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
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