Horn Antenna for 2.45 GHz Microwave Oven Magnetrons

Overview

When constructing EMP, HERF, or radar devices based around microwave oven magnetrons, it is often useful to have a horn antenna on the antenna's feed to provide the proper free-space impedance matching and parabolic dish illumination.  Not having a good antenna impedance match or dish illumination will often result in lower overall antenna gain and wierd sidelobes in the antenna's radiation pattern.  Now, the chances of finding a commercial feed horn at a hamfest are going to be very low, so your best bet is to try and make one yourself.

A fairly high-quality, medium-gain, pyramidal horn antenna operating at 2.45 GHz isn't as hard to construct as you may think.  All the materials are available at most hardware or hobby stores.  The horn antenna's layout dimensions, equations, and construction techniques are available in Chapter 18 of the ARRL Antenna Handbook.  Since the ARRL can go fuck themselves, a copy of the handbook's section entitled "A Horn Antenna for 10 GHz" will be included here.  The design is easily scaled to other frequencies.

Read the article first to get a good idea on what you'll need to do for laying out and marking the four main pieces of the horn.  The article's instructions are kinda confusing, but it will start to make sense if you layout everything onto a large piece of cardboard.  Two of the horn's sides will need to be "a little longer" to allow soldering on the outside seams of the horn.  This is to help improve the horn's electrical specifications when operating at microwave frequencies as solder is actually a poor conductor.

This horn antenna will be made from a 4" x 10" piece of 0.008" thick tin sheet stock (K&S Engineering #254).  While this particular tin sheet is a little too thin for proper mechanical stability, the ease of soldering will more than make up for it.  You will also need to salvage the little "magnetron-to-waveguide" assembly often found in most older microwave ovens.  This will be used to mount the magnetron and to also allow for some custom L-brackets which the horn will attach to.

When mounting the horn on a parabolic dish, the horn should be adjusted so that the focal point is just a little bit inside the horn's main opening.  To reduce the possibility of sidelobes, the focal length should also be a whole number of 1/2 wavelengths.  This keeps the proper phase relationship with the "direct" and "reflected" signal coming from the feed.  Horn antennas are naturally wideband, and should work over the entire frequency range of the waveguide feed.  It should also be able to handle all the power you can throw at it.  A piece of polystrene from the hobby store can be glued to the horn's opening to make it somewhat water resistant.

2.45 GHz Horn Dimensions

One Wavelength at 2.45 GHz : 4.82 inches
          Target Horn Gain : 14 dB 
     Gain as a Power Ratio : 25

                "L" Length : 7.88 inches
                "A" Length : 5.34 inches
                "B" Length : 4.33 inches
                "S" Length : 2.88 inches

       Waveguide's H-Plane : 3.50 inches
       Waveguide's E-Plane : 1.70 inches

Construction Notes & Pictures

Magnetron waveguide assembly and aluminum L-brackets.

The waveguide assembly was salvaged from an old microwave oven.  Use a chisel and a hammer to break the spot welds holding the waveguide to the microwave oven's chassis.

The waveguide's internal dimensions are 3.5 inches wide (H-plane) and 1.70 inches high (E-plane).

The L-brackets are made from 1/2" aluminum angle stock.  The brackets will be used for mounting the horn antenna and can also "square up" the opening of the waveguide in case it was not cut straight.

Temporarily attach the four L-brackets to the waveguide as shown.

This part is quite critical, as the waveguide opening will need to be square, without any gaps to prevent dangerous RF leakage.  Drill a series of evenly-spaced holes for the mounting screws.  Be sure the mounting screws to interfere with each other.

Alignment of the L-brackets for the final horn mount.

Note how the top bracket was used to square the waveguide opening as it was cut a little crooked.

Use a belt sander to further square up and debur the L-brackets.  Mark each bracket and waveguide side with an engraver when disassembling.  This will help keep everything in alignment when construction is finished.

Layout and cut the tin stock.

Layout the dimensions per the ARRL Handbook article.  You should layout and mark everything ahead of time on a piece of cardboard to practice with.  Move onto cutting the actual tin when all the dimensions appear O.K.

A digital calipers and carbide scrib will be really useful for doing the layout marking.  A pair of heavy-duty scissors can be used to cut the tin as it's very thin.

Align the four main pieces that make up the horn itself, being sure the side that attaches to the waveguide is mechanically solid and square.

Tack solder the corners to hold everything in place.

Note how crooked the horn's opening is.  This can be trimmed down by gently placing the horn on a belt sander.

Cut a piece of tin for mounting the horn to the L-brackets.

Use a nibbling tool to cut out the center opening so it matches the internal dimensions of the waveguide.

Using a combination of attaching the mounting screws and soldering, solder all the outside seams around the horn antenna.

The mounting hardware will be used to "square up" and secure the thin pieces of tin while you solder them in place.

Again, don't worry to much about the large opening of the horn, just concentrate on making a good, RF-proof seal between the waveguide and the horn's mounting plate.  There should be no gaps in the seam and no protrusions sticking up into the waveguide's opening.

Horn and waveguide internal view.

The L-brackets were secured to the waveguide assembly using #4 stainless steel hardware.

Use a small file to trim the inside of the horn and the lip of the horn-to-waveguide transition.

The right-panel came out a little lopsided...

Finished view.

Painted black as the tin is very shiny and can rust.

The finished tin horn isn't very mechanically rigid and will easily bend or dent, so you'll need to be careful handling it.  Brass or copper sheets can be used for the horn's construction instead.  This will increase in horn's strength and maybe even the electrical performance, but with an increase in cost.

When used with a 800 watt, 2.4 GHz magnetron, the effective radiated output power with just this horn will be around 20 kilowatts.