On Sun, 17 Jan 2010 16:02:35 +0000, Manfred Mornhinweg wrote:
>Yes. 43 material is an extremely bad choice for that application. This
>material's best application is actually for absorption purposes, when
>fighting RFI! It can also be used quite well in low impedance, broadband
>transformers, as long as you are aware of its characteristics, and
>design the transformers accordingly.
Yes.
>The basic material characteristic to keep in mind is this: Adding a
>ferrite core to a coil will drive up its inductance, but also make an
>equivalent resistance appear in parallel with it (caused by the loss in
>the core). Depending on the exact material, and the frequency, this
>resistance part can be anything from negligible to overwhelming!
Yes.
>With 43
>material, the resistance added tends to be about as large as the
>inductive reactance added, at a frequency of just a few megahertz. In
>the upper HF range, 43 material actually adds more resistance than
>inductive reactance! That's why it's so good for absorbing RFI!
>Note that despite this characteristic, you can still use 43 material
>quite well for boradband transformers covering the whole HF range. For
>example, if you make a bifiliar transformer that will be used to
>transform from 50 ohms down to 12.5 ohms, you can easily wind enough
>turns on a small toroid to get 1000uH of total inductance.
Yes -- BUT -- winding those turns also adds parallel capacitance, which
forms a parallel resonance with the inductance. You can see MEASURED DATA
from an excellent lab in Appendix One of
http://audiosystemsgroup.com/RFI-Ham.pdf
This tutorial includes data for coils made of single conductors on #31, #
43, #61, #77, and #78 made by Fair-Rite, for coils of 1-14 turns.
There is a related Power Point file on my website,
http://audiosystemsgroup.com/publish/htm
that shows how to use curve-fitting to determine approximate values of R,
L, and C in the resulting equivalent circuit.
>On 160
>meters, that will give you a reactance of more than 10 kiloohm, and the
>parallel loss resistance will be even larger, so that this transformer
>will have next to no loss. And on 10 meters, the inductive reactance
>will have risen to almost 200 kiloohms, while the equivalent resistance
>might be as low as 10 kiloohms (don't take this too literally, I haven't
>checked the data sheet, I'm just giving estimated numbers out of my
>head). Note that using this transformer as an inductor would be
>absolutely terrible, because it would have a Q factor of only around
>0.05! But in that transformer, the loss caused by the 10 kiloohm
>resistance in parallel with the 50 Ohm circuit impedance is totally
>negligible! That's why 43 ferrite does work well in broadband
transformers.
The above analysis is seriously flawed, because it overlook the
contribution of stray capacitance, as discussed above. The Q of the
resulting resonant circuits wound on #31 Fair-Rite cores is on the order
of 0.4 at resonances in the MF and HF range. For #43 ferrites, the Q is
slightly higher. Above resonance, a parallel resonant circuit looks like
a lossy capacitor. Below resonance, it looks like a lossy inductor. At
resonance, it looks like a resistor.
>To design a ferrite cored plate choke, you should start with a type of
>ferrite better suited for low loss operation at HF. Among the
>inexpensive, widely available ferrite types, this could be 61, but there
>are definitely better types than that.
YES! Fair-Rite #67 is another candidate. In general, you trade
permeability for low loss -- that is, low loss materials also have lower
values of mu.
Ferrite materials also vary widely in their resistivity. Some are quite
good insulators, while others have substantial conductivity. If the
material you're using is good conductor, it may be important that the
coil is insulated. See the Fair-Rite website for data sheets for each
material.
>And then you have to design the
>whole thing, considering the AC flux density you will be putting through
>the ferrite, and the resulting loss. You also need to consider DC flux
>density, and make sure it won't saturate the ferrite, but this is
>usually not a problem in such chokes, because the AC flux density limit
>for acceptable loss is so low that the resulting DC flux density is
>child's play!
Not necessarily. If there is enough DC current in the choke, it can cause
saturation.
>And for a given frequency, the AC flux density is essentially defined by
>turns number, and cross section of the core. A thicker core, or more
>turns, both result in lower AC flux density.
>And then you have to decide whether it's even worth using a core! A
>simple solenoid core (ferrite cylinder), only causes a rather small
>increase in inductance, because at least half of the whole magnetic loop
>is still in the air! To really obtain a significant size reduction of
>the plate choke, you would need to use some sort of ferrite core that's
>closed, or perhaps has a small air gap (whether or not this is necessary
>depends on factors such as the permeability of the core).
>A core made of low loss ferrite and having a size in the ballpark for a
>legal limit plate choke might need something like 300 turns. And on 43
>material, even more. So it's debatable whether it actually offers an
>advantage! Sure, it will have much higher inductance than an air cored
>plate choke, but by itself that's often not a real advantage, since the
>inductance of the choke can be easily absorbed in the impedance matching
>network.
300 turns on a core will set up a lot of stray capacitance, that shorts
out the choke. Remember old-fashioned RF chokes wound "pancake" style?
The purpose of that winding style was to reduce stray capacitance.
>A ferrite core in the plate choke might be an advantage, specifically
>BECAUSE of its loss, when such loss helps to eliminate self-oscillation
>at frequencies where the matching network completely decouples the
>antenna. But then, you can get much the same effect by placing a
>resistor across an air cored plate choke!
YES. It is the RESISTANCE at resonance of the ferrite choke that
suppresses current flow, NOT its inductance.
73,
Jim Brown K9YC
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