Many issues affect cellular signal strength including:

• Distance to the nearest cell tower
• Interference from nearby sources
• Frequency of your cellular carrier's signal
• Antenna placement
• Antenna gain & directionality
• Antenna cable length, number of connectors and other RF loss factors
• Sensitivity of your modem's radio
  

Some simple changes you can make to improve reception include:

• Remove any nearby sources of interference such as other radio equipment, CRT's, lights, motors, microwaves, TV's, etc.
• If you are not using an external antenna, make sure the 3G card's internal antenna is not enclosed in metal (e.g. the Card-Guard, an equipment cabinet, etc).
• Use an external antenna and place it outside or near a window.  Try the antenna in different locations - small differences in position can dramatically affect signal strength, esp. indoors where multipath signal reflections and other sources of attenuation and interference are common.
• Place the antenna as high as possible, but use the shortest possible coax length.
• Use a higher gain antenna.  Most "desktop" cellular/PCS/GSM/UMTS antennas under 6 inches provide from unity to 2 dBi gain.  Larger antennas can increase this to 5 - 10 dBi.
• Determine if your antenna requires an external metal ground plane.  Many magnetic antennas were designed to for optimal performance when attached to a flat metal surface.
• Install a cellular signal amplifier between the LAN-Cell and your antenna.
  
  

For more in-depth information on antennas, placement and amplifiers, please see our TechNote: Cellular Antenna below