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P. Gorham, SLAC SalSA workshop 1
Saltdome Shower Array: Antennas & Antenna Modeling
Peter Gorham
University of Hawaii at Manoa
P. Gorham, SLAC SalSA workshop 2
SalSA antenna requirements
Must fit down a borehole, between 2 and at most 4.5 to 5” diameter
Must pass tension member through its center (clamp on or integral)
Dual polarization a high priority (linear polarizations seem like the best bet for bandwidth)
Should be fully compensated for lithostatic pressure
Wide bandwidth: 100% or more single-mode if possible
Easy to make, low cost, no integral electronics
P. Gorham, SLAC SalSA workshop 3
Antenna test & modeling
UH SalSA group has dedicated anechoic chamber 14’ by 10’ by 24’, good 100-20,000 MHzFoundation slab can support large salt stack for embedded antenna testsTest equipment: time domain response to ~10ps; spectral analysis to 20 GHz
Antenna modeling:NEC2, NEC2_salt (embedded dieletric version): Beam patterns, impedanceXFDTD (commercial package): time domain response
P. Gorham, SLAC SalSA workshop 4
Dual polarization Antenna studies
Vertical polarization is easy
“Fat” dipoles are excellent for this
Horizontal polarization is tough in a narrow boreholeUse complementary slot antenna, but matching the vertical is difficultBest solution: probably adjacent (but separate) antennas
Test data
P. Gorham, SLAC SalSA workshop 5
Fat dipole results in salt
4” diameter by 30 inch length, copperUsable from 50MHz to 1 GHz (better than model predicts)Single mode from 50-350MHz
120 MHz
370 MHz
180 MHz
530 MHz
Frequency, Hz/MHz
SWR (predicted)
SWR (measured)
Gain, dB
50 ohmfeedpointcoupling
P. Gorham, SLAC SalSA workshop 6
Finite difference time domain, slot-cylinder
reference antenna with parameters:cylinder length = 81.5 cmcylinder radius = 5.1 cmslot width = 30 mmslot length = 68 cmplaced in air space without HDPE
P. Gorham, SLAC SalSA workshop 7
Slot-cylinder in salt and air
Dramatic effects when immersed in salt, but surprisingly, HDPE core gives best bandwidth
SWRs at R = 150
0
2
4
6
8
10
12
14
16
18
201.
50E
+08
2.04
E+
08
2.59
E+
08
3.13
E+
08
3.67
E+
08
4.22
E+
08
4.76
E+
08
5.30
E+
08
5.84
E+
08
6.39
E+
08
6.93
E+
08
7.47
E+
08
8.01
E+
08
8.56
E+
08
9.10
E+
08
9.64
E+
08
Frequency
SW
Rs
filled with air, in air
filled with HDPE, in air
filled with salt, in salt
filled with HDPE, in salt
P. Gorham, SLAC SalSA workshop 8
More slot-cylinder in salt
50 ohms impedance match in salt is tolerable, probably even better with HDPE core.
slot37.id -- cylinder length = 81.5 cm, cylinder radius = 5.1 cm, slot width = 15 mm, slot length = 79 cm
02
468
10
121416
1820
1.00
E+
08
1.46
E+
08
1.92
E+
08
2.38
E+
08
2.84
E+
08
3.30
E+
08
3.76
E+
08
4.22
E+
08
4.68
E+
08
5.14
E+
08
5.60
E+
08
6.06
E+
08
6.52
E+
08
6.98
E+
08
7.44
E+
08
7.90
E+
08
8.36
E+
08
8.81
E+
08
9.27
E+
08
9.73
E+
08
Frequency
SW
Rs R = 50
R = 100
R = 150
P. Gorham, SLAC SalSA workshop 9
Slot-cylinder Antennas
Best response at feedpoint impedance of about 150 to 200 ohms, need 3:1 or 4:1 transformer balun
Antenna model shows good efficiency from 175 MHz up to more than 700 MHz
But: very sensitive to diameter--this assumes about a 4 inch borehole or more
P. Gorham, SLAC SalSA workshop 10
Slot-cylinder predicted beam patterns
Antenna is single-mode from 180-280MHz, goes multi-mode after that2-3dB of front-to-back asymmetry favoring the side with slot
180 MHz 210 MHz 280 MHz
330 MHzantenna
P. Gorham, SLAC SalSA workshop 11
Summary
4 inch diameter dipole and slot-cylinder antennas look good
Dipole ~80cm high, good Vpol single-mode response from <100-350MHzSlot-cylinder of order same length will get good 180-300 MHz Hpol single-mode, but with ~few dB front-to-back directionalityPolarization separation for both is outstanding (-20dB cross-pol at all angles)
Helical antennas investigated but do not provide clean polarization separation
Become elliptically polarized at off-principal axesBandwidths relatively narrow
Combination of fat dipoles and slot-cylinders seems a viable configuration for SalSA
