iMac: unable to connect to local wifi router

The problem is being unable to wirelessly connect to a local wifi router, which has no encryption.  
The iMac does not see any nearby networks.  However, my windows notebook  sees 2 nearby networks, and is currently connected to the local wifi router.
The local router is a Linksys WRT54G.

When I click on the wifi symbol on the iMac, a menu is displayed:
Turn AirPort Off
No network selected (greyed out)
Other ...
Create Network ...
(checked) Use Interference Robustness
Open Internet Connect ...

What I want to see is a list of nearby networks that can be chosen, but have not yet been able to make that happen.

If I try to define a Closed Network with the proper name and no wireless security, an error box is displayed:
  "there was an error joining the Airport network "networkname".

If I try to define a new network, a "Computer-to-Computer" box is displayed.  The Name is already prepopulated with a descriptive name of the computer, and Channel is Automatic (11).

If I open Internet Connect, an "AirPort" box is displayed.  AirPort Power: On.  Network: No network selected.  Signal level is zero.  Base Station ID: Not Available.  (checked) Show AirPort status in menu bar.  Status: Not associated with any network, status not available.

OS is Mac OS X  Version 10.4.11
Processor: 1.6Ghz PowerPC G5

AirPort Card Information:
Wireless Card Type: AirPort Extreme
Current Wireless Network: wireless network not available

The Network Utility reports:
en1 is the wifi adapter
IP Address: (--> strange, because my windows wifi adapter gets

ifconfig shows the following for en1:
ether (mac address)
media: autoselect (<unknown type.) status: inactive
supported media: autoselect

Your help is greatly appreciated!

Who is Participating?
strungConnect With a Mentor Commented:
Try downloading and opening iStumbler and see if it shows any active networks. If not, my guess is that the antenna has come unclipped from your Airport card.

Which model iMac do you have?
ChrisEddyAuthor Commented:
The antenna cable was indeed disconnected.  And thanks for the tip on iStumbler, interesting and useful tool.  
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