Tom said:
"I can't think of any way to build a directional coupler that won't
show high SWR when the load is shorted. Can anyone think of a
way?"
That is not the problem.
Let me quote my Ten-Tec manual.
"ALC serves three major functions: assures maximum power from the
transmitter without critical adjustment of the input drive, prevents the
amplifier from being overdriven into the non-linear, distortion producing
area, and serves as a power limiting device which protects the output
transistors. It does the first two very well, but the third only
partially..." (Note many SWR protection circuits use the ALC circuitry)
"The broad-band transformer system used with transistors transforms the 50
ohm load impedance not higher, but much lower (in the order of 4 or 5 ohm)
to match the transistor output impedance. Since this transformation is
fixed in design, any reactive component in the load impedance is applied in
a transformed way to the collector circuit...In the case of transistors,
where the collector internal impedance is only a fraction of an ohm,
extremely high currents can be demanded of the power supply, especially with
mismatched loads well below 50 ohms." (Note - a short is well below 50
ohms)
"...The SWR reading gives no indication of reactive components, nor can it
separate the resistive from the reactive components. It is calibrated with
a pure resistive load and therefore has its greatest accuracy with pure
resistive loads."
"... It can be seen that a 3 to 1 SWR on the low side of 50 ohm will ask the
amplifier to deliver much more power than a 3 to 1 SWR on the high side.
Since the amplifier does have a finite value of output impedance, the amount
of power delivered to the load with efficiency will change with load.
Unless the load is near the design value, the transistors will heat up
unnecessarily without delivering an more power to the antenna."
The directional coupler cannot be viewed outside of the entire transmitter
package. I am planning on pulling my Atlas out and do some testing into a
short to see what happens.
Colin K7FM
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