Hi Gary,
Am 13.01.2013 16:51, schrieb Gary - G4WGT:
Hi Stefan,
I have thought about LF & MF antenna coil switching but I am very wary
about the very high voltages on the LF coil due to my short vertical
antenna. At the moment I have 3 screw connectors to change over :-
1 - Main coil inductance
2 - Matching transformer input
3 - Matching transformer output
I have also thought about using 4mm banana plugs to make the change
over easier & quicker.
But not sure about using banana plugs, are they suitable ?
...at least i'm using one for LF and it still isn't molten or evaporated
;-) Since i have to manually change the coil to the antenna wire i would
notice when the contact becomes loose, so i see no problem, so far.
But for a remote antenna switch i would have to use something special.
Not an ordinary relay of course.. The distance between the contacts
should be at least 20 cm.
One idea would be a use a motor that turns an arm (isolated between
motor and arm). The arm is stiff and there is a thin wire connected to
it, coming from the coil. The arm turns until it touches the electrode
of the antenna (here it is the upper part of the HV isolator, see
https://dl.dropbox.com/u/19882028/MF/MF_vs_LF.JPG). The force to get a
goot contact could be achieved by using a magnet, maybe from an old HDD.
Of course the force must be lower than the force the motor torque can
be, so the ferromagnetic element which touches the HDD magnet must be
quite small because these magnets are strong! If the length of the arm
is 30 cm and the force is say 2 N than the drive motor must generate at
least 0.6 Nm, of course ;-)
As i was young i built a similar arrangement to discharge a 5 uF / 30 kV
capacitor via remote. The arrangement was floating (on a few cassette
boxes), including a small battery. The motor was activated by a
transistor and a light dependent resistor (LDR) and a hand lamp :-)
73, Stefan/DK7FC
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