Indian Journal of Radio & Space Physics
Vol. 35, April 2006, pp. 129-132
RF loss profile measurement for a high gain, broadband helix TWT
Vikas Kumar
Amity Institute of Telecom Technology & Management, Sector-125, Noida (U.P.) 201 301, India
and
Anil Vohra
Electronic Science Department, Kurukshetra University, Kurukshetra, Haryana 136 119, India
and
Vishnu Srivastava
Microwave Tubes Area, Central Electronics Engineering Research Institute, Pilani, Rajasthan 333 031, India
E-mail: vk_aggarwal@rediffmail.com, vohras@sancharnet.in
Received 14 January 2005; revised 4 January 2006; accepted 16 February 2006
Helix type structure is widely used as slow wave structure in the travelling wave tube (TWT) due to its natural wide
bandwidth. Reflections present in the structure may cause oscillations and hence instability in the device. In order to
minimize the reflections, a coating of lossy material is done on the helix support rods in a TWT. The loss introduced by such
coating has significant effects on the characteristics of the device. In the conventional methods the loss due to this coating
(also known as the loss profiles) can be measured only on a single rod before fabrication of the slow wave structure (SWS)
assembly. In the present work a new and convenient method has been developed for the measurement of loss profiles of a
fabricated SWS assembly. Using the new method an experimental study has been made on a high gain TWT that is made in
two sections with sever in between. The loss profiles along the axis of helical SWS have been measured at the sever end.
This measurement is found to be very helpful to ensure the accuracy of the final fabricated assembly.
Keywords: Travelling wave tube, Helix, Reflections
PACS No.: 84.40 Fe
IPC Code: H03F; H03 G3/00
1 Introduction
Helix travelling wave tube1,2 (TWT) as shown in
Fig. 1 is a high gain and broadband microwave
amplifier, which is commonly used in communication
systems and satellites. The major components of a TWT
are: an electron gun, a helix slow wave structure (SWS),
PPM focusing system, input and output couplers and the
collector for collecting the spent beam. It is known, by
the analysis of the travelling wave amplification, that an
amount of power is reflected back from the output due to
mismatch through the SWS. If there is a mismatch at the
input also, a portion of the signal will be reflected back
towards the output. This may induce oscillations caused
by feedback signal. To avoid the possibility of these
oscillations caused by mismatch at the input and output
termination in the helix TWT, the helical SWS is
severed in two sections1 as shown in Fig.2. The helix is
grounded at sever end and the reflection at this end is
damped by using proper attenuation. In the two-section
TWT of Fig. 2, each section will be unstable when the
following condition is satisfied:
(G−L−ρL− ρI) >> 0
...(1)
where,
G = Small-signal gain of the section
L = Circuit loss of the section
ρL = Return loss at the output end
ρI = Return loss at input end
Helix inside the barrel is normally supported by
rectangular (or cylindrical) rods of the dielectric
material such as Alumina (Al2O3), Beryllia (BeO) or
anisotropic Pyrolytic Boron Nitride (APBN) having
high thermal conductivity. In SWS assemblies, used
for measurements, helix has been supported by three
rectangular rods of APBN. These rods have been
coated with carbon by Pyrolytic deposition3 for tip
loss. The loss profile is properly contoured to avoid
any reflection caused by it4. The tip loss coating in
helix is shown in Fig. 3 along with the cross-sectional
view.
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INDIAN J RADIO & SPACE PHYS, APRIL 2006
Fig. 1⎯ Schematic of TWT
Fig. 2⎯ TWT in two sections with sever in between
Fig. 4⎯ Block diagram of the measurement set-up
Fig. 3⎯ Tip loss coating in a SWS
In conventionally used methods loss profiles are
measured only on a single rod before the fabrication
of complete SWS assembly. This is done by placing
the support rod inside a flat waveguide with hole and
the output power is sensed corresponding to input by
using a power meter. The present paper discusses a
simple and convenient method for measuring the loss
profile of a fabricated helical SWS assembly
incorporating all the support rods.
2 RF measurement set-up
Measurements have been done by measuring the Sparameters using the Hewlett Packard 8510B Vector
Network Analyser5. The set-up was consisting of an
oscilloscope, a colour plotter and a signal generator of
frequencies 45 MHz-40 GHz. The block diagram of
the measurement set-up has been shown in the Fig. 4.
In measurements, port-1 probe of the S-parameter test
set-up has been used to provide RF input to the SWS
assembly under test. Whereas the port-2 probe has
been conneced to a stainless steel rod of diameter
1.1 mm and length 44.5 mm through a proper adaptor.
A Teflon bush is used as an adaptor in order to
maintain the symmetry of the sensor probe inside the
structure and to ensure that the sensor probe does not
touch the helix. The body on which the measurements
are to be carried out is placed inside a metallic block
having a round hole with a hollow cylindrical body of
Teflon in the centre. The port-2 probe by which the
VIKAS KUMAR et al.: LOSS PROFILE MEASUREMENT FOR BROADBAND HELIX TWT
131
Fig. 5⎯ Schematic of the body under measurement
measurement rod is connected is mounted on a slider
assembly and can be moved inside the SWS of the
body. The schematic of the body under measurement
is shown in Fig. 5. The present method is used to
measure the profile of the loss only along the supports
rods and is not intended to measure the absolute loss.
Hence, any change in the signal strengths due to the
presence of probe would affect all the measurements
in a similar fashion and will not be affecting the shape
of the loss profile.
3 RF measurement test results
In order to make the measurements the RF input
is applied through probe 1 of the test set-up and
loss is then sensed through probe 2 as shown in
Fig. 5. Output is in the form of the loss parameters
S11 and S21, which can be seen on the oscilloscope
screen of the analyzer. In the measurements, the
input and output SWS assemblies of a 60 W C-band
space TWT have been taken and a set of 80
readings was taken for each case at an interval of
0.5 mm distance, i.e. moving the sensor probe along
the axis. The loss profiles as obtained are shown in
Figs 6 and 7. Figure 6 shows the variation of S21
parameter with the distance along the coating in the
SWS assembly-I of TWT. The maximum loss
obtained is 80 dB at a distance of 5 mm along the
coating. The graph then shows a gradual variation
along the axis of helix and attains a minimum value
of 10 dB at the distance of 35 mm. Similarly in
Fig. 7, the variation of the S21 of assembly-II is
shown along the axis. The maximum loss in this
case is 56 dB at a distance of 11 mm along the axis,
which then decreases and gives minima at a
position of 36 mm along the axis. The results are in
close agreement with the tapered coating, as the
loss decreases with movement along the axis of the
helix.
Fig. 6⎯ Loss profile of the input body (SWS assembly-I)
Fig. 7⎯ Loss profile of the output body (SWS assembly-II)
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INDIAN J RADIO & SPACE PHYS, APRIL 2006
4 Conclusions
The loss profile, being a very critical aspect of
achieving high gain and high efficiency4,6 of a TWT
with good stability, has been studied. The loss profiles
of the two sections of a 60 W C-band helix TWT are
studied using the present method and good results are
found. The presented method is found to be much
helpful for the measurements of loss profile and can
be used to detect and improve the errors regarding the
losses during the fabrication of a helix SWS.
Acknowledgements
Part of the work was done at CEERI, Pilani. One of
the authors (VK) is thankful to the Director, CEERI,
Pilani, for granting him permission to work at MWT
Area, CEERI, Pilani.
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