Saturday, 14 May 2016

U3S Beacon Kit (2) - I search for more RF Power

I have been running the new G0UPL U3S Beacon Kit I built last year for some months now, and I'm very pleased with it's performance.  On 30 metres, I have been seen by W4HBK in Florida, EA8BVP in the Canary Islands, KL7L in Alaska, and SA6BSS in Sweden.

I have been running a pair of BS170's on 5V, and on 30M they produce 270mW of RF to the antenna.

I had read an article by Hans Summers G0UPL, the kit's designer, regarding the efficiency of the U3S PA, and a method he had tried to increase the RF Power from the unit without modifying it too much.  This consisted of changing the configuration of L1 the inductor  feeding the Drains of the PA MOSFET's.  See Hans' original article HERE.  I could easily reproduce his "safe Power" limits on 40M and 30M, but further reading elsewhere convinced me I could get more from these limits by increasing the efficiency of the PA.  A couple of sources I used were HERE and HERE.

I used a single Amidon BN43-202 2 hole Balun Core with 2T of miniature 50 ohm coax wound in it.  The inner of one end was joined to the outer of the other end and became the centre tap. The free outer was connected to the 5V rail and the free inner at the other end to the output coupling Capacitor C5.  The Centre Tap was connected to the 2 BS170's Drains.  The core lies on it's side, there is plenty of space for it on the board, without touching the board above, the OCXO.  I anchored it in place with a piece of double-sided sticky tape.
Drawing of new L1

With 5V on the BS170 drains (See how to set drain current by Hans' Summers "Safe Quiescent Current"), however I set the Quiescent Current to just 0.0 mA I get:

40 Metres                 420 mW
30 Metres                 390 mW

I replaced the spare toggle Switch with a single pole 3 position switch, and added a 7809 9V regulator, as per my earlier Blog.  I can now switch the PA to "Low power" - "Off" - "High power".

With 9V on the BS170 drains (Quiescent Drain Current set as above) I get:

40 Metres                1100 mW
30 Metres                1050 mW

In either case the 2 BS170's are running well within their limits, only getting warm to the touch.  No finger tips or BS170's were harmed in this experiment.

Pete Mulhare
14 May 2016

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