On Sun, Feb 25, 2024 at 01:19 AM, Howard wrote:
Apologies if this has been discussed. I searched and did not find anything relevant.
Are there are any plans to implement the SWR protection circuit from the QMX in the QDX? The QDX is perfect for certain applications due to the absence of controls.
I agree that it is unlikely that Hans will want to find time for incorporate a reflectometer into the QDX with associated extra software in the controller.? Part of the problem is that, unlike the QMX, there is no easy way of reducing the supply to the final amplifier in the QDX. This would mean that the only way for a reflectometer to operate would be to completely shutdown the transmitter by removing the RF drive. By itself, this may protect the final amplifier, but would give little help to resolve the poor SWR situation.? A better way would be switching to reduced power operation, which might allow matching adjustments to be made before going back to full power.
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It could be possible to make an outboard unit to implement such protection. Transmitter output would pass through a tandem bridge reflectometer (as at the output of the QMX).? Rather than using a processor to monitor the forward and reverse waves, I think an op-amp based circuit might be better because it would operate far faster than any processor based system.? Speed of operation is essential to give maximum protection to the output BS170 transistors (although it still may not be fast enough for high voltages across the transistors). The reflectometer circuit would be arranged to trip at an appropriate SWR (perhaps 2:1) whereupon a MOSFET switch in series with the 12V QRX supply would open to put a string of silicon diodes to reduce the supply to around 8V. This voltage would reduce output to around 1W which would lessen the stress on the output transistors. Once operating at reduced power, antenna matching could be adjusted as required, returning the QDX to full power afterwards by pressing a reset button. Note that this assumes the QDX is built for 12v operation rather than the 9V option.
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With regard to the op-amp circuit, the simplest arrangement would be to just monitor the reverse wave detector and trip the protection when its absolute voltage exceeds that for a 2:1 SWR.? However, it may actually be better to monitor both forward and reverse wave to that the trip would operate at a defined SWR (i.e. for example 2:1). This would require additional complexity in the op-amp circuitry.
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The diagram below shows the elements of the outboard protection unit.
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Peter Lee
G3SPL
