[Moon] Question to the 3cm TWTA Experts around

Peter Hansen PH at Focon.dk
Tue Nov 11 15:53:35 CET 2014


I am working on a Traveling Wave Tube project for my 3cm work. I am tired of people not hearing me :O). I have a NEC 14Ghz TWTA that was used for uplink business.
Tubes are a LD4581A from NEC. That tube is a periodic permanent-magnet focused TWT with Depressed Collector. It is a single stage collector tube. Output is WR75 and input is SMA.

All tubes make good power on 14Ghz. 650-700W. On the output side there is a tuning section in the TWT. The first screw is placed 11mm from the output pin from the TWT. Next screw is 16mm from the center and the last on is 21mm. Those screws are in use on 14Ghz. When trying to get it on 10.4Ghz I removed the screws and get 700W output from 2mW drive on 10.368Ghz. Helix stays low until around 600W so that is all fine.

Now to my real problem I try to get the other tubes working on 10.368Ghz as spares. They behave not at all like the first one. The second tube was killed by I guess bad match on the output side. So now I would with care like to get the last tube going. But that tube does not perform on 10Ghz as well. They are all equal number and also equal elapsed filament hours.

I used a network analyzer to measure the RL backwards into the output waveguide (direct on the TWT) with no power on . The good on for 10Ghz have a good match on 10.2-10.5Ghz. But that figure is that something real that I measure ?.
I did also measure the shortening block on the output TWT. It is placed only 3.5mm from the center pin. That seems very short for me also on 14Ghz.

The other one does with all tuning screws in the same place as the good tube only only work on 13.7Ghz. I see a similar dip in Returnloss there at 13.8Ghz. It does make some power on 3cm but poorly and with high helix.

So why do I see such a big difference on those tubes and are there a way I can pull the 13.8Ghz tube down on 10.3Ghz.
There must be someone outthere that have done this before on similar amplifiers.

I can add any pictures needed for easy understanding.

Working conditions on the TWT is:
Ehelix 11.5KV
Vcollector 5.8kv
I collector 400mA
I Helix max 15mA (triplevel)
Anode/grid voltage 5.8kv

If someone have tubes laying around I would be a happy buyer. Good to have spares.

73 Peter OZ1LPR
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