From: jrwalliker@gmail.com   
      
   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.   
   John   
      
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