From: jl@glen--canyon.com
On Mon, 5 Jan 2026 16:27:20 +0000, liz@poppyrecords.invalid.invalid
(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.
Would it have to run long-term?
A little relay will switch in about a millisecond.
If you don't mind the complexity, use a logic-level isolator and a
diode. Just be careful about the EMI issues.
A PV optoisolator driving a phemt would be fun. The SAV541 goes to
about 2 ohms with +0.7 on the gate.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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