From: liz@poppyrecords.invalid.invalid   
      
   Bill Sloman  wrote:   
      
   > 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.   
      
   I don't need a particularly high on/off ratio and running a relay at 200   
   c/s for long periods isn't a good idea.  Further down the line the   
   signal will be synchronously detected, so any delay in switching will   
   upset the null, which is critical for the system to work properly.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)