Broadband VHF Radiometry with a Notch-Filtered Antenna Exhibiting a Large Impedance Mismatch Richard H. Tillman* and Steven W. Ellingson Virginia Tech, Blacksburg, VA July 24, 2015
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Introduction • Some applications for VHF Low Band (30-100 MHz) : • • • •
Radio science, including radio astronomy, riometry, ionospheric studies. Spectrum sensing, including diagnostic and dynamic spectrum access. Government/military applications. Regulatory monitoring.
• Now commonplace in radio astronomy, but typically relying on low RFI levels.
• Can achieve similar performance with simple antennas in locations with higher RFI.
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Need for Filters DTV CH3
• Without input filtering, a sensitive radiometer is susceptible to nonlinear effects. • Input filtering helps ensure the lownoise amplifiers operate linearly, but complicates radiometric calibration.
CB
FM
AM
100 kHz RBW, 3.3 ms integration. 3 m straight dipole, 1.5 m over ground. 150 Ω receiver input impedance. 50 dB gain, 30-80 MHz 3 dB passband.
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Connecting Network Impedance transformer Notch filters for CB steps up the FEE input (27_MHz) and FM (88impedance to 150 Ω. 106_MHz)
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System Model 𝑇𝑠𝑘𝑦 ≈ 9120 K
𝑓 39 MHz
−2.55
• From Circuit Theory
𝑇𝑎𝑛𝑡 = 𝜂 𝑇𝑠𝑘𝑦 + 𝑇𝑔𝑛𝑑 Connecting Network
𝑇𝑔𝑛𝑑 ≈ 150 K
𝑍𝑎𝑛𝑡
𝑍𝑅𝑥 𝐺𝑇
Receiver
𝐺𝑅𝑥 𝑇𝑅𝑥
𝑃𝑅 4𝑅𝑎𝑛𝑡 𝑅𝑅𝑥 𝐺𝑇 ≜ = 𝑃𝐴 𝑍𝑎𝑛𝑡 + 𝑍𝑅𝑥 2 Note 𝐺𝑇 ≠ 1 − Γ𝑎𝑛𝑡 2 𝑆𝑜𝑢𝑡 unless 𝑋𝑎𝑛𝑡 or 𝑋𝑅𝑥 is 0. • Measured PSD
(1)
𝑆𝑜𝑢𝑡 = 𝑘𝐺𝑅𝑥 𝐺𝑇 𝑇𝑎𝑛𝑡 + 𝑇𝑅𝑥
(2)
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System Model 𝑇𝑠𝑘𝑦 ≈ 9120 K
𝑓 39 MHz
−2.55
• From Circuit Theory
𝑇𝑎𝑛𝑡 = 𝜂 𝑇𝑠𝑘𝑦 + 𝑇𝑔𝑛𝑑 Connecting Network
𝑇𝑔𝑛𝑑 ≈ 150 K
𝑍𝑎𝑛𝑡
𝑍𝑅𝑥 𝐺𝑇
Receiver
𝐺𝑅𝑥 𝑇𝑅𝑥
𝑃𝑅 4𝑅𝑎𝑛𝑡 𝑅𝑅𝑥 𝐺𝑇 ≜ = 𝑃𝐴 𝑍𝑎𝑛𝑡 + 𝑍𝑅𝑥 2 Note 𝐺𝑇 ≠ 1 − Γ𝑎𝑛𝑡 2 𝑆𝑜𝑢𝑡 unless 𝑋𝑎𝑛𝑡 or 𝑋𝑅𝑥 is 0. • Measured PSD
(1)
𝑆𝑜𝑢𝑡 = 𝑘𝐺𝑅𝑥 𝐺𝑇 𝑇𝑎𝑛𝑡 + 𝑇𝑅𝑥
(2)
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Experiment System Backend Receiver 𝐺𝑅 ≈75 dB, 𝑇𝑅 ≈ 800 K. between 30-80 MHz (3 dB bandwidth). Spectrum analyzer is a Rohde & Schwarz FSH3.
