of course, if you want to enter the modern era, you can always use a digital adjustable cap: pSemi (formerly Peregrine Semi) has a very good dirt line (quantity 1, $0.75 each), which can be used in pF with SPI interface Level. Can be used in the GHz range. They are QFN (obviously), so you need to design the circuit board and have certain soldering skills, but they are not difficult.

I can go out and buy a chip with a precise capacitance that can be adjusted to within 50 fF (!), which is ridiculous, but hey, this is modern technology for you.

Using reverse-biased LEDs as variable capacitors is also an art. It works well especially for older people.

That's crazy. I just quickly searched for "digital adjustable capacitors" and came up with more manufacturers. Some of IXYS may be great for ham radios.

They all seem to have "low" resolution, such as 4-5 bits, but this may be enough to adjust the antenna matching circuit or dial in a rough filter cutoff frequency across the entire frequency band.

What a cool device! Thank you for bringing it up.

This seems interesting:

Wide capacitance range from 6.6pF to 37.553pF

0.063pF small step size

Digital selection of up to 1024 capacitance values


It is more complicated than the IXYS upper limit. This is not surprising, because stacking 10 or more capacitors in the chip will obviously give you a lot of parasitic effects.

Below 100-200 MHz, they may be ok, but on UHF, they are not. The capacitance range in the 70 cm frequency band is not 6.6 pF, such as 37.553 pF, which is approximately 6.6 pF to 70 pF, with gaps. See the diagram of the relationship between capacitance and control code on slide 8? That is low frequency. Do you know why you have to drop 4 big steps? Because at low frequencies, they will adjust the maximum capacitance value too small, so at high frequencies they will not be annoying (of course still bad), I am not sure what to use a capacitor with a Q value of 3 -5 Yes).

The frequency of the pSemi chip is more regular, especially as a shunt capacitor, until about GHz. The capacitance range of the 70 cm frequency band is still within 5-10% of the low frequency range.

The advantage of pSemi devices is that they have an equivalent model equivalent to the chip. Therefore, by connecting fixed capacitors in series or in parallel, the adjustment range of the capacitor can be easily controlled within the required range, although this is very important. Remember that if for any larger pads (ie, fixed capacitors/inductors/whatever), do not leave copper under the component pads, the parasitic capacitance will be very significant (e.g. ~pF level ).

For the ham frequencies reached via VHF, IXYS is acceptable, but for the 70 cm band, you do want to use pSemi. The Q value of pSemi DTC at high frequencies is 3-4 times higher.

When I was young, I made a crystal box with tin can capacitors. I put the aluminum foil on the kraft paper, wrapped it in Saralan paper, rolled it up, and then slid it into the tin can. You slide the paper in and out of the can to adjust the capacitance. The description is in one of the old Time-Life science books.

Do you use it for all tunings? Or do you also have a large wire wound inductor with a slider?

Wow, this is a long time ago. In retrospect, the inductor is an electromagnetic coil wound on a pencil. Or it could be one of those old Radio Shaek variable indicators, with magnets on long screws. It may be the former. I don't think I had enough money to buy something from Radio Shack.

We need a high temperature stable capacitor for the converter and fixed on a certain length of PTFE coaxial cable

Not exactly the same, but even in some very expensive kits, DIY covers are still used

"Work has been done using fractals to arrange linear capacitors on IC substrates, but we are not sure how this will translate to PCB layout. We have seen many other PC tracking components such as antennas, shunt resistors, inductors and transmission lines. "

You should study various numerical values ​​and analysis methods to simulate these small capacitance values ​​on the pcb. This is especially true at high frequencies. If accuracy is required, a numerical method is needed and the relevant properties of the material need to be specified. If you use a good method such as FDTD or FEM and visually observe the results (electric field/magnetic field), you will begin to understand that components such as inductors, capacitors, resistors are very similar at high frequencies, but differ only in the materials involved proportion. It takes some time and patience, but once it clicks mentally, it will be very beneficial.

Check out the open source OpenEMS (FDTD).

If you have a lot of money, take a look at HFSS or High Frequency Structure Simulator (FEM) for commercial use. (Faster).

"Of course, the pitch of the manual winding is not very accurate, and my cutting or measurement is not very accurate, but this can reach about 1 pF per centimeter. Obviously, your results will depend on the type of winding and wire used. [Harry· Harry Lythall] suggested folding a single wire, securing it with pliers, and twisting it. Then cut the loop when it's done."

Take the longer enameled magnet wire, fold it in half, clamp one end of the ring with a vise or pliers, and clamp the other "end" in the drill chuck. Slowly twist to the required twist density, but be careful not to damage the enamel. Cut off the ring end and strip the wire at the other end. Test with a capacitance meter. Calculate the required length so that it is slightly larger than the required capacitor length. The clamp end can be fine-tuned in the circuit as required.

It has been a while, but I seem to remember to notice the slot engraved on the ceramic disc cover to adjust its value.

Doesn't the slot of the ceramic capacitor provide a path for the arc? There are special capacitors.

But I have seen tips about using small ceramic capacitors and cutting them down to get smaller capacitors. But the inside is exposed to a humid environment. Maybe I have read that you can use epoxy to hide things. It's like applying for a resistor to get different values. This is possible, but only in emergency situations.

If there is memory. I believe the cut caps (grooving and trimming) were the lowest-priced RC cars in the 1980s. These are toys that the child will destroy within a week or two, so I guess that depending on the life expectancy of the toy, the life of the hat may not be important.

I only did some simple repairs, such as re-welding broken connections and melting/sewing any tiny cracks in the runner's plastic.

Then I gave them to friends with children.

I kind of miss the boxes in the radio shed!

Cordless phones and clock radios are becoming victims, usually designed for easy repair of broken PC boards.

Will the capacitance change if you bend the twisted pair?

I want to know if this is an inexpensive way to measure how much the robot's arm is bent....

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