How to troubleshoot why ath0 can't connect to a passwordless wireless network?

[Yuri]

I have a wireless network without password that my linux box easily  
connects to.

On FreeBSD 'ifconfig ath0 up scan' command shows it. 'ifconfig ath0  
ssid <my-ssid> up' brings interface to 'associated' state. But  
dhclient fails set it up.

I have another device on the same system: ral0. It connect to this  
network without problems.

What can I do to understand what may be a problem with ath0 in my case?

Yuri