From: jl@glen--canyon.com   
      
   On Mon, 5 Jan 2026 22:17:39 +0000, liz@poppyrecords.invalid.invalid   
   (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.   
      
   I guess you could have a feedline of the proper number of wavelengths,   
   and apply the switched short at the end. That puts all the electronics   
   in one place, out of the rain.   
      
   I suppose I should Spice that.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-DOS v1.05   
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