Biomedical Engineering Reference
In-Depth Information
TABLE 6.1
Cable Losses
a
c
a
d
a
total
e
dB/ft
dB/m
dB/ft
dB/m
Z
0
D
/
d
dB/ft
dB/m
Dielectric
1.0
0.253
0.8300525
0.068
0.2230971
77
3.59
0.321
1.0531496
Air
1.36
0.29
0.9514436
0.079
0.2591864
66
3.59
0.373
1.2237533
Air-filled
Teflon
2.1
0.367
1.2040682
0.0985
0.3231627
53
3.59
0.46
1.5091864
Teflon
1.0
0.279
0.9153543
0.068
0.2230971
50
2.3
0.347
1.1384514
Air
1.36
0.308
1.0104987
0.079
0.2591864
50
2.64
0.387
1.269685
Air-filled
Teflon
2.1
0.368
1.2073491
0.0985
0.3231627
50
3.35
0.468
1.5354331
Teflon
2. For a constant impedance
Z
0
, the loss decreases as e approaches unity
while the diameter of the inner conductor increases to maintain the con-
stant impedance.
These effects are shown in Table 6.1 for various cables having an outer con-
ductor of 0.025 in. with various dielectric constant materials (typical loss factor
of 0.0001) operating at 2450 MHz.
The following is an example of a cable-antenna assembly design and
testing. The assembly consists of lengths of miniature coaxial cable terminated
in a gap, whip, or helical antenna (Fig. 6.1) in order to radiate microwave power
to treated tissue during balloon angioplasty experiments. The gap is approxi-
mately 0.025 in. wide and is located approximately 0.31 in. from the short-
circuited end of the cable. Two cable sizes have been used for most of these
assemblies. The basic characteristics of these two commercially available semi-
rigid coaxial cables are given in Table 6.2.
A typical test procedure for the cable-antenna assembly included the fol-
lowing steps. After assembly, the cable and antenna were first checked for con-
tinuity and leakage. Recording the total DC resistance with the gap shorted
(short circuited) is also recommended as a simple check on the cable-antenna
potential microwave loss. The cable-antenna assembly was tested under high-
power conditions at 2450 MHz to verify its ability to handle the power without
breakdown or excessive heating. The cables were surrounded by a test catheter
and a simulated arterial environment during this test. For example, the cables
were wrapped in wet sponges over their full lengths during the tests. The gap
antennas were inserted into a plastic pipette containing deionized water. The
pipette was wrapped in a saline-soaked sponge for RF loading purposes. The
temperature at the outer pipette surface was monitored as evidence of the RF
heating from radiated power. The wetness and contact of the sponges varied
considerably, causing the temperature rise to be only qualitative. All units
were subjected to an input of 30 W for 30 s, monitored for excessive reflected
power, and inspected for damage after removal. The results are presented in
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