850 MHz Wireless Broadband Modem Antenna

[Geoffrey]

Recently I built and tested a home-made antenna for the wireless
broadband modem given to me by Telstra Bigpond (Maxon BP3-EXT). The
Bigpond software indicates signal level with five vertical bars. Using
the modem antennas (there are two, a primary and a secondary) signal
level was usually 3 bars,sometimes 2 (<3 is poor). With the new antenna
(fitted in place of the modem's primary antenna) level is always at the
maximum of 5. The higher signal level should give fewer packet errors
and therefore faster overall data transfer.
The antenna consists of two quad (full-wave) loops made from cupper wire
each connected in parallel and mounted above an aluminium reflector
plate. A soldering iron is needed to fabricated the wire and to connect
coaxial cable.
The design frequency was about 850 MHz based on the quarter wave
antennas used by the modem. Because of the reflector the quad dimensions
are smaller than those calculated for free-space conditions. The actual
dimensions were arrived at by testing with an RF Vector Network Analyser
(VNA)and the distance to the reflector adjusted for best SWR (1.3). The
impedance of the antennas was close to 50 ohm. The antenna was connected
to the modem via a metre of 50 ohm high-quality coax suitable for the
modem's SMA connector.
This antenna is not a new design. I have seen one made using two diamond
quads. My antenna is two square quads; equally good and simpler to make.
Email me if you are interested in looking at a drawing of the design and
a picture. I have drawings as .odg and .xsf files. If required I could
send a .jpg file.