why is this inductor necessary?

Hi Guys
In TI CC2430 product, their chips have a differential output. They provides two solution for dipole antenna and monopoly antenna each.
For the dipole antenna, I am wondering why L321 in the attached pic? What is the purpose? ALso, why do they also need L331 to connect DC and middle of the dipole?

Second, for the monopoly antenna solution, can anyone explain the matching network? Basically I am wondering how can they get the network?

Finally, in those day, people are unlikely to use off-chip discrete components, what is the reason for that?
I am thinking it is because of the price. Also, even off-chip discrete component has better electrical quality , but it will be hard to model the connection parasitics. Are those the two reasons people try to avoid off-chip components?
Thanks

L321 is used to compensate some parasitic capacitor, to have a real differential impedance.
L331 is used as choke, from TX-RXswitch pin they bias the RF_N and RF_P pins, at least in TX mode.
The 2 lambda/4 sections acts as phase shifting the signal by 180 deg, the additional L341, C341 make some matching impedance to 50 Ohm, plus some additional filtering.

The reason of integrate as much as possible is usually the space and cost saving.

The problem of the parasitic you mention, is not so relevant. The design can be easily tuned and tested in many ways, so a lot of optimization is possible.

I hope it can help.

Mazz

Why do you need the choke inductor ?
To remove supply noise ?
Thanks

L331 is used as RF choke, from TX-RX switch pin so that you dont get RF going down into the TX-RX switch pin.
The TX-RX switch pin will have DC coming out of it to (as Mazz said) bias the RF_N and RF_P pins. You need to keep the RF out of that DC Supply.

cheer
Dave

Thanks guys, it is really helpful.