Query about Tx Power

[David Goodenough]

I have a pair of stations, one in Master mode and the other in Managed mode.
On the Master I set the essid and the channel, on the managed I set the essid.

They communicate happily.

iwconfig report extreemly low txpower, but they communicate properly which 
is reasonable as they are close together,

I have another station, further away (the master has an omni aerial, the 
two managed direction yagis).  It is also a managed station, and iwconfig
reports a low power, and it can not hear the master station.

So I tried setting the power explictly on the master to the maximum that 
this card support (100mW) and promptly the two stations that could previously
talk to each other now can not.

How is power supposed to be managed?  Are you supposed to set it or do
all the stations try all available powers and fix on the one that is 
the least needed?  

Which then brings me to the next question.  I am only setting model,
channel (for the master) and essid.  Am I supposed to set others, or
will they default sensibly?  Also is there an order in which they
have to be set, and are any of them only setable before you start to
use the card for communications? Does this vary from card to card.

If it matters I am using Senao cards (NL-2511CD plus ext2) running
on a 2.4.20 linux kernel with the level 14 hostap driver.

Thanks for any guidance anyone can give

David