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> Horn Antenna for the 5 and 2 GHz ISM Band
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.
As with any antenna, the dimensions of everything depend
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.
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.
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.