From: liz@poppyrecords.invalid.invalid   
      
   John R Walliker  wrote:   
      
   > On 05/01/2026 08:56, Liz Tuddenham wrote:   
   > > I am looking for a device that can be placed across the centre of a   
   > > dipole aerial tuned to 150 Mc/s, so as to short-circuit it at about 200   
   > > c/s.  The impedance at that point is around 75 ohms, so 5 ohms would be   
   > > as good as a short and 500 ohms would be as good as O/C; therefore the   
   > > maximum capacitance of the O/C device would have to be around 2pf.  The   
   > > power level is negligible - probably microwatts.   
   > >   
   > > The control signals could be any convenient voltage but they would have   
   > > to be isolated from the RF, either by a choke or by some other form of   
   > > galvanic isolation.  As the switching frequency is so low, I had even   
   > > wondered about a cheap photovoltaic panel illuminated by a few LEDs.   
   > >   
   > > Does anyone know of a suitable device, especially one with built-in   
   > > isolation such as a high-side driver for power control?   
   > >   
   > A simple switching diode like a 1N4148 should be suitable.  Drive it   
   > through isolating resistors of maybe 1k each from a +/- 30 to 50V source   
   > so that the diodes are passing around 20mA when conducting and   
   > are reasonably reverse biased when off.  It would still work with   
   > zero volts in the off condition, but reverse bias will reduce the   
   > capacitance. You will also need a couple of series isolating capacitors.   
      
      
   That's a good easy solution.  Chokes in series with the resistors should   
   give even more RF isolation and allow lower resistor values so that it   
   could be driven from a lower voltage (it may have to be field portable).   
      
      
   --   
   ~ Liz Tuddenham ~   
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