I don't know what commercial designs use, but you can simulate a 50 ohm
quarter wavelength line with a lowpass pi network (shunt C, series L,
shunt C). The reactance of each component should be equal to the Z0 of
the "line" (e.g., 50 ohms). This will behave just like a quarter
wavelength line at the design frequency, and will be a good
approximation over a typical amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input to be
grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that will be
messy. Commercial equipments just use an LC circuit. I am looking for a
pointer to the design of this cicuit.

Thanks,
Bernard