On Tue, 2008-06-24 at 15:35 +0800, Marinus Loewensteijn wrote:
> Just bought a 228 ATU. Front is immaculate, bottom and top have some
> scratches but a paint job will take care of that.
>
> The SO239 sockets are well used so those are replaced with N-sockets in line
> with all my other gear.
>
> When the cover was off it showed that the toroid / switch has the plastic
> melted on one side. Further inspection revealed that the capacitor
> connections were swapped around: the TX is connected to the antenna side and
> the antenna is connected to the TX side. This may have had an influence on
> not optimal tuning practices but more likely it has been used with a badly
> matched vertical antenna and too much power.
>
> I've been looking for an alternative configuration that will minimize /
> avoid this problem of high losses in the main inductor.
>
> What I have discovered so far is that the greatest losses in a T-match appear
> due to: (too) small variable capacitors combined with low frequencies and low
> impedance.
>
The losses in a T-match are greatest when the loaded Q of the match is
higher than necessary for the impedance transformation needed. This a
condition easy to achieve because there are many L and C combinations
that will tune a particular load. The one with the lowest loss is the
one with the lowest loaded Q. It may be of benefit for finding this
situation (also should show the greatest bandwidth, but that's affected
by the bandwidth of the antenna) to include a watt meter on the antenna
side and pick the match components that give the maximum output power.
That's where the advocates of an L match often win. There is only one
value for L and C for matching a given load at a frequency. Its the
lowest loaded Q that will do that match.
> I intend to use this ATU for coax-coax matching for "tuned" antennas in order
> to get a 1:1 SWR across the band.
>
> I am considering to use an 1:4 impedance transformation at the output using a
> meaty toroid (T184-2) and have the tuner "see" a nominal 200 Ohm output
> towards the antenna which gets "down transformed" to 50 Ohm for the coax.
>
So long as that toroid isn't upset by high voltage to lead to core
saturation, it should work. I've contended that for a general purpose
tuner that a 1:4 balun on the input so that the tuner's basic impedance
matches to 200 ohms can take a smaller set of switched C and L to match
a load impedance that can vary from a few ohms to a kilohm or so. But
I've not built one that way.
> Using the ARRL TL software indicates this should at a minimum half the losses
> in the main inductor while at the same time the capacitor values become well
> within a range of far more practical values.
A low impedance load always demands a large load size capacitance, no
matter what the circuit. Adjusting the raw impedance with a transformer
can be a benefit.
>
> I know that there will be additional losses in the impedance transformation
> toroid but that should dissipate without causing damage.
>
> ( Since I will be working with matched antenna's I do not expect any great
> reactances or abnormal high impedances on the feeder. The impedance
> transformation toroid "should" therefor run within a reasonable load area and
> "should" not have extreme losses. )
>
> Any feedback on the above proposed change will be greatly appreciated.
>
> 73, Marinus, ZL2ML
>
Question remains wondering if the insertion loss of the tuner will be
less than the mismatch loss from the Tentec PA feeding the antenna
directly.
73, Jerry, K0CQ
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