Introduction


GSM Cell Phones in the US operate in the 1850-1990 MHz spectra.  Wireless Ethernet b/g operates at about 2.4 GHz.  Examples of building a BiQuad antenna for each frequency band are detailed below.  BiQuads are simple to build and can get 10-12 dB gain.  They can be mounted on common satellite dishes and get as much as 20-32dB gain.  BiQuads are appropriate and useful in rural areas, and are probably neither in areas well served by wireless providers. If you are an urbanite and want to isolate your wireless signal or add a little range to your existing gear, look here. 

WiFi BiQuad antennas are slightly smaller than the Cell phone antennas, but otherwise they are nearly identical.  Standalone BiQuads have about a 50 dg field of view alone, and when integrated with a parabolic dish, about a 4 dg view.   Standalones also should have a larger backplane-see the Trevor Marshall link below for details.

I have composed two paper templates to help in the fabrication of the antennas.  They are included them at the end of this document.   Print out a template, and verify it's dimensions by comparing the printed millimeter scale to a real ruler-shrink or expand the printout as neccesary to get it to scale.  Once you have the template sized, you can use it to measure all the components and make the BiQuad.  They look Way Too Big on your screen, but when printed at 300 DPI, the templates are the right size.

The antennas detailed below are well suited to support a "backhaul" point-to-point wireless channel linking distant buildings at 54 mbps.  The access points I've described support WDS, a repeater/mesh mode that allows access points to act as bridges while still offering roaming/wireless clients infrastructure support.  Since the antennas have such a small field of view, they are not well suited for wide coverage or supporting roaming clients over a large area.  To do that, you need a high gain omnidirectional antenna like the 32 slot "Slotted Waveguide" antenna described here.  They are a lot harder to build, but get about 15 dB gain across a 360 dg plane.

The BiQuad design as described here is not perfect.  These antennas are really designed for tweaking and easy manipulation.  If you have certain knowledge of the parabolic dish you are working with, you won't need the graceful hueristics offered by the sliding tubes I've implemented-you can just build to the right dimenstions with confidence.  Theoretical optimums suggest that one can get at least another 10-20% gain.  Consider these designs as B+/A- grade efforts that are relatively easy to build and manage, but by no means the best one could do.

Most of the dishes I've acquired lack the feedhorn, so I have had to guess at the dimensions and then tune the antenna.  If you have the original feedhorn and mount, you may manage to use it to design the BiQuad mount and skip all the tuning and hueristics.  Absent that,  I use PVC and copper pipes to mount the BiQuad on the dish feed arm.   Using the pipe design, one can slide the BiQuad towards/away from the dish, up/down on it's pipe post, rotate the antenna on the feed arm, and on it's stand pipe.  This gives you all the degrees of freedom you need but two (the relative angle of the backplane to the parabolic dish and the distance of the BiQuad from the backplane) so you can tune the dish easily.  The original angle of the feedhorn to the dish face needs to be replicated, and the distance between the backplane and antenna tweaked for the best SWR (Standing Wave Ratio) & gain.  The typical best SWR distance between the antenna and the backplane is 15-17 mm, but small changes can affect the antenna efficiency.

Caveat:  Microwaves will cook you, so you should minimise your exposure to them.  Use basic sense when you operate these devices.  Don't presume an antenna's radient behavior-it might not be typical.  You can test signal levels with a wifi portable.  Check the gain calculators to determine safe levels and distances.  If you turn up the power, stand farther away. Power off the transmitter before you stand in front of the dish or you might get cooked.  Mount them safely away from people. 



The first photo below shows a pole mount for the dishes.  They are more easily mounted on the sides of buildings and the like.  The poles are nice for consolidation and may be required for signal.  If you can use an existing structure, it is a lot less work.

Below left is a 2.4 GHz BiQuad antenna driving the wifi-G protocol.  The access point (grey box below center) is in WDS mode, doing 54 mbs on channel 6 across 250m (w/ 125m' winter forest).  The right hand is a cell antenna.   Note the difference in cable types.  The WiFi channel outruns my full duplex thinnet (10B2) by a factor of 3, while running in half-duplex mode (repeating packets from one client to another) across a span that previously required 2 repeaters and about .25km of Thinnet cable (with attendant ditching, lightning protection, maintenance, and 110v AC reticulation). 

a


The left hand dish antenna is an unfinished cell phone on a pole mount.  The righthand antenna is mounted directly to the building.
a  a  a  


A plain BiQuad antenna-this one gets about 10 db gain.
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Here are good references on BiQuad design and construction.

Trevor Marshall BiQuad

Biquad Construction

Building a Biquad Antenna

More Biquad Design

Trevor Marshall Antenna Design 101

Quad_Biquads

Wifi_Through_Woods

Bicircle Antenna


These links are pretty useful-you can detail all the components and distances in a Wlan, and have the signal levels and EIRP calculated-a good first step to staying legal.

Gain Calculator #1

Gain calculator #2


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