P. Gorham, SLAC SalSA workshop1 Saltdome Shower Array: Antennas & Antenna Modeling Peter Gorham University of Hawaii at Manoa

P. Gorham, SLAC SalSA workshop2 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 workshop3 Antenna test & modeling UH SalSA group has dedicated anechoic chamber 14’ by 10’ by 24’, good ,000 MHz Foundation slab can support large salt stack for embedded antenna tests Test equipment: time domain response to ~10ps; spectral analysis to 20 GHz Antenna modeling: NEC2, NEC2_salt (embedded dieletric version): Beam patterns, impedance XFDTD (commercial package): time domain response

P. Gorham, SLAC SalSA workshop4 Dual polarization Antenna studies Vertical polarization is easy “Fat” dipoles are excellent for this Horizontal polarization is tough in a narrow borehole Use complementary slot antenna, but matching the vertical is difficult Best solution: probably adjacent (but separate) antennas Test data

P. Gorham, SLAC SalSA workshop5 Fat dipole results in salt 4” diameter by 30 inch length, copper Usable from 50MHz to 1 GHz (better than model predicts) Single mode from MHz 120 MHz 370 MHz 180 MHz 530 MHz Frequency, Hz/MHz SWR (predicted) SWR (measured) Gain, dB 50 ohm feedpoint coupling

P. Gorham, SLAC SalSA workshop6 Finite difference time domain, slot-cylinder reference antenna with parameters: cylinder length = 81.5 cm cylinder radius = 5.1 cm slot width = 30 mm slot length = 68 cm placed in air space without HDPE

P. Gorham, SLAC SalSA workshop7 Slot-cylinder in salt and air Dramatic effects when immersed in salt, but surprisingly, HDPE core gives best bandwidth

P. Gorham, SLAC SalSA workshop8 More slot-cylinder in salt 50 ohms impedance match in salt is tolerable, probably even better with HDPE core.

P. Gorham, SLAC SalSA workshop9 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 workshop10 Slot-cylinder predicted beam patterns Antenna is single-mode from MHz, goes multi-mode after that 2-3dB of front-to-back asymmetry favoring the side with slot 180 MHz210 MHz280 MHz 330 MHz antenna

P. Gorham, SLAC SalSA workshop11 Summary 4 inch diameter dipole and slot-cylinder antennas look good Dipole ~80cm high, good Vpol single-mode response from < MHz Slot-cylinder of order same length will get good MHz Hpol single-mode, but with ~few dB front-to-back directionality Polarization 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 axes Bandwidths relatively narrow Combination of fat dipoles and slot-cylinders seems a viable configuration for SalSA