No one appears to have addressed my original query re: servo cancelling the
DC in a toroid core. Rather wandering back to the general plate choke topic...
So, I'll wander a bit too!
While experimenting with all those chokes, one of the curious things I
noticed was that if I took a large ferrite washer (4.4 cm dia. x .4 cm thick of
unknown material) and placed the flat side adjacent to a 70uH solenoid wound
coil (3" of B&W 3016), it almost completely dampened the series resonances
(first one circa 25 MHz)!!
I went looking around the 'net for an explanation and found US patent
#3,835,370 that states that this effect is well known!
The magic washer exhibits rather high resistivity (circa 10^7 ohms per cube)
and is only weakly attracted by a magnet. Otherwise, it looks like plain ol'
ferrite though, it is unlike any other core material in my inventory. It is
also very low mu -- 32 turns of wire produced only 3uH! Anyone know what
material this might be??
I'd wager that this scheme could be usefully employed with almost any stock
solenoid plate choke to eliminate those pesky resonances.
Further searching found #3,982,814 & #2,692,372 both of which use a secondary
Nichrome winding to similar effect!
A final point of interest: #3,980,975 employing concentric coils. I tried
this with a pair of B&W coils and it kinda of worked. A bit of judicious turns
counting and positioning may be in order.
Note: Patents may be easily downloaded via: free.patentfetcher.com
73 & Good evening,
Marv WC6W
## OK... why wouldn't just plane 16-24 ga magnet wire, wound like a
solenoid, on say type 43 rod work ? [ like polyimide /kapton magnet wire..
with the 10-15 kv insulation] Why would .5 to 2 amps of dc plate current
saturate the type 43 material ??
It's a RF plate choke.. so should present a high Z.
## I have see pix of the harris plate chokes... and they used a HUGE chunk
of ferrite inside a form.. with the usual magnet wire [ with way less turns
than a typ ham choke] wound on top of the same form. It worked down to 1.6
mhz.. and no series resonances from 1.6 to 32 mhz.
Jim VE7RF
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