From: liz@poppyrecords.invalid.invalid   
      
   john larkin  wrote:   
      
   > 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.   
   >   
   > Why not switch between two receive antennas?   
      
   I want one aerial that can be swung on a rotator and used for direction   
   finding, there is no advantage to having two of them if a single piece   
   of hardware can be made to do the job with the addition of just one   
   extra reflector.   
      
      
   > There are lots of cool RF switch chips. We use one that switches   
   > cleanly in 5 ns.   
      
   At 200 c/s switching frequency, 1N4148 sounds far more attractive -   
   eapecially as I have a drawer full of them.   
      
      
   --   
   ~ Liz Tuddenham ~   
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