A TWTA (travelling wave tube amplifier) is the high power amplifier at the end of many uplink chains: DSNG trucks, teleports, flyaway systems and earth stations. A used or refurbished TWTA can give years of reliable service, but the value of the unit depends almost entirely on the condition of the tube and the high voltage power supply. Below are the points that tell a good unit from a tired one, and the questions worth asking before ordering.
1. Tube hours and elapsed time counters
The travelling wave tube is a wear part. Most modern TWTAs keep elapsed time counters, typically for unit on time, transmit (beam on) time and standby time. Transmit hours say the most about tube wear. A photo or screenshot of the counters from the front panel or the web interface is a reasonable request for any used amplifier.
Standby time also counts. With the beam off, the cathode is still heated. CPI, for example, states in the manual of its Ka-band outdoor TWTA that leaving the amplifier in standby for more than 4 hours is not recommended, because TWT longevity can be reduced.
2. Helix current and helix voltage
Helix current is one of the most telling health indicators of a TWT. A healthy tube shows a low and stable helix current, with RF drive at rated output power. A helix current that rises over time points to an ageing tube or a focusing problem. The amplifier controller shows both values, and every amplifier has a factory set helix over current fault limit. Comparing the current reading with the test data of the tube, or with the value logged when the unit was in service, gives a clear picture.
3. Output power, gain and frequency range
The rated output power must be reached across the full band the buyer needs, for example 13.75 to 14.5 GHz for standard Ku-band or 12.75 to 14.5 GHz for extended Ku-band. Check:
- saturated output power at the low, middle and high end of the band
- small signal gain, and the gain adjustment range
- for units with a linearizer: intermodulation or spectral regrowth with the linearizer on and off
- whether a built in BUC is included, and its input frequency (L-band) and reference requirements
4. Fault and meter log
Most TWTAs keep a log of meter readings and faults. A log with repeated arc faults, helix over current faults, high reflected RF or over temperature faults deserves a closer look. CPI advises in the same manual to log meter readings daily during operation, as a rising trend in reflected power or unexplained faults can point to a deteriorating condition.
5. Cooling, mechanics and monitoring and control
- Cooling: fans and air filters wear out. CPI lists fan replacement after 50,000 hours of operation as a service task.
- Outdoor units: check for corrosion, damaged seals and the condition of the waveguide output flange.
- Monitoring and control: confirm the M&C interface works (serial and/or Ethernet), and whether a remote control panel or a 1:1 redundancy controller is part of the deal.
- Power: check the required mains voltage and phase for the site.
6. Long storage
A TWTA that has been on the shelf for a long time should be powered up according to the manufacturer’s procedure and tested under load before shipment, not only switched on. A recent test report with output power and helix current is worth more than a description of the unit.
7. New O.E.M, ex-demo, refurbished or used
On the AI-SAT website the condition is stated in the product title: NEW O.E.M, EX-DEMO, REFURBISHED or USED. The exact test status, the configuration and any warranty are confirmed in the written offer. Used and pre-owned equipment is covered by the DOA terms in the refund and returns policy, unless a longer warranty is agreed in writing.
8. Shipping
TWTAs are heavy and contain high voltage parts, so they travel in proper crating. Prices on the AI-SAT website are Ex-Works Netherlands. Freight, insurance and import duties come on top, and a forklift may be needed to unload larger units at the site.
Short checklist
- Photo of the elapsed time counters (transmit and standby hours)
- Helix current at rated output power
- Output power across the required band
- Fault and meter log
- Recent test report
- Configuration: BUC, linearizer, redundancy controller, M&C
- Condition and warranty confirmed in writing
TWTAs in stock at AI-SAT
AI-SAT has new, refurbished and used TWTAs from CPI, Xicom, Spacepath and others in stock, including the Xicom XTD-750K 750W Ku-band TWTA (refurbished) and the Xicom XTD-400C 400W C-band TWTA (refurbished). All units by band:
For test data, tube hours or a quote on a specific unit, contact AI-SAT at sales@ai-sat.com or via the contact page.
