Layers in a Resonant Cavity?

[kiutukri]

I want to build a Resonant Cavity to play with frequencies around 100-700 mhz. I'd like to try different shapes, so I was thinking in using a cheap malleable material like aluminum foil.

Questions:

- I think one layer of aluminum foil might be too thin.. If I paste a couple of layers will do the job?

- How to calculate the thickness?

Many thanks in advance.

 

[davenn]

I want to build a Resonant Cavity to play with frequencies around 100-700 mhz.

so you really want to build a whole bunch of cavities ?
you do realize that a cavity resonant at 100 MHz isn't going to be resonant at 200 MHz, 500 MHz or 700 MHz etc

Do you also realize how big a resonant cavity is at 100 MHz ?
just an approximate ... one for 146 MHz is around 3-4 ft long and ~ 5 inches in diameter

so I was thinking in using a cheap malleable material like aluminium foil.

So what were you going to put the foil against for support ? ( one layer from an RF point of view would be ok)
Brass tubes are the commonly used method for cavities. Have used 100's of commercial ones over the years
for VHF - UHF repeater installations


There's lots of design info on google ... have some fun learning about them


Dave

 

[kiutukri]

so you really want to build a whole bunch of cavities ?
you do realize that a cavity resonant at 100 MHz isn't going to be resonant at 200 MHz, 500 MHz or 700 MHz etc

Do you also realize how big a resonant cavity is at 100 MHz ?
just an approximate ... one for 146 MHz is around 3-4 ft long and ~ 5 inches in diameter



So what were you going to put the foil against for support ? ( one layer from an RF point of view would be ok)
Brass tubes are the commonly used method for cavities. Have used 100's of commercial ones over the years
for VHF - UHF repeater installations


There's lots of design info on google ... have some fun learning about them


Dave

Hi, Dave,
Thank you for your answer. I'll start building just one then I'll see.
Yes I do realize, that's one of the reasons I want to use a flexible material so I can fold it when I'm not using it. I'm planning to use wood as support, like a canvas frame.

What about my questions?

- I think one layer of aluminum foil might be too thin.. If I paste a couple of layers will do the job?
- How to calculate the thickness?

Thanks.

 

[marcusl]

Very thin. Search "skin depth" and calculate it--you can let us know what you find.

You'll have a much bigger problem with the seams, because of the insulating oxide layer on aluminum. Use copper foil instead and solder the seams to get good conductivity all around.

 

[davenn]

You'll have a much bigger problem with the seams, because of the insulating oxide layer on aluminum. Use copper foil instead and solder the seams to get good conductivity all around.

yup agree totally ... the inside of the cavity cannot have any breaks in the surface ... it must be electrically continuous

Yes I do realize, that's one of the reasons I want to use a flexible material so I can fold it when I'm not using it. I'm planning to use wood as support, like a canvas frame.

I really don't think that is going to be feasible. The cavities need to be sturdy because any variations will cause changes in the resonance of the cavity

Dave

 

[kiutukri]

Very thin. Search "skin depth" and calculate it--you can let us know what you find.

You'll have a much bigger problem with the seams, because of the insulating oxide layer on aluminum. Use copper foil instead and solder the seams to get good conductivity all around.

Hi Marcus,

Thank you for your awesome answer.

I've found this online calculator http://www.rfcafe.com/references/calculators/skin-depth-calculator.htm

According to this, using Copper, for 100 mhz I'll only need a skin depth of: 6.54 μmeters or 257.59 μinches. A thickness of 0.006543 mm sounds totally doable to me.

Even aluminum could do it: 8.19 μmeters, 0.00819 mm. The thickness of a standard aluminum foil is 0.016 mm

I am doing something wrong?

 

[davenn]

According to this, using Copper, for 100 mhz I'll only need a skin depth of: 6.54 μmeters or 257.59 μinches. A thickness of 0.006543 mm sounds totally doable to me.

Even aluminum could do it: 8.19 μmeters, 0.00819 mm. The thickness of a standard aluminum foil is 0.016 mm

yes, skin depth and the thickness of the aluminium foil isn't a problem and it isn't YOUR problem

Your problem lies in the last part of my last post, which you have yet to acknowledge


Dave

 

[kiutukri]

yes, skin depth and the thickness of the aluminium foil isn't a problem and it isn't YOUR problem

Your problem lies in the last part of my last post, which you have yet to acknowledge


Dave

Is there any way to calculate that?

 

[davenn]

Is there any way to calculate that?

why, there's no need ... you already have done so anyway

and as I said, it isn't the problem you need to solve ... you problem is going to be all mechanical