Horn Antenna for the 5 GHz and 2 GHz ISM Band

This is one of those things I built where I didn't really need it, I just wanted to see if I could do it. I was reading through the microwave section of the ARRL Antenna Book and I thought it would be really neat to build a pyramidal horn antenna. The only problem is, I don't have an ameteur radio license (yet), so I'd have to use the microwave ISM bands. If you're not familiar with the term, it stands for Industrial, Scientific, and Medical, basically a range of frequencies set aside for unlicensed use, originally for things like medical diathermy machines, microwave ovens, MRIs, etc... The ISM frequencies most people are familiar with are the parts of the S (2.4 GHz) and C (5 GHz) microwave bands used for WiFi, cordless phones, and other fun stuff. Basically what this means is that there's a lot of high quality radios with high data transmission capability, built in error correction and encryption, and with the right antenna, long range, that are widely available and really cheap.

Horns and Other Antennas

Commercial, carrier grade microwave links can and do span dozens of miles. AT&T used to use them for its Long Lines, and a variety of TV, cell phone, and utility companies currently use them for backhaul of bulk data. The most common today are probably the shrouded dish type antennas like the ones in my logo up there, along with the unshielded dishes which look a lot like satellite antennas. Another common form is the horn antenna, basically just a section of waveguide with a flare at the end. AT&T used to use a type of this antenna, called the Hogg horn, extensively. Although you're probably more familiar with them as the 'feed horn' of a satellite antenna.

A horn is a more directional and higher gain antenna than the Yagi type antenna commonly used for TV reception, and less directional with lower gain compared to a dish type antenna. They're the perfect choice for taking a 5 GHz WiFi link and extending it. A pyramidal horn has similar operating characteristics to a round horn, but it easier to construct.

Planning

As with any antenna, the dimensions of everything depend on the wavelength. Wavelength is that distance a wave travels in one cycle, so the higher the frequency the shorter the wavelength. First we need to know the wavelength (λ), which for 2.4 GHz (S band) is 125 mm, and for 5.5 GHz (C band) is 55 mm.

The Waveguide/Coax Adapter

A waveguide has two planes, the "E" plane and the "H" plane. I don't remember exactly where I heard this, but I believe the minimum H plane dimension necessary for a waveguide to conduct a signal is 0.7× λ. In rectangular waveguides the E plane is usually 2/3 the size of the H plane. Calculating this we get a minimum size of 90mm × 60mm if we want to support S band as well as C band. The length of the waveguide should be at least 1λ, so we'll make it about 130mm. The coax adapter will need to be placed ¼λ from the back wall of the waveguide, so go with 31mm for S band and 14mm for C band.

The Horn

An ideal horn would be as long as possible, with a very shallow flare angle, so our goal is to make the horn as long as possible without getting ridiculous. First we have to determine what kind of gain we need.  Now, let's say we're looking for a horn with 18 dBi of total gain in the C band, which is equivalent to 14.5 dBi in the S band. First we have to convert the decibel figure into a ratio, with the equation P₂/P₁ = 10^(x/10) where P₁ is the reference power, P₂ is the power being measured, and x is the measurement in dB, which works out to 64. The length of the horn in wavelengths, L in the diagram, is equal to 0.0654 × gain, in this case 4.1856λ, multiplied by the wavelength is 230mm. The H plane aperture, H in the diagram, is equal to 0.0443 × gain = 2.8352λ, multiplied by the wavelength equals 156mm. The E plane aperture, E in the diagram, should be 0.81 × H = 126mm.

Construction

I built mine out of sheet metal, I had some scrap glavanized steel laying around from a ductwork project, so I figured it would work out pretty well. The first thing I did was make a model out of construction paper (pictured) to get all of the angles right.

I wanted the horn to be one piece, with only one seam. Once I got everything worked out I used regular tin snips to cut it out, and a pair of seaming pliers and a board to bend it into shape. The whole thing is held together with three pop rivets. The waveguide was made in the same fashion, out of a single piece of sheet metal folded over and held together with six rivets. I attached the waveguide with two more rivets through a pair of tabs that I left sticking out of the waveguide. On the interior, I smoothed over the seam between the waveguide and the horn with some aluminum foil tape.

Although most of the nerds people who build these things use N connectors for the coax interface, I chose to use TNC connectors instead, since they're cheaper, smaller, and offer pretty much the same performance at these frequencies. The coax adapter is a standard female TNC bulkhead connector, with a ¼λ stub soldered to the center conductor. If you want a dual band antenna, you can put in two separate feeds side by side, one 15mm for C band and one 30mm for S band.

 I have a jumper that I made from about four feet of Belden 8214 RG8 (which is so chunky it could probably jump start a car) with a TNC connector at one end, for an antenna, and an RP-TNC connector at the other end for the access point. I've only hooked this up a couple of times, but the results seem promising. I set it up in my yard and walked over 100 meters away and the signal level was still good. I'll do some more thorough measurements in the future and update this page with my findings. If everything works well, I may even try to build a replica Hogg horn and setup a long range point-to-point link.

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