Walter Blanchard schrieb:
Only just saw the postings re Decca coil screening.
My Decca pal tells me he lined Decca Tx coil huts with copper (thin - 20g)
to maintain constant dielectric environment and constant phase. The whole
operation of Decca depended on maintaining accurate phase. Note - NOT
grounded. Unlined huts caused considerable losses and phase shifts when wet
outside. Timber huts used in N. Canada not too bad in winter when dry but
lossy in summer when wet. VE's take note. Absolutely essential that proper
bonding maintained between sheets - solder not screws. Does NOT cause
losses if not grounded. Unimportant for amateurs because phase shifts don't
matter, although if out in the open a conductive box around coil could
prevent nearby wet bushes catching fire.
A point about corona. Electrical discharge off a point can exist without
being visible.
I referred some time ago to the "green" edition of Terman for coil data.
This referred to the 1943 edition of Terman - the one before transistors
got in.
I gather that when medium-wave went commercial in the UK back in the 60's
copies of this edition were fetching £100 a time.
Walter G3JKV.
Hi Walter,
quotation Terman ed.1943, page 128 on shielded air coils:
...shields... increase the losses as a result of energy dissipated
in the shield by the eddy currents. However, if clearence between
the coil and its shield is everywhere at least equal to the radius
of the coil, the reduction in inductance and in the decrease in
coil Q by a copper or aluminium shield will not be serious.
end of quotation.
grounded or not grounded, the eddy currents in the shield cause losses
(short-circuit).
on this reflector some OMs discussed whether not used windings of a
loading coil must be shortened or not. in both cases the (eddy)
current in those windings will cause losses.they also cause some wave
reflection. the reflection caused by not shortened windings is vy stronger.
I could observe that when trying out how to transport clean sine lf
into the aerial. I removed all not used windings. the result was impressive.
regards
Uwe/dj8wx
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