One of the characteristics of the lowpass pi network Roy suggested is
that it provides a DC path between input and output. (Well, it IS a
lowpass, after all!) That characteristic can be a blessing at times,
in that it lets you use the same DC bias current for both the TX path
series PIN diode and the RX path shunt diode. But some other
arrangement might benefit from DC isolation between the two nodes, and
you can get that by changing to a highpass configuration. You have a
choice of a couple of them using three parts: a PI configuration or a
T configuration. In both, the shunt branches are inductors and the
series branches are capacitors. So a PI gives you DC-grounded input
and output nodes, and a T gives you DC-isolated nodes. And you can
also use a T lowpass with series inductors and a shunt capacitor. In
each case, all the Xls and Xcs are equal to the resistive reference
impedance. In other words, for example, at 150MHz and in a 50 ohm
system, C = 21.2pF and L = 53.1nH. Each of the four configurations (pi
and T, lowpass and highpass) behaves at the design frequency
essentially like a 1/4 wave transmission line.

Cheers,
Tom

Roy pointed out to me in email that the highpass versions have a
positive phase shift, which is different behavior than a transmission
line. However, they do have the characteristic that they reflect an
open circuit as a short, and a short as an open, and an Ro load as Ro,
which is the behavior you want for the PIN switch. The phase shift in
that application should not be important. In general, both the pi and
T versions, and for both highpass and lowpass configuration, reflect a
load Z as (Ro^2)/Z, I believe, where Ro is the reactance of each
component.

Cheers,
Tom