Thursday, January 09, 2014

The Sisterhood Of The Traveling Wave Tube

A traveling-wave tube is a special kind of vacuum tube used in electronics to amplify RF signals.  That's probably why it's also referred to as a traveling-wave tube amplifier (TWTA). The power gain a TWTA can be greater than 40 dB and operating frequencies can range anywhere from 300 MHz to 50 GHz, bandwidth is highly variable.  They are mostly used to make receivers more sensitive. It's used pretty heavily in radar and microwave gear used for satellite communications. They are not that expensive anymore either, you can find some used models on Ebay for under $100 now.

The TWT was invented twice. Once it was invented by Nils Lindenblad at RCA and patented in 1940. And again two years later by Rudolf Kompfner at a secret British radar lab under Prof. Mark Oliphant at the University of Birmingham. John Pierce, and Lester M. Field at Bell Labs made a number of refinements Lindenblad's model. After the end of WWII in 1951, Kompfner went to Bell Laboratories and made further improvements to the TWT based on his own research.

The TWT amplification is controlled by the legnth of the helix, the voltage on the helix, the electron beam diameter and the power input. By design the helix can actually be a  ring loop, ring-bar, or other "coupled cavity structure."  We all know what a helix is (thank you DNA) but some of those others my be less clear. A Ring loop looks like a series of connected flat washers and a ring loop is like connected segments of pipe. That third category, the "coupled cavity structure" has some variations but it's just a series of segmented but connected resonant cavities. These different constructs have different impedance, resonance and bandwidth.

The electron gun produces and then accelerates an electron beam down the axis of the tube. A surrounding magnet provides a magnetic field along that axis focusing the beam. The helix (ring loop, ring bar or etc) lies at the center of the tube and provides a transmission line for RF. An attenuator prevents signal reflection from traveling back up the helix.