Author Topic: Caps for RF transmitting loops, that handle power well?  (Read 2638 times)

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Offline cdev

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Caps for RF transmitting loops, that handle power well?
« on: April 20, 2021, 02:19:51 pm »
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. ) :(
« Last Edit: April 20, 2021, 02:23:55 pm by cdev »
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Offline TimFox

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #1 on: April 20, 2021, 03:26:05 pm »
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.
 

Offline TheMG

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #2 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.
 

Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #3 on: April 21, 2021, 12:59:33 am »
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.

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.

Some of them have quite complicated internal structures..  But still its not rocket science.
« Last Edit: April 21, 2021, 02:06:02 am by cdev »
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Offline radiolistener

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #4 on: April 21, 2021, 08:53:15 am »
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.
« Last Edit: April 21, 2021, 09:33:46 am by radiolistener »
 

Offline A.Z.

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #5 on: April 21, 2021, 09:58:23 am »
I am building a loop antenna that I hope to transmit into in the future

A transmit loop antenna may be a nice experiment, but beware, the voltage at the ends of the top capacitor will be VERY HIGH, and if using the loop indoors you'll have a LOT of RF around you

Quote
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?

for TX loops, usually vacuum variable capacitors are preferred for a number of reasons, including the fact that, if the loop is placed outside, the capacitor won't arc due to a bit higher humidity in the air; never tried using fixed capacitors but given the voltages I believe that some "door knob" capacitors (see here for some examples) would fit; keep in mind that a tuned loop has a very HIGH "Q" so you'll also need to find a way to remotely tune it (and avoid getting burned when touching the variable cap shaft), again, it may be a nice experiment, but sincerely I'd rather use a multiband antenna like an OCFD and a separate, dedicated RX antenna which then may be a loop (RX ones are fine in my book)

Then ok, if you absolutely want to go for a TX loop, check this one



 ;D

« Last Edit: April 21, 2021, 05:06:33 pm by A.Z. »
 

Offline HB9EVI

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #6 on: April 21, 2021, 04:03:47 pm »
+1 for a vacuum cap; the smaller the loop and the lower the operating frequency, the higher the voltage over the cap; on the higher SW band, it might work with silver mica caps, but 160m on a 1m diameter loop no chance; but it's a rather unusable setup as well, efficency will be well below 1%
 

Offline geggi1

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #7 on: April 21, 2021, 07:29:48 pm »
I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.
 

Offline TheMG

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #8 on: April 21, 2021, 08:15:02 pm »
I wonder why vacuum variable capacitors are so expensive? It must be difficult to make them in terms of time or ... something..

Low demand, since they are used commercially in just a limited number of niche applications. This means they aren't mass produced, and thus cost a lot more.
 

Offline sidspop

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #9 on: April 21, 2021, 11:12:05 pm »
There is an interesting and informative video on Youtube done by a gent who used 22mm copper tube, plastic tube and 15mm copper tube to create a trombone, driven by a motor to slide in and out. A bit agricultural possibly, but works fine.


 

Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #10 on: April 22, 2021, 12:19:06 am »
Don't worry folks, I am not even licensed yet and when I am I fully intend to put any loops far away from my body, like in the back yard, hanging from a tree, or between two trees most likely. A safe distance even from my friends the squirrels.

The video is indeed of the guyI mentioned and his dual magnetic loop. And its a great idea, in terms of cost. I think building a variable capacitor is the way to go.

There is an interesting and informative video on Youtube done by a gent who used 22mm copper tube, plastic tube and 15mm copper tube to create a trombone, driven by a motor to slide in and out. A bit agricultural possibly, but works fine.



What did you mean by use of the term "agricultural" here? Reminding one of farm machinery perhaps? Inventive ?

The design is good because the "pea" of a magnetic look lke this one is likely to be very sharp, so the tuning is best when its got a reduction gear.

If ou are using  a standard "bread slicer" capacitor that means that the peak is likely to be so sharp you will just blast through your desired frequency in a second or less and will be unlkely to be able to hear the noise peak as it blasts by.

But with the long linear motion, that seems likely to be almost ideal. .
« Last Edit: April 22, 2021, 12:31:25 am by cdev »
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Offline radiolistener

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #11 on: April 22, 2021, 02:29:48 am »
I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.

I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.

Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
« Last Edit: April 22, 2021, 02:39:19 am by radiolistener »
 

Offline A.Z.

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #12 on: April 22, 2021, 12:17:14 pm »
check this

http://sm0vpo.altervista.org/antennas/frameant.htm

not perfect, but will get you on the air on 160/80 and since it's easy to put together, it will allow you to evaluate a loop and decide if you want to go on
 
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Offline M0HZH

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #13 on: April 22, 2021, 03:28:45 pm »
A typical "magnetic loop" antenna gain figure is -10dBd to -20dBd, for a correctly installed dipole (which achieves 8+dBi on the main lobe).

They are attractive when space is extremely restricted, especially on the 160m/80m/40m bands. However, on lower bands the resonance bandwidth becomes extremely narrow, to the point that it barely fits the SSB spectrum (2.5kHz or so) and tuning is absolutely critical.

On higher bands (20m and higher), better performance, simpler and much cheaper solutions (including shortened verticals or even random wires & tuner) are more viable.

They are mostly a novelty rather than a real solution, but they could work. The complexity makes it less than ideal for beginners.
 

Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #14 on: April 22, 2021, 03:49:51 pm »
That kind of coil is easy to make and works well. Multiple strands of wire definitely will mean higher Q.

check this

http://sm0vpo.altervista.org/antennas/frameant.htm

not perfect, but will get you on the air on 160/80 and since it's easy to put together, it will allow you to evaluate a loop and decide if you want to go on

I find the online calculators are not so accurate.
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Offline geggi1

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #15 on: April 22, 2021, 04:52:57 pm »
I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.

