Modifying the RW1127 and similar TWTs for 24GHz - F1CHF
fashioned from expanded polystyrene to hold it in place. This modification applies to both input and output. Next, the SMA assembly is replaced with waveguide.
Modifying the RW1127 and similar TWTs for 24GHz Some notes by Brian G4NNS updated after the EME conference. During a visit from Johannes DF1OI he explained how he and Ulli DK3UC had modified Siemens RW1127 TWTs for operation on 24GHz. Transforming this 2W 12GHz tube to a potential 40W 24GHz tube sounded almost too good to be true, the more so as the modification he described was quite simple. At the EME conference at Wuerzburg I was able to watch Ulli carry out the most delicate part of the conversion. This had already been done for the tubes I had received. Step one is to remove the clamps supporting the SMA barrels at the input and output to provide better access. Tubes in this series have SMA input and output connectors and the structure of the SMA connectors includes impedance matching at the design frequency. The next task therefore is to remove the SMA assemblies. These unscrew but are held with Loctite. To avoid the use of undue force which will destroy the tube, the SMA barrels should be heated with a hot air gun until they turn with minimal force. Next, the most delicate operation, is to remove the matching disks from the input and output pins.
The discs are held in place by (usually two) spot welds. These are filed or ground away to free and remove the disks. If access to the welds is not possible with a file, a small grinding tool on a multi tool such as Dremel can be used. Care must be taken not to damage the pin but it may be necessary to file or grind away any obstructions which stop the disc from sliding off.
Next an M2 (or similar sized) full nut is dropped onto the protruding pin and a washer is fashioned from expanded polystyrene to hold it in place. This modification applies to both input and output.
Next, the SMA assembly is replaced with waveguide (WG20 / WR42). This modification requires two 3mm thick brass plate with two M3 fixing holes to match the fixings for the SMA assemblies and an 8mm clearance hole to fit over the probe assembly.
The waveguide has an 8mm clearance hole for the probe assembly and a 3mm hole on the opposite side for the probe to protrude into.
Tight fitting plugs (sliding shorts ) are fitted to the end nearest the probe and flanges to the other ends of the waveguide sections. The plugs were machined very slightly over size and then lapped very carefully on fine emery paper until they were a tight but sliding fit in the waveguide.
The plates and waveguide sections are carefully aligned (I used an old 8mm drill to assist with this) and soldered together. (I used a thin layer of solder paste to ensure that good contact was achieved between the waveguide and the plate). Note:- Plugs fit at the left hand end near the probe not as shown. And maybe chokes would be better. I believe Johannes used some shorting pins held in place by magnets instead of sliding shorts which are a bit of a fiddle to make and to adjust.
Modification of the RWN32x PSU The PSU switches should be set for the type of tube you are using according to the table printed on the power supply cover. Additionally it is helpful to drill a hole in the cover to provide access to the Helix adjustment potentiometer which is the right hand one in the picture P900-1.
PSU Interface and Monitoring circuit. The RWN32x series power supply provides monitoring of Helix and Cathode current, Alarm indication and reset and a Standby / TX or PTT input. These signals are isolated within the PSU but I provided some additional isolation of the reset, PTT and alarm signals and also buffered the current indicators. I did this mainly because the Helix current indicator provides 1V / mA into 100K and I did not have a suitable meter (20uA for 2mA FSD with 100K). Also I wanted to be able to monitor these signals locally when testing the system on the bench (or perhaps for terrestrial /P operation) or remotely in the shack some 30m cable run from the EME antenna system and the PSU. So maybe my circuit adds unnecessary complications but I include it in these notes in case it helps
Note that the meters are acting as voltmeters and the series resistor values will need to be selected to suit.
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