On Thu, 5 May 2005 09:50:53 -0700, "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...

Suggest that you visit he

http://www.microsemi.com/brochures/p...hapter%202.pdf

in particular page 10.



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