From: bill.sloman@ieee.org   
      
   On 5/01/2026 8:52 pm, 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).   
      
   A reed relay could be even better. They take about a millisecond to open   
   or close, though the actual makes and breaks happen a lot faster.   
      
   A mercury wetted reed relay doesn't have any contact bounce, which could   
   be an advantage.   
      
   A slightly bizarre way of managing the magnetic field to open and close   
   the relay would be to put two permanent magnets on wheels on either side   
   of reed,and rotate the two wheels in opposite directions at 200Hz.   
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)