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.
>
> 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.
Each dipole will have to have a waterproof connecting box at the centre
point, so a few RF electronic components could easily be contained
inside it. The switching connections then can be made at 200 c/s by
ordinary twin flex, with no need for co-ax and all the standing-wave and
phasing problems it would cause because of the variable terminating
impedance.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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