Application of Goubau Surface Wave Transmission Line for Improved Bench Testing of Diagnostic Beamline Elements Page: 2 of 3
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Proceedings of PAC09, Vancouver, BC, Canada
Horn diameter and taper are somewhat empirical, with
the final horn radius roughly equal to the radius
encompassing the 1/r field characteristic. Nomographs
can be used to simplify the design process, subsequently
leading to experimental optimization; ranges between X/4
and , are typical.
Taper is a function of limiting higher order modes,
while maintaining the proper planar wavefront .
Empirical diameter-to-length (D/l) ratios have been
published, ranging from 0.4 to 1, which serve well as
design baselines .
A prototype G-line was loosely patterned after
Goubau's UHF SW description, using the inner conductor
of RG-174 coaxial cable, and brass shim stock for the
launchers . A design nomograph predicted a nominal
wire impedance of 225 Ohms for the polyethylene-coated
conductor . A second nomograph was consulted, which
prescribed a horn radius of 5 cm, given a = 20 cm and
conductor with 1.5 mm diameter (wire plus dielectric) .
A D/l ratio of 0.4 was empirically chosen for the horn,
resulting in a cone diameter of 5 cm and a length of 14
cm. No other form of matching was included in the
preliminary design. Insertion and return losses were
measured using an Agilent E5017 network analyzer, and
although the performance was sub-optimal, the SW
mechanism was clearly present.
Subsequently, collaboration resulted in locating and
adapting a commercial system intended for distributed
wideband communication systems, under the trade name
RadWire by Rubytron . Preliminary measurements of
the flared horn resulted in a diameter of 10 cm, and D/1
ratio of 0.54. The non-linear, exponential-like taper
facilitates a broadband impedance transition from 50
Ohms to the higher wire impedance . The inner
conductor is tapered, mating with the supplied enamelled
#14 AWG conductor. A nominal 250 Ohm impedance is
predicted for the 1.6 mm wire diameter .
The RadWire system was assembled atop an optical
bench, along with an X-Y stage, possessing <10 um
resolution. Reciprocal s-parameter measurements were
performed, in order to fully characterize the G-line
configurations, and evaluate the launching process. In
addition, all tests were performed with and without a
sample 4-wire BPM (terminated), typical of JLAB
installations, in order to observe the effects of SW
perturbations. Aside from energy extraction at 1497.0
MHz from the nominal 15 dB coupling factor, minimal
impact was observed over the entire 8 GHz frequency
The RadWire is remarkably broadband, exhibiting a
nominal 3-4 dB insertion loss, and flat 521, as shown in
Figure 2, consistent with predictions in the literature .
Figure 2: Insertion loss (S21) plot of 1.6 mm diameter
RadWire, demonstrating broadband SW characteristic
with 3-4 dB nominal loss.
Return loss was nominally greater than 10 dB (VSWR
= 2:1) over the 1 - 8 GHz sweep range, a desirable trait
for pulsed RF measurements. A VSWR of 2:1 implies
88% of the energy is transferred to the transmission line
system. Figure 3 shows the S11 plot, exhibiting minimal
structure, thus implying good transmission line
I Acve ChfT-a 2 Resporse 3 tEr u MrnL ArnsE 5 Insrr State
In an effort to more closely model the 200 um diameter
of the CEBAF electron beam, #34 AWG enamelled
magnet wire ( 6 = 160 um) was substituted for the
RadWire conductor, and again measured. Extensions were
required to complete the taper, and clear enamel was
appliedft the soldered surfaces to ensure the presence of
dielectric. S21 data is shown in Figure 4.
T03 - Beam Diagnostics and Instrumentation
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John Musson, Keith Cole, Sheldon Rubin. Application of Goubau Surface Wave Transmission Line for Improved Bench Testing of Diagnostic Beamline Elements, article, May 1, 2009; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc834300/m1/2/: accessed June 18, 2021), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.