[HBR] Some info on coil "Q" ..."reflector activity filler"

[Sandy and Kees Talen]

Here is some experimentation with coil Q on one of the other reflectors. 
It started with some comments about old receivers using DCC 
(double cotton covered) wire and how that attained higher "Q" but it
had to be sealed, etc. You might be interested in it. All the comments
are mine.

73s  Kees K5BCQ

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I agree that paraffin is an excellent way to reduce moisture intrusion,
it's easy to use, and the "dipped in hot paraffin" would reduce existing 
moisture content. Dipping a coil would also allow the sealant to flow 
into all the spaces ....much better than painting/smearing/globing on a 
sealant. Many of the older multi- layer coils used cotton served Litz
wire 
and you HAD to "seal off" the entire coil. 
 
On the use of enameled wire, I think its fine for larger inductances, 
however the enameled wire allows very close adjacent spacing which 
increases the turn-to-turn capacitance and lowers Q. For single layer 
coils, if you "space wind" the turns tightly with another wire or string
in between and remove it later, you don't have the problem. You do 
have the problem of really having to stabilize the separated turns with
paraffin, Duco cement, etc. Wonder what thicker insulation on the wire 
would do ?
 
On multi- layer coils wound with enameled wire a basket weave pattern 
reduces turn-to-turn and layer-to-layer capacitance and enameled wire
allows a lot of wire in a small volume. Q can be increased by using 
multi strands of smaller wire (like Litz wire ....skin effect) at
frequencies
below about 3Mhz. It's a substantial improvement in Q to use two strands 
of #39 AWG vs one strand of  #36 AWG, or better yet .....six strands if 
#42 AWG vs one strand of #36 AWG. All of those wire combinations 
have the same current handling capability ....but substantially different
Qs. Many (small) higher quality IF transformers are wound with two 
strands. Larger, older ones use Litz wire, are dipped, etc. 
 
One other hint from the youthful past ...if you ever catch the hot
paraffin
on fire do NOT try to put out the flame with water. 
 
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 Here is some data on two coils:

 - 1-1/4" coil form (amber pill bottle)
 - both coils 13.8uH +/- 0.1uH
 - both coils #28 AWG wire
 - coil "A" 16 turns are adjacent
 - coil "B" 20 turns are on .025" centers +/- (spacer was removed)
 - both coils, no adhesives/coatings to hold the turns in place

 At 3.4MHz  ..... coil "A"  Q=107, coil "B"  Q=175

Granted, these are unloaded Q measurements on a Boonton 260-A and 
loaded Q will be less, however the improvement will be carried over
and the "trend" is there.
 
What does this mean for the signal (and, again, loaded Q will be less
dramatic) .....roughly:
 
Sensitivity ...the Boonton uses a 20mVAC "e" signal and measures "E" 
with a very sensitive voltmeter, so a Q of "107" means "E" is 2.14VAC. 
And for a "175" Q, "E" is 3.5VAC ....higher sensitivity.
 
Selectivity ....Q is also the -3db voltage BW divided into the frequency
(Q = Fo/-3dbBW  ...or -3dbBW = Fo/Q). For a Q of "107" the -3dbBW 
is 31.8Khz. For a Q of "175" the -3dbBW is 19.4Khz .....better
selectivity.
 
By the way, coating the "A" coil with "CG Clear Ice" fingernail polish
drops the Q to "97".
 
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Coil dope........
 
I tried some E6000 Craft Glue on the coil with a Q of 
"175" ...absolutely no change in Q as you put the material 
on, as it dries, or 12 hrs later, even for a very heavy 
application of glue. I think it's some kind of silicone 
based product .....waterproof, clear, and remains flexible.
 
My wife didn't even tell me she had some "real good coil
dope" material in her crafts pile.