Antenna Front 150 m coax Backend Receiver End
Receiver from System Model
Spectrum Analyzer
Amplifiers and Filters Front End Bias
Battery Powered
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Antenna • Using a circuit model for the antenna impedance (T.G. Tang et al., IEEE Trans. Antennas Propag., 1993). 3.048 m 1.524 m
150 Ω 100 Ω 50 Ω
1.2 m × 2.4 m ground screen
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Front End Antenna Terminals
HELA-10
HELA-10
Bias-T
Relays CB & FM Noise Source
Antenna
30—80 MHz
Bias and Control
𝝂 Range TRx Input P1dB (MHz) (K) (dBm)
Int. Cal.
This Work
Straight Dipole
30-80
700
+11 Yes 20 dB gain
EVLA*
Cassegrain Reflector
50-86
710
−13 Yes
LWA1**
Bowtie V-Dipole
10-88
225
−18 No 36 dB gain
LOFAR
Straight V-Dipole
10-80
?
? No
* Includes 2 dB cable loss between antenna feed and FEE. ** LWA1 has five outriggers with internal cal.
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Experiment Result • To estimate the sky temperature from Eqn. 2 on Slide 5 (System Model) : 𝑇𝑠𝑘𝑦 =
1 𝑆𝑜𝑢𝑡 𝑇𝑅𝑥 − 𝜂𝑇𝑔𝑛𝑑 − 𝜂 𝑘𝐺𝑇 𝐺𝑅𝑥 𝐺𝑇
Measured Measured 𝜂=1 𝜂 = 0.8 Expect 𝑍𝐴 model accuracy to deteriorate above ∼ 60 MHz.
Model, assuming a hemispherical antenna pattern and a spacially uniform intensity (H.V. Cane, MNRAS, 1977).
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In Summary Achieved a sensitive, linear measurement of the galactic background noise over a ∼90% bandwidth using a narrow band antenna and a poorly matched front end in a high RFI environment. • Radio science, including radio astronomy, riometry, ionospheric studies. • Spectrum sensing, including diagnostic and dynamic spectrum access. • Government/military applications. • Regulatory monitoring.
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Backup
ARX Calibration • Hot and cold loads were used to calibrate the ARX gain and noise temperature out of the measured PSD. 𝑆𝑜𝑢𝑡 − 𝑆𝑐𝑜𝑙𝑑 𝑆𝑐𝑎𝑙 = 𝑘 𝑇ℎ𝑜𝑡 − 𝑇𝑐𝑜𝑙𝑑 + 𝑇𝑐𝑜𝑙𝑑 = 𝑘𝐺𝑇 𝑇𝑎𝑛𝑡 𝑆ℎ𝑜𝑡 − 𝑆𝑐𝑜𝑙𝑑 𝑆𝑜𝑢𝑡 𝑆ℎ𝑜𝑡 𝑆𝑐𝑜𝑙𝑑 𝑆𝑐𝑎𝑙
𝐺𝑇 vs Impedance Mismatch Efficiency Recall
4𝑅𝑎𝑛𝑡 𝑅𝑅𝑥 𝐺𝑇 = 𝑍𝑎𝑛𝑡 + 𝑍𝑅𝑥 2 which may also be written as 2 𝐺𝑇 = 1 − Γ 𝑎𝑛𝑡 where ∗ 𝑍𝑎𝑛𝑡 − 𝑍𝑅𝑥 Γ 𝑎𝑛𝑡 = 𝑍𝑎𝑛𝑡 + 𝑍𝑅𝑥 However, 𝑍𝑎𝑛𝑡 − 𝑍𝑅𝑥 2 𝐺𝑇 ≠ 1 − Γ𝑎𝑛𝑡 , Γ𝑎𝑛𝑡 = 𝑍𝑎𝑛𝑡 + 𝑍𝑅𝑥 unless either the antenna or radiometer input impedance is real valued.
Expected
Measured???
