[TenTec] OT: Toroid ratings / selection (was TenTec 228 ATU)

[Dr. Gerald N. Johnson]

On Sun, 2008-06-29 at 08:49 +0800, Marinus Loewensteijn wrote:
> 
> >> Found a construction description of a 1:9 unun with a FT114-61 toroid that 
> >> spans several bands. Will try this setup with a low pass L-C matching network 
> >> since it will have very low losses.
> 
> >But not zero losses. That toroid will survive 100 watts, but not 1.5 KW.
> 
> The TenTec 228 ATU was sold as a 200 Watt ATU. If matching 50 Ohm coax to
> 50 Ohm coax (I think this is called conjugated match) without a reactance with 
> the components then the loss is around the 25% mark. I feel 200 Watt is
> very optimistic, would rate it more like 50 - 100 Watt. The 100 Watt capability
> of the FT114-61 can be lived with since I am going to use it behind the Omni
> 546C.
> 
> >> The backup plan will be to build an S-Match and then use a Guanella 4:1 output
> >> unun for matching the lowest impedances. According to some information the 
> >> losses in properly build Guanella are low and it can handle up to a 1:5 SWR.
> 
> >Switching in a low to medium Z output transformer can take care of low
> >feed Z antennas, nothing new there, is especially handy when feeding
> >short loaded verticals.
> 
> >Maybe for your loads, you want an output transformer wound to have a tap
> >for 12.5 ohms (4:1 from the 50 ohm winding),  no transformer for 50 ohm
> >load, and 1:4 for the 200 ohm load region. It would have 3 or four
> >winding. I might be lazy and make it with four windings in quad filar,
> >just enough turns so that two windings in series gives the right winding
> >for 50 ohms. Then the connection between those two windings is the 12.5
> >ohm load point. The other two windings in series and connected to the 50
> >ohm input gives a 200 ohm load point at the extreme and a 9 * 12.5 or
> >112.5 ohm load tap. More winding taps can give more load values to
> >minimize the impedance ratio of the tuner and so give it the best chance
> >for low loss and broad bandwidth.
> 
> This was my first approach however output baluns start to behave strange when
> there are reactances and impedances present that are quite some distance 
> removed from what it was originally designed for. Losses can get quite high
> with resulting overheating of the un-un / balun.

In this application the impedances are forecast to be low and so that
output transformer has a better chance.
> 
> Trouble is that one may have an acceptable SWR at the antenna, e.g. a SWR 
> of less than two, but by the time this arrives at the ATU it may well have changed
> into something higher / lower due to the transmission line length. Lower is not
> the problem, it is the higher that gives the grieve.
> 
> >However the minor losses of this
> >output transformer may be more than that of an L, Pi, or T match
> >covering that load range made with large low loss parts.
> 
> The Q has little influence on the efficiency, it is just the T-match design. To get 
> broadband matching networks  to function properly with very low impedances 
> requires impractical capacitor values.

To the contrary, loaded Q has a great deal of influence on the
efficiency. In any tuned circuit (and the tuner IS a tuned circuit) the
circulating current within the tuned circuit is Q times the input or
output current. So the resistive losses in the coil are raised by the
factor Q.
> 
> For a long time I have wondered about when to use a ferrite and when to use a 
> powdered iron wondered in power matching. Have read untold, and often heated, 
> discussions on this subject. 

If you want broad band, you want ferrite. If you want narrow band and
resonances you use powdered iron. Powdered iron generally doesn't come
with a high enough permeability to make a broadband transformer, but the
core material is more linear.
> 
> I have made for myself notes on the use of them and I'll relate them here. It may
> help others who have been just as confused as I have been.
> 
> (The caveat this is my impression and have no scientific proof to back this up.)
> 
> Ferrite is selected to to minimize the number of windings required for a given 
> inductance. Often the material selected is the one that gives the lowest number 
> of windings. In general ferrite toroids span in the balun - unun application 
> several octaves more satisfactory use than the powered iron counterparts.
> 
> However, due to the ferrite material characteristics, the material with the least 
> number of windings has the highest amount of loss. 
> When a ferrite toroid overheats then it loses it's permeability and the toroid
> is no longer usable. 

Depends on the ferrite, sometimes the permeability returns, if the
heating doesn't crack the core.
> 
> When an iron powered toroid is overheated and then cools down it will just
> continue to work, it has not been destroyed. Core losses in ferrite are higher
> than the core losses in iron powder. In ferrite, if the wrong material is selected,
> there can be 1%. Powdered iron is more forgiving and does not run above 0.1%.

True.
> 
> If you drop a ferrite then you can just superglue this back together again and
> it will work fine. Make the join as thin as possible. An iron powered cannot be 
> glued back and is a write off.

Both can be clued. Powdered iron is often embedded in some plastic or
phenolic.
> 
> Often the wrong ferrite is selected in order to minimize the number of windings
> and the result may be that the ferrite material goes into non-linear function
> which causes all kinds of distortion in the transmitted signal. However for this
> to happen you need high amounts of power and often the toroid overheats 
> first due to not being a pure resistive match.

Ferrite can be nonlinear at all levels, but with the transformer turns
tightly coupled, changes in the core have very small effects until the
core reaches saturation.
> 
> Ferrites are far more unforgiving of high SWR than a powered iron core.
> 
> Once again - this is my understanding from what I have read and if I am wrong
> then please correct it.  Perhaps it may shed light on the correct use for others
> too rather than having one author touting his horn of how wonderfull his idea
> is and that nothing else will do.
> 
> 73, Marinus, ZL2ML

73, Jerry, K0CQ