I'm gathering from what you've posted that you don't have many
design/analysis tools available. Have a look for "RFSim99"--a Google
search should find it for you. It's freeware. It will let you make a
model of many linear RF circuits quite easily and get a decent feel for
what to expect in the way of performance. You can model a PIN diode
quite reasonably as a low-value resistance when "on" and as a
capacitance when "off". The data sheet(s) for your diode(s) should
list reasonable values to use.

Then you can easily see what sort of isolation you'll get versus
frequency, for example, and get an idea about the losses. If you have
any question about losses in inductors, estimate their Q at the
operating frequency and put in a resistor to account for that.
Capacitors, at least decent RF caps, have low enough loss that it can
usually be ignored.

RFSim99, in addition to the simulation stuff, has some "tools"
accessible from pulldown menus that will design simple filters for you,
calculate inductance, capacitance, frequency or reactance--enter two of
those and see the other two--and some other useful RF things.

Hope this helps some!

Cheers,
Tom


Bernard wrote:
Thanks this helps. I went into the wrong track because I ignored the
shunt C
at the antenna point. Looking at it as a pi network is definitly
helping
(not that I know the formula in top of my head, but I will find it
!).

Now, the next problem will be to test the efficiency/tuning of the
network.
I will need to correctly evaluate the value of the equivalent C's a
the
antenna point and at the grounded diode point...

Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
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