From: legg@nospam.magma.ca   
      
   On Mon, 5 Jan 2026 09:52:49 +0000, liz@poppyrecords.invalid.invalid   
   (Liz Tuddenham) wrote:   
      
   >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).   
      
   Bias voltage should be greater than anything (ac peak) on the line.   
      
   RL   
      
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