Low Cost microwave focusing

[geekodude]

Hello All:

Is there any commercial off the shelf microwave focusing apparatus from 5 to 1000W @ 2.45GHz? I may be dreaming here but I would like some sort of rod whereby I can focus the radiation onto materials. Any ideas would be appreciated.

 

[Higgler]

I don't think you'll find any commercially
(see the ps below).

I've done what you are looking for in medical research a decade ago.
I used a cable to a notch antenna radiating into an elliptical reflector to focus on a spot of chicken breast. We had approximately a 1" diameter area that was cooked on the boneless chicken breast. Power was from a microwave oven with rf power coupled off into a coax. using a coax. to waveguide transition. The coaxial cable went through a new door we made out of aluminum and added a type n f-f adapter through the door to a cable outside which connected to the notch antenna (which we also designed and built). Worked pretty well and we could change the transmit average power on the microwave duty cycle control and set the maximum power by moving the waveguide to coax adapter closer or further from the microwave source aperture.

to focus it into a smaller spot, you need to change air to dielectric. You could probably get down to a 1/3 inch diameter spot with dielectric = 10, but 0.1" diameter would cost an added $30K in ceramic high dielectric. The part would be probably 6" long, 6" diameter and triangular in shape, or look like a large spinning top a child would play with, except the bottom would be somewhat flat. There's a bit more complication too on the sides.

You can't just change the elliptical reflector larger or smaller to change spot size. Laws of physics says your medium must change from air to something denser.

What size spot do you need to make?

kch
PS: GTRI (part of Georgia Tech) has a lense system to focus a spot as you discuss, but it's really expensive and you get a 5-10" diameter spot at 5-10 feet away. It's for dielectric testing, etc.

 

[VEBill]

Focusing microwaves brings to mind the Luneburg lens. These are formed using layered hemispherical shells of dielectric material (often inexpensive foam, although the finished products cost a fortune). I'm not sure if there would be any problem with high power.

Another option is a simple dish reflector.

Either way - with 1kw - take precautions.

 

[geekodude]

Thank you both for the quick responses. For prototyping purposes, a 1 inch diameter will suffice.

But if anybody else can suggest another off the shelf solution, that would be great.

If not, I'll have to roll up my sleeves and do what Higgler did with his project.

Merry XMAS

 

[Higgler]

The 1 kw full power may be a challenge if you try to make your own antenna? You may want to purchase a waveguide to N transition as the antenna.

Our ellipse was cheap. Purchased a 12" diameter Globe of Earth, cut it in half right through Equador, cut 4 slits in it and changed the hemiphere into an ellipse. Added metal tape on the inside.

Here's an antenna option.

http://www.pasternack.com/Pdf/PE9824.pdf

Not sure about the power rating, not specified. Maybe look around and compare prices. WG Cutoff is 2.078 GHz, so it'll work at 2.45 GHz.
The next size larger adapter (1.7-2.6 GHz band) might be too large and increase your spot size.

Good luck,
Kevin

 

[VEBill]

If you need a dish reflector, then satellite TV dishes of various diameters (18" and up) are very cheap or even free.

Merry Xmas.

 

[Higgler]

Ve1BLL, I believe those dishes form a plane wave in space for far field gain. It'll take an elliptical shape to focus a foot away from the reflector I think.

He could maybe start with satellite dishes and modify them with added shapes though to get an elliptical shape. Although some dishes like DirecTV have offest reflectors, you only have the upper part of a parabola, hence making it an ellipse might not be doable.

Kevin

 

[VEBill]

Good point - thank you.

 

[geekodude]

Wow thank you folks! Can somebody please submit a block diagram of components needed. This in all aspects is a new thing for me. Anything will help. Any of your favorite references or books on the subject would be beneficial. I will start of with low watts for safety reasons.

 

[Higgler]

What is your application for this?

If you have small objects you want to just heat alot. Maybe the "focusing" requirement isn't necessary, and you can just concetrate your energy on your material simply.

i.e. Open your microwave oven and place you materials at the center of the RF power source for one. It will heat more from one side than another.

You could try to form an ellipse or hemisphere around the microwave power source so that all the energy stays confined and heats your material. Or attach a waveguide to your microwave oven and place your mateial in the center of the waveguide. That'll concentrate it. For higher concentrations, your normal waveguide could transition to ridged waveguide and all the power would be inside one small area.

This would be cheaper and faster than a semi-optical reflector type system. When I was working on our, we needed to head one spot inside an MRI medical imaging machine, so we needed minimum metal and couldn't use these heating options.



kch