From: jl@glen--canyon.com
On Tue, 6 Jan 2026 10:11:14 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
>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.
Well, the ADRF5024 costs us $108 in quantity.
Take a look at the TS3USB30. It's a damned fine fast DPDT switch for
20 cents.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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