An experimental, field replaceable travelling wave tube (TWT) with low profile, TEM
transitions has been developed with the aim of combining the cost advantages of field
replaceable TWTs with the bandwidth advantages of TEM transitions. Field replaceable
TWTs are cheaper because their periodic permanent magnet (PPM) stacks (which are
expensive) are easier to produce and can be reused. Standard TEM transitions have a
high profile and prevent TWTs from being plugged into PPM stacks. The stacks of
packaged TWTs are therefore built onto the tubes, using split ring magnets, before the
tubes are potted. Upon failure, such tubes are discarded together with their focusing,
coupling and cooling structures.
Two types of low profile, TEM transitions were investigated: a ring and a plug transition.
In the former, the helix was joined to a metal ring that was sandwiched between two
ceramic rings in the vacuum envelope. Due to its high shunt capacitance, this transition
was narrowband. In the plug transition, the helix was joined to a small metal plug in the
wall of a ceramic tube. Plug transitions with a return loss of better than 10 dB over an
octave were made but some technological difficulties were encountered. Metal/ceramic
joining with active solder is therefore discussed.
The development of graphite attenuators was attempted but graphite of high enough
resistivity to achieve the attenuation and matching required was not available. Thin film
attenuators were therefore used instead. Some good attenuators were made but the
molybdenum films were sensitive to processing conditions.
An empirical procedure was developed to magnetize PPM stacks to a wanted profile.
The stacks that were designed and built resulted in good beam focusing. TWT tests
were made using an automated test station that was built. Maximum RF gains and
output powers of 20 dB and 43 dBm respectively were measured.
The TWT design, construction and testing was a combination of materials science and
microwave work. Some simulations were used to solve electromagnetic problems but
much of the work was practical, involving trial and error and many measurements. / Thesis (Ph.D.)-University of Natal, Durban, 1994
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/6907 |
| Date | January 1994 |
| Creators | Vassilopoulos, Nicolaos. |
| Contributors | Nattrass, H. L. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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