> (3) for a given winding wire cross sectional area, litz wire will give
> higher Q, but it is often simpler to just double the area
There are a lot of wive's tales about Litz wire, one being that the
parallel wires increase surface area. Factually, the exact opposite
happens. For a given conductor diameter Litz wire increases wire
loss resistance compared to a solid conductor the same diameter,
because Litz wire reduces the surface area.
The Hustler KW mobile coil is a prime example, if the Litz wire is
replaced with a solid conductor with no other changes Q improves
significantly.
I've never seen a case where Litz wire increases system Q above
VLF, or where Litz wire has ever improved Q in a single-layer coil at
any frequency.
Litz wire is useful at low frequencies where the skin depth is large
compared to conductor diameter, and when considerable flux is
cutting the turns causing eddy currents.
The reason Litz wire improves Q *in some specific cases* is the
wire is "sliced" into small insulated sections. This reduces eddy
currents in the wire (like it does when a transformer core is "sliced"
and insulated in layers)....but that sure isn't an issue in single-layer
coils at HF and higher! Litz wire is useful in multi-layer transformers
at low frequencies, such as switching-supply transformers operated
around 50 kHz or so.
> ferrite core :
> (1) forget the 43 material with the ur over 1000 - in your dreams
> buddy - this material is great for broadband tightly coupled
> transformers especially the "balun" 2 holed beads - but even at MF the
> Q will be about 10 or even less - when i tried the stuff it was so
> poor that i didn't even bother to measure it - it is just not suitable
> for either resonant or non-resonant circuits - that is why it is used
> around coax cables etc to absorb the shield currents
Look at the graphs of core characteristics, and you will see the
problem is the loss factor. In 43 material, Q reaches unity around 5
MHz. You want a core with a high ratio of permeability to loss
factor **at the operating frequency** in resonant systems or in
systems that depend on the core having high flux densities.
43 Material is quite good for non-resonant transformers at high
frequencies (all the way to VHF and higher) and is commonly used
in that application. It is not good in Q-critical resonant systems on
higher frequencies.
> those big rods from amidon years ago and it just clutters up my junk
> box now (2) go for the 61 material, the ur is about 100 at the 1-2 MHz
> mark and is what is normally used in am broadcast receiver loopsticks
The problem is permeability vs loss factor, not ui. The Q of 61
material is around 300 or higher at 1 MHz. That makes it a good
choice. As a matter of fact, Q peaks at 2 MHz and is around 400!
73, Tom W8JI
(W8JI@akorn.net)
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