From: bill.sloman@ieee.org   
      
   On 6/01/2026 3:27 am, Liz Tuddenham wrote:   
   > 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.   
      
   Mercury wetted reeds are good for 100 million operations, and the   
   degradation is a gradual increase in contact resistance.That's 138 hours   
   of continuous operation at 200Hz, and the reed capsule isn't all that   
   expensive or hard to swap.   
      
   > 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.   
      
   A variable delay in switching might upset the null, but the delay is   
   essentially controlled by the mass of the reed, the distance it has to   
   travel and the magnetic force which is getting it to move, all of which   
   should be pretty consistent.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)