On Sat, 2008-06-28 at 12:31 +0800, Marinus Loewensteijn wrote:
> > From: "Dr. Gerald N. Johnson"
> > Subject: Re: [TenTec] TenTec 228 ATU
>
> > Do you know that switch was melted feeding a reasonable load or was it
> > used to feed a short wire with a very low radiation resistance with more
> > than the tuner's rated power?
>
> In the Tentec 228 ATU the toroid is encased in plastic and hte windings
> are silverplated copper. The plastic is sued to keep the windings from
> touching each other and then there is just a wiper connecting to each of
> different windings. Hence the "toroid-switch" naming I used earlier.
>
> According to the ARRL Antenna Compendium Vol 3 the losses on 80 m
> can with a coax:coax matching of 50 Ohms still account to 25% losses in
> the inductor. It does not need a lot of power to melt the plastic since the
> toroid cannot get cooled down sufficiently.
>
> > I've tried to build a 1:9 transformer with large toroids and found it
> > was barely useful on one band where I was wishing for 450 ohms on the
> > high side. 1:9 is quite practical for 5.5 to 50 ohms, but very difficult
> > for 50 to 450 ohms because the high impedance takes so many turns to
> > achieve an inductive reactance a few times larger than 450 ohms that the
> > wire approaches a quarter wave long which sets the high frequency side
> > of the transformer pass band. Again something achievable in a quarter
> > inch toroid, but hard in a 2" toroid.
>
> Was this current balun or was it a voltage balun?
I'm talking transformer. This is a voltage device.
> Were reactances
> presented to the toroid?
No, just a 50 ohm resistor and I tested the input Z with a RX-meter and
it was as narrow as a Pi network. Hardly covered one band. It was fairly
large using a couple 200 sized ferrite toroids, not powdered iron. I was
wishing for it to be big enough to handle more than a KW of RF power
from a couple large tubes.
> The toroid I have in mind is the T184-2, a rather
> late comer to the scene but having a greater cross section than the well
> known T200-2. I can live with a maximum frequency of 14 MHz and if
> (as we say here down under) "push comes to shove" then I do not
> really need 450 Ohms. 300 would be sufficient and I could always use
> a 50 : 75 Ohms transformer and then use a 1:4 balun to get to 300.
> Being at the input it means that no reactances should be present which
> should make construction a lot easier .
Well, no reactances once the tuner is tuned. Are you going to check SWR
between the tuner and the transformer or through the transformer? At
least a test it may prove interesting to compare the reactance of the
tuner's load on the transformer to that seen through the transformer.
E.g. how much reactance is necessary from the tuner to compensate for
the foibles of the transformer.
>
> > The trade off in raising the operating Z of the tuner is that at the LF
> > end, it will require more inductance which is fundamentally lossier than
> > more capacitance at high frequencies.
>
> Just had an email from PA0FRI telling me that some Germans looked at
> his S-Match design and found it very efficient. I built one and used the
> T184-2 core because it would give me the lowest number of windings and
> hence the lowest distributed capacitance of all the "mid sized" cores.
> Power handling is about 25% higher than the T200-2 and the uL is about
> double of the T200-2 (from memory). A very respectable toroid, only a
> small amount bigger than the T157-2.
>
> I do not like to go into "unchartered waters" with a design. If a 1:9
> transformation is going to be very difficult then perhaps I should
> just make an unbalanced version of the S-Match and use a 4:1
> step down toroid on the output (to match from 200 Ohm down to 50
> Ohm). I know the S-match will be a darn side more efficient than a
> t-match.
>
> As it stands at this moment I want to have the capability to adjust
> a 1:4 SWR which means I need to be able to handle 12.5 - 200 Ohms.
And if you make the input Z of the tuner a few hundred ohms, you put it
into an impedance step down mode which limits the bandwidth more than if
the input Z was 50 ohms and you are matching 12.5 ohms. That's not
forward progress, but reverse progress. I like the idea of the higher
input Z, if I'm going to tune an antenna fed through 300 ohm line and
use the same antenna at all multiples of 1/2 wave resonance including
full wave resonance.
>
> 73
> Marinus
>
>
Fact is there are many possible tuner circuits that will work. Some may
be inefficient (like a resistive impedance matching attenuator), many
may have comparable efficiencies, but the differences between them will
often be smaller than the differences over horizon HF propagation loss
measured at ten second intervals.
73, Jerry, K0CQ
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