From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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