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 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 workshop

4. 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 workshop

5. Fat dipole results in salt 120 MHz 180 MHz Gain, dB • 4” diameter by 30 inch length, copper • Usable from 50MHz to 1 GHz (better than model predicts) • Single mode from 50-350MHz 50 ohm feedpoint coupling 530 MHz 370 MHz SWR (predicted) SWR (measured) Frequency, Hz/MHz P. Gorham, SLAC SalSA workshop

6. 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 workshop

7. Slot-cylinder in salt and air • Dramatic effects when immersed in salt, but surprisingly, HDPE core gives best bandwidth 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. 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 180 MHz 210 MHz 280 MHz • Antenna is single-mode from 180-280MHz, goes multi-mode after that • 2-3dB of front-to-back asymmetry favoring the side with slot antenna 330 MHz 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 <100-350MHz • Slot-cylinder of order same length will get good 180-300 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 P. Gorham, SLAC SalSA workshop