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myfiles
Hello all,
My post on the braid was questioned (and reasonably so).
Here is a post I sent to the list late last year that might help
explain my stance on this one.
M
------- Forwarded Message Follows -------
PERFORMANCE OF WIRE
The following is a summary of a brief investigation into wire
performance at low radio frequencies.
Six wires of various types were tested. Copper strap was not
available for the experiments. The wire coming closest to strap in
terms of RF compatability was an 80 strand LITZ wire, each strand
being 0.2mm in diameter.
Test frequency was about 195kHz. Skin depth at this frequency
is about 0.15mm.
The experiment consisted of using a 17 foot (+-1 inch) sample
of each wire connecting a secondary coil (aspect 2, close-wound with
0.56mm diameter wire) to earth.
An untuned primary was used to couple a signal into the secondary
and coupling was greatly reduced (k approx 0.01) to minimize shunting
of secondary impedances by the signal generator. The oscilloscope was
loosely capacitively coupled to the top of the secondary.
Estimated accuracy for Q is +-2%. If the wire under test changed
signal level at resonance from the previous wire, the signal generator
level was adjusted to compensate before bandwidth was measured to keep
scope readings constant. As tests progressed, wire previously tested
was remeasured to check for change in experimental conditions.
The wires were :
(1) Two lengths of coax cable (braid only) connected in parallel
(2) 1.2mm enamelled solid section copper wire (~twice the diameter of
the coil winding)
(3) 0.3mm enamelled copper (about 1/2 diameter of windings)
(4) 0.063mm enamelled copper (about 1/9 the diameter of the windings)
(5) plastic coated stranded hookup wire (conductor diameter = 0.44mm)
(6) 80 strand LITZ (as above)
In a latter part of the experiment, the polyethylene wrapped inner solid
conductor of a piece of coax was measured as Wire(7) (diameter 0.59mm).
RESULTS
WIRE (1) (2) (3) (4) (5) (6) (7)
____________________________________________________________
Q = 172 189 189 177 172 193 185
Within the margins of experimental error, wires 2 and 3 behaved
identically. A marginal improvement in performance was noted for (6)
as compared with 2 and 3 (about 0.5% greater Q).
Reduction in skin effect became quite noticeable for wire 4, causing
about 4% attenuation over wires 2,3 and a drop in Q of around 5%.
Of most interest however was the behaviour of plastic coated wires
1 and 5. Wire 5 caused signal attenuation of around 10% on 2,3 and a
drop in Q of about 8%.
The performance of the twin braid was bizarre and for me unexpected.
It showed attenuation of around 7% and a drop in Q comparable with wire 5.
This prompted further investigation using a single length of coax. With
the outer covering on it performed worse than the twin coax, showing a
decline in Q of about 11.5% and attenuation of around 10%. With the outer
sheath removed, Q improved marginally (to a decline of around 10%). Remov-
ing the polyethylene covered central conductor and using the braid alone
showed a drop in Q of about 5% and attenuation of around the 5% despite
having overall copper thickness comparable with the LITZ wire. This res-
ult shows poor skin performance - the central core interfered substan-
tially with the outer braid.
CONCLUSIONS
Plastic covered stranded wire is a no-no in RF applications, even at
quite low frequencies. I don't know what type of plastic covered the wire
but it was probably PVC. The solid conductor polyethylene covered wire
was a much better performer. An important note here : all wires were
trailing along the garage floor on their way to the earthing stake. What-
ever the capacitive coupling between the wire and earth, it was swamped
by the earth itself.
Braid is a no-no. Although this has often been said in this forum, I
found this result surprising as coax is made for use in RF applications
although not in the way in which I used it. It is suggested that the best
coax cable would use an outer conductor equivalent to a pipe cross-section
(i.e. no woven conductors). Coax of this type is used in ethernet applic-
ations although that which I have seen has a spirally wound aluminium
outer. One would hope that good conductivity exists between each turn of
the spiral (a bare wire running the length of such an outer to connect
each spiral to its neighbour hardly seems adequate).
The thinnest solid enamelled wire outperformed both braid and stranded
wire.
Leaving aside considerations of wire heating with high currents, the
solid section wires performed well in small diameters. Departure from
this was noted as significant for a thickness of 0.063mm (c.f. skin depth
of 0.15mm at the test frequency).
Personally, I'll never use braid again in an RF application!! Imagine
how a few feet of this detracts from primary performance if it is included
within the tank circuit.