From: liz@poppyrecords.invalid.invalid
Bill Sloman wrote:
> On 6/01/2026 3:27 am, Liz Tuddenham wrote:
> > Bill Sloman wrote:
> >
> >> On 5/01/2026 8:52 pm, Liz Tuddenham wrote:
> >>> John R Walliker wrote:
> >>>
> >>>> On 05/01/2026 08:56, 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?
> >>>>>
> >>>> A simple switching diode like a 1N4148 should be suitable. Drive it
> >>>> through isolating resistors of maybe 1k each from a +/- 30 to 50V source
> >>>> so that the diodes are passing around 20mA when conducting and
> >>>> are reasonably reverse biased when off. It would still work with
> >>>> zero volts in the off condition, but reverse bias will reduce the
> >>>> capacitance. You will also need a couple of series isolating capacitors.
> >>>
> >>>
> >>> That's a good easy solution. Chokes in series with the resistors should
> >>> give even more RF isolation and allow lower resistor values so that it
> >>> could be driven from a lower voltage (it may have to be field portable).
> >>
> >> A reed relay could be even better. They take about a millisecond to open
> >> or close, though the actual makes and breaks happen a lot faster.
> >>
> >> A mercury wetted reed relay doesn't have any contact bounce, which could
> >> be an advantage.
> >>
> >> A slightly bizarre way of managing the magnetic field to open and close
> >> the relay would be to put two permanent magnets on wheels on either side
> >> of reed,and rotate the two wheels in opposite directions at 200Hz.
> >
> > I don't need a particularly high on/off ratio and running a relay at 200
> > c/s for long periods isn't a good idea.
>
> Mercury wetted reeds are good for 100 million operations, and the
> degradation is a gradual increase in contact resistance.That's 138 hours
> of continuous operation at 200Hz, and the reed capsule isn't all that
> expensive or hard to swap.
I don't need the good on/off properties of a reed relay in this
appication and I certainly don't want to have to replace any of the
components after 138 hours of operation.
> > Further down the line the
> > signal will be synchronously detected, so any delay in switching will
> > upset the null, which is critical for the system to work properly.
>
> A variable delay in switching might upset the null, but the delay is
> essentially controlled by the mass of the reed, the distance it has to
> travel and the magnetic force which is getting it to move, all of which
> should be pretty consistent.
The whole purpose of the system is to null the synchronous detector,
so anything that upsets it or needs calibration to start with (or after
a change of components) is unsuitable.
Reed relays are not the answer to this particular problem.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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