I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.

Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
The voltage is not only dependent of the TX power but also on the size of the loop. A larger loop will give a lower voltage. By making the loop as large as possible while still being a STL the voltage can be kept under the dielectric strength of the coax.
A fixed capacitor can also be made of a sheet of 1,6mm PCB material or two sheets of thin copper separated by a sheet of picture frame glass.
 

Offline radiolistener

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #16 on: April 23, 2021, 01:30:31 am »
The voltage is not only dependent of the TX power but also on the size of the loop.

The voltage depends on Q factor and the coil impedance.

Since half wave dipole Q factor is about 10, the voltage is multiplied by 10 times.
Since magnetic loop Q factor is about 2000, the voltage is multiplied by 2000 times.

Magnetic loop impedance is about Z=0.25 Ω. If you put 100 W into load with Z=0.25 Ω, you will get U=sqrt(100*0.25*2)=7 Vpk. And since LC has Q=2000, the voltage after 2000 cycles at resonant frequency will be about U=7*2000=14000 Vpk.

This is how magnetic loop works. It uses high Q at resonant frequency to accumulate oscillations amplitude to a very high voltage  in order to compensate low efficiency of a small size radiator. As a drawback it needs 2000 oscillations in order to radiate the same energy as antenna Q=1. This is why bandwidth of magnetic loop is reduced 2000 times.

And this is why full size antenna with lower Q is more safe and has less energy loss on heating.
« Last Edit: April 23, 2021, 02:05:10 am by radiolistener »
 
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Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #17 on: April 23, 2021, 07:11:46 pm »
The voltage is not only dependent of the TX power but also on the size of the loop.

The voltage depends on Q factor and the coil impedance.

Since half wave dipole Q factor is about 10, the voltage is multiplied by 10 times.
Since magnetic loop Q factor is about 2000, the voltage is multiplied by 2000 times.

Magnetic loop impedance is about Z=0.25 Ω. If you put 100 W into load with Z=0.25 Ω, you will get U=sqrt(100*0.25*2)=7 Vpk. And since LC has Q=2000, the voltage after 2000 cycles at resonant frequency will be about U=7*2000=14000 Vpk.


That's a very high voltage that I am sure would fry most home-made capacitors Ive built. How far a gap would that jump, I wonder. A lot.

Sometimes my loops when receiving a strong signal develop enough power to make the reciever completely lose its ability to differentiate signals from one another.

Quote
This is how magnetic loop works. It uses high Q at resonant frequency to accumulate oscillations amplitude to a very high voltage  in order to compensate low efficiency of a small size radiator. As a drawback it needs 2000 oscillations in order to radiate the same energy as antenna Q=1. This is why bandwidth of magnetic loop is reduced 2000 times.
And this is why full size antenna with lower Q is more safe and has less energy loss on heating.

What do you mean by "full size" ?

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Offline radiolistener

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #18 on: April 24, 2021, 08:06:43 am »
What do you mean by "full size" ?

The half wavelength
 

Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #19 on: April 30, 2021, 01:20:09 pm »
I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.

I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.

Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
The voltage is not only dependent of the TX power but also on the size of the loop. A larger loop will give a lower voltage. By making the loop as large as possible while still being a STL the voltage can be kept under the dielectric strength of the coax.
A fixed capacitor can also be made of a sheet of 1,6mm PCB material or two sheets of thin copper separated by a sheet of picture frame glass.
A sheet of plexiglass would make a good dielectric. I've seen some easy to make homemade caps made which probably cost almost nothing made with plexiglass and copper sheeting and or even tape. Have two sheets hanging down with bungee cords on the back to defaultto wide open.  The control of the capacitance can be adjusted by increasing or decreasing the amount of fulling on two pieces of parachute cord. (unlikely to stretch much and lose calibration. )
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Offline radiolistener

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #20 on: April 30, 2021, 03:07:10 pm »
A sheet of plexiglass would make a good dielectric.

As I know, fiberglass has smaller loss than plexiglass, but it depends on material.
« Last Edit: April 30, 2021, 03:08:46 pm by radiolistener »
 

Offline TimFox

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #21 on: May 01, 2021, 02:32:13 pm »
Other plastics available in sheet or plate form, such as polyethylene and polypropylene, have very low dielectric loss and high dielectric strength (but cannot be glued easily).
 

Offline langwadt

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #22 on: May 01, 2021, 03:19:42 pm »
why not glass?
 

Offline cdev

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #23 on: May 25, 2021, 08:00:19 pm »
Glass is probably a very good dielectric, with a material as good as glass the main determnant of the voltage capacity is the distance between the two conductors. The further apart they are the more voltage the capacitor can handle without flashover.

Glass might be very good.
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Offline tkamiya

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Re: Caps for RF transmitting loops, that handle power well?
« Reply #24 on: May 25, 2021, 09:48:34 pm »
Looking at eBay, there seems to be a good number of vacuum FIXED caps available.  For example, 100pF 25KV for $35 from Russia.  50pF 20KV from US is $75.  Doesn't seem to be excessively expensive.  Otherwise, I think one of more practical options are door knob capacitors.  They were often used in tank circuit on a very large transmitters. 

I don't think you ever gave a specific number.  What kind of power are you thinking?  QRP at 10 watts?  Or the usual 100 watts?  That makes huge difference in what's suitable and what's not.

Home brew approach isn't bad either.  Overlapping copper pipe with teflon insulation will work reasonably well.  Or, if power is low enough, air insulation will be even better.
 


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