[TenTec] TenTec 228 ATU

[Marinus Loewensteijn]

> 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 
torid 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? Were reactances 
presented to the toroid? 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
knwon 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 .

> 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 goind 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 teh 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.

73
Marinus


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