Jim,
> Perhaps type 43 material is the wrong stuff to be using for a
> plate choke...esp considering the typ plate load Z's involved ??
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.
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! 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. 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 problem is that a plate choke is definitely not working at a low
impedance. Regardless of what inductance the choke has, the equivalent
parallel loss resistance introduced by any magnetic core must be high
enough to cause negligible, or at least acceptable, loss at the high
impedance and at the frequency used. Just calculating a ferrite cored
plate choke to give enough inductance, and letting the loss care for
itself, is a recipe for disaster.
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. 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!
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.
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!
I think a designer of an amp should in any case consider using ferrite
in the plate choke, and weigh its advantages and disadvantages. But I
wouldn't be surprised at all if most cases would end up with an air
cored choke being the most (cost)effective solution.
> Type 43.... 1" OD bead's, slid over 393 Teflon coax makes for
> excellent Choke baluns.
Only if you use enough of them to make the resulting loss resistance
pretty high, relative to 50 ohms. About 20 of these cores should already
be quite good. But try using just one or two, and see them melt the
coax! If instead you use a lower loss material, you can get away with
fewer beads.
> The 1/2" x 8" long type 43 rods work
> great for Bifilar's. But in both those cases, the Z is only
> 25-50 ohms.
Exactly. That low Z is the point. Such a rod wound with enough turns
might end up with a loss resistance of a kiloohm or so, which causes
negligible loss at that low circuit impedance.
Manfred.
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