Dear LF Group,

I have attached a picture of GBR's wooden loading coil taken last year - the
hexagonal spiders are about 5m across, the litz wire conductors are about
50mm thick and I believe are 6561 strands of 36swg wire. The inductance is
of the order of a few  millihenrys. The papers describing the original 1926
station indicate the coil Q was a few thousand; the timber used was
"American Whitewood", whatever that may be - it was reckoned to be much
lower loss than other types of wood. The plates attached to the coil frame
said it was rated for 1000A at 16 - 22kHz.

John Rabson wrote:

But didn't the helix house burn down once?

I think that was in 1943, when the original wooden roof caught fire and
gutted the top part of the building containing the loading coil. The lower
floor housing the transmitter was repaired fairly quickly, but it was not
possible to replace the coil immediately, so a temporary outdoor loading
coil was used for a while. The coil in the photo seems almost identical to
pictures of the original 1926 loading coil, except that pairs of spiders are
paralleled up, presumably because the later antenna top loading had more
capacitance than the original, and required less inductance. Also in 1926
the antenna feed was connected through a big glass window in the wall,
wheras last year it passed through the (now concrete) roof. You can see the
copper sheet shield above the coil; also there was a grid of copper wires
covering the walls, which connected to the earth bus-bar, which can be seen
going round the wall.

Cheers, Jim Moritz
73 de M0BMU