From: pcdhSpamMeSenseless@electrooptical.net   
      
   Liz Tuddenham  wrote:   
   > Phil Hobbs  wrote:   
   >   
   >> On 2026-01-05 12:43, Liz Tuddenham wrote:   
   >>> Phil Hobbs  wrote:   
   >>>   
   >>>> On 2026-01-05 11:27, Liz Tuddenham wrote:   
   >>>>> john larkin  wrote:   
   >>>>>   
   >>>>>> On Mon, 5 Jan 2026 08:56:35 +0000, liz@poppyrecords.invalid.invalid   
   >>>>>> (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?   
   >>>>>>   
   >>>>>> Why not use a relay?   
   >>>>>   
   >>>>> The high on/off ratio of a realy isn't needed and the 200 c/s switching   
   >>>>> will wear it out.  Also, I need to synchronously detect the signal and   
   >>>>> any delay in switching will upset the null point.   
   >>>>   
   >>>>   
   >>>> If you're planning on using diode switching, I'd suggest doing it at the   
   >>>> receiver  end of the feedline.  That'll make it easier to protect from   
   >>>> surges, and let you do a better job of filtering the reverse bias   
   >>>> voltage--otherwise, any duty cycle asymmetry in your lock-in will let   
   >>>> low frequency junk into your front end.   
   >>>   
   >>> The dipole is the reflector of a Yagi-Uda receiving array, so normally   
   >>> there would be no connection to it at all.   
   >>   
   >> So you're trying to find the exact null to get rid of a strong signal   
   >> coming from the reverse direction?  Or what?  Inquiring minds want to   
   >> know. ;)   
   >   
   > I want to have two reflectors, one each side of the usual position, and   
   > short them alternately so the lobe of peak sensitivity shifts from side   
   > to side.  By synchronously detecting the carrier level, it should be   
   > possible to make a direction-finding system based on the point of   
   > maximum sensitivity.  This may give better results on weak signals than   
   > the usual system which is based on a null.   
   >   
   > Basically a Lorenz blind-landing system in reverse.   
      
   Sure thing. Bright-field interference works a lot better than dark field in   
   optics too. The trouble with surfing a null is that the SNR is zero there,   
   making the tracking sloppy.   
      
   Phase-sensitive detection fixes that in electronics, but that’s harder to   
   do without having a phase reference from the transmitter.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
   --- SoupGate-DOS v1.05   
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