Electronics > RF, Microwave, Ham Radio

Caps for RF transmitting loops, that handle power well?

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cdev:
I am building a loop antenna that I hope to transmit into in the future, so I am wondering about good fixed capacitors for RF at medium power. Best caps for power?

If I used fixed non-variable caps I can parallel it with a single variable cap to support a wider range. Ceramic, probably?



Preferably high Q. Mica? Vaccum? Ceramic? What are the best/cheapest caps for filters, loops and other high Q applications if you're on s budget?

(Unfortunately seems most vacuum caps are too high in price for me now, although its clear they are the best ones to use if you have them. ) :(

TimFox:
How much RF current do you intend to put through the capacitor?  In a "tank" circuit, the circulating current is higher than the input or output current of the resonant network.
There are ceramic and porcelain capacitors with wide ribbon leads, and sometimes a traditional NP0 non-monolithic disc ceramic capacitor, such as the (former Sprague) Vishay Ceramite will do, because of their relatively large area.
The power dissipated in the capacitor will depend on the current and the ESR, which depends on the Q of the capacitor at the resonant frequency.  At not very high frequency, the old (huge) mica capacitors  (for example, https://www.rfparts.com/capacitors/capacitors-transmitting/cm55-01-600.html) could handle large currents due to their large volume for dissipating the heat.

TheMG:
You must also consider that the voltage across the tuning capacitor(s) on a transmitting loop will be very high. It can be over 1kV with as little as 10W of transmit power!

Also the capacitor ESR and dielectric losses must be kept to an absolute minimum to maintain a high Q and thus achieve high efficiency.

The issue with using fixed capacitance with a smaller variable capacitor is that you will have limited tuning range. Might be able to get away with it if designing a monoband loop, but otherwise you'll need a much larger capacitance range.

On a budget, the most practical and functional is an air-variable capacitor. There are many ways of constructing air-variable capacitors yourself for relatively low cost. Air variable capacitors have low loss, high Q, can handle high voltages depending on the spacing between the plates, and can offer a very wide tuning range from a single capacitor (covering 3-4 bands on a single loop is achievable). Power levels of 100W or a bit more are achievable on a suitably constructed air variable capacitor, but the required plate spacing makes these quickly get unreasonably large if you're going for 500W, 1000W... that's where vacuum variable capacitors really shine.

Vacuum variable is the best of the best, especially for high power, but of course more expensive and you can't build one yourself.

cdev:
I wonder why vacuum variable capacitors are so expensive? It must be difficult to make them in terms of time or ... something..

The effect of a high -Q magnetic loop is hard to beat. Its my understanding that they beat out almost any other HF antenna and often their size is the most manageable especially for the lower frequency bands. If somebody has a high location they can really perform spectacularly well. They even work surprisingly well indoors.

I know that they often develop surprisingly high voltages across the variable capacitors. I have a huge collection of variable capacitors of many kinds and except for two that are not already in use none of them would be appopriate for use with a transmitting loop. I'd have to make one, which I agree would be the best way to go considering the cost. My problem as far as making the usual style is I dont have any CNC or similar metal fabrication gear. If I did I would try my hand at making a butterfly cap.


--- Quote from: TheMG on April 20, 2021, 06:21:16 pm ---You must also consider that the voltage across the tuning capacitor(s) on a transmitting loop will be very high. It can be over 1kV with as little as 10W of transmit power!

Also the capacitor ESR and dielectric losses must be kept to an absolute minimum to maintain a high Q and thus achieve high efficiency.

The issue with using fixed capacitance with a smaller variable capacitor is that you will have limited tuning range. Might be able to get away with it if designing a monoband loop, but otherwise you'll need a much larger capacitance range.

On a budget, the most practical and functional is an air-variable capacitor. There are many ways of constructing air-variable capacitors yourself for relatively low cost. Air variable capacitors have low loss, high Q, can handle high voltages depending on the spacing between the plates, and can offer a very wide tuning range from a single capacitor (covering 3-4 bands on a single loop is achievable). Power levels of 100W or a bit more are achievable on a suitably constructed air variable capacitor, but the required plate spacing makes these quickly get unreasonably large if you're going for 500W, 1000W... that's where vacuum variable capacitors really shine.

Vacuum variable is the best of the best, especially for high power, but of course more expensive and you can't build one yourself.

--- End quote ---

Some of them have quite complicated internal structures..  But still its not rocket science.

radiolistener:
for magnetic loop usually vacuum capacitors are used. As alternative you can use air capacitor with extended gaps between plates. Also air capacitor should not use moving mechanical contacts like it is used in air capacitors for a usual tube-receivers. For example you can use two sections connected in series through a rotor to avoid moving mechanical contact.

Vacuum capacitor provide you with better Q. But on the other hand very high Q leads to very narrow bandwidth, very sharp resonance and extreme high voltage and current in antenna. So, it will be more hard to tune and requires more expensive capacitor.

By the way, using magnetic loop for transmission is not safe, because it has extreme high field strength near the loop.

Just look at simulation for a magnetic loop at 14 MHz with a 100W power. The black line on the color grade is official safety limit.

As you can see, the magnetic loop has:
- E field strength more than 15000 V/m, while safety limit is just 130 V/m
- H field strength more than 60 A/m, while safety limit is just 0.34 A/m

For comparison, half wave dipole has 8 times smaller E field strength and 15 times smaller H field strength.

I don't recommend to use more than 10-15 W power with magentic loop. And when TX, keep away from antenna as much as you can.

Some years ago I was played with magnetic loops and it was a funny when you keep the gas-discharge lamp in your hands and it glows like it is plugged to the mains. It was just 10 W power and I had a many very painful burns on my fingers when tried to tune air capacitor. But later I read more about electromagnetic flux, antenna design theory and underlying physics and I came to the conclusion that it is not safe for the health. Especially if you use more than 10-15 W power with a shortened antennas.

Using a full size (half wave dipole) antenna is much more safe and give you better results. Full size antenna has much better efficiency , much less noises, it's more easy to match it and it has much more safe field strength near it.

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