Measurement of the Radio Emissions from the Galactic Poles R.H. Tillman∗ May 22, 2014

Contents 1 Introduction

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2 System Model 2.1 Sky Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Receiver Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 The Data

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4 Results and Future Work

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∗ Virginia

Tech, email: [email protected]

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1

Introduction

This memo reports on an attempt to use our system to measure the galactic polar region radiation spectrum. The purpose of this measurement is two-fold: 1) to demonstrate understanding of the system model, and 2) to demonstrate galactic-noise dominated performance of the RF receiving system. The measurement was performed at Pandapas pond, near Blacksburg, VA. Only one antenna was necessary to achieve the measurement goals, and thus a broad band, incoherent Spectrum Analyzer (SA) was used for data collection, as opposed to the S60. The rest of this report is organized as follows. Section 2 presents both the model of the sky and the receiving system. Section 3 shows the data, and the meager RFI mitigation scheme. Section 4 removes the system response from the data and compares to the model.

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System Model

2.1

Sky Model

The expected antenna temperature for a dipole antenna is given by [1] Tsky =

c2 1 Iν 2 2k ν

(1)

where k is Boltzmann’s constant, c is the speed of light, ν is frequency in Hz, and Iν is the intensity, which for the galactic polar region is [2] −0.52 −0.8 Iν = Ig νM Hz + Ieg νM Hz

(2)

where Ig = 2.48 × 10−20 , Ieg = 1.06 × 10−20 , and νM Hz is the frequency in MHz.

2.2

Receiver Model

The receiver consists of three main components: 1) a straight wire dipole antenna, 2) front end electronics (FEE) [3], and 3) an analog receiver providing signal transportation (cable), filtering, and signal gain to interface with the digitizer. The system gain, consisting of components 2 and 3, is shown in Fig. 1, along with a polynomial fit to the frequency range of interest. The system is currently configured to operate in the S60’s first Nyquist zone (NZ1), which is why the band pass does not go all the way to 80 MHz. The amount of power transferred from the antenna to the receiver is characterized by the impedance mismatch efficiency IME, defined as 2

IM E , 1 − |ΓA | where ΓA =

ZA − Z0 ZA + Z0

(3)

(4)

is the antenna reflection coefficient, and ZA (Z0 ) is the antenna (pre-amplifier) input impedance. In this work, ZA is modelled using a lumped element model [4]. The presence of the notch filters in the FEE (see Figs. 2 and 3 in [3]) complicates this definition. To circumvent this, we consider the notch filters as part of the antenna, such that ZA is the cascade 2

Figure 1: Modelled (blue) system gain, and a fit to it (red) for the frequency points in the measurement. of the notch reactances cascaded with the dipole impedance. This is analogous to putting a coil at the end of a monopole antenna; the coil cancels the monopole’s reactance, but is still considered part of the antenna and not a separate device.

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The Data

Figure 2 shows time, frequency, and spectrograph plots from the measurements. Each pixel has a time-frequency resolution of 33 ms × 100 kHz. The time series/power spectral density (PSD) are simply the average of the pixels at a given time/frequency, respectively. As a crude means of mitigating the RFI effects on the spectral measurement, pixels with a measured power 1 dB above the PSD at the respective frequency were flagged. The masked dataset is shown in Fig. 3.

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Results and Future Work

Figure 4 shows the PSD with the system responses removed. The Cane model is also plotted, and satisfying agreement is seen between the two. The mean offset from the model is primarily attributed to the fact that the Cane model really represents a minimum in the sky temperature which varies diurinally. The offset at the lower frequencies (< 35 MHz) is attributed to the system temperature of the FEE (see Fig. 9 in [3]). The slight curvature in the measured temperature is due to a close, but imperfect, system model due to parasitics in the filter elements.

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Figure 2: Time, frequency, and spectrograph representation of the raw data at the system output.

Figure 3: Time, frequency, and spectrograph representation of the masked data.

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Figure 4: (solid) Calibrated and (dashed) modeled antenna temperature. The FEE’s three state calibration capabilities will remove both the gain and the noise temperature from the measurement. The next logical step is to perform this measurement again with the calibration running. This will require a revision to the system model, since the notch filters are after the switch [3].

References [1] S. W. Ellingson, J. H. Simonetti, and C. D. Patterson, “Design and Evaluation of an Acive Antenna for a 29-47 MHz Radio Telescope Array,” Antennas and Propagation, IEEE Transactions on, vol. 55, no. 3, pp. 826–831, March 2007. [2] H. V. Cane, “Spectra of the non-thermal radio radiation from the galactic polar regions,” MNRAS, vol. 189, pp. 465–478, Nov. 1979. [3] R. Tillman, “Design and evaluation of a HELA–10 based FEE with 3–state switched calibration,” Tech. Rep. 6, Apr. 2013. [Online]. Available: https://filebox.vt.edu/users/hankt5/ Public/ [4] T. Tang, Q. Tieng, and M. Gunn, “Equivalent circuit of a dipole antenna using frequencyindependent lumped elements,” Antennas and Propagation, IEEE Transactions on, vol. 41, no. 1, pp. 100 –103, Jan 1993.

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