Homemade Interferometer

From AstroBaki
Jump to navigationJump to search

Reference Material

<latex> \documentclass[11pt]{article} \usepackage{graphicx} \usepackage{amsmath} \usepackage{fullpage} \begin{document}

\subsection*{The antenna} The interferometer consists of two dipole antennas. The dipoles are made out of single sided copper circuit boards. The dielectric on these boards was FR4 with a dielectric constant of K=4.3. They were milled so that only 4 strips of copper remained (each being ~5). These formed two perpendicular dipoles (one for each polarization). The dipoles are tuned to pick up 750MHz (in the middle of the ultra high frequency (UHF) band). Above and below the dipole plane are two circular metal sheets or radius ~2.5. These sheets are use to increase the frequency response of the dipoles by a few hundred megahertz.

\subsection*{The Signal Path} The interferometer consists of two dipole antennas; here we describe the signal path for one of the two polarizations available to each antenna.

The signal path begins at the sky, whose temperature at 750 MHz is approximately 10 K (obtained through models of the sky), which corresponds to a power of approximately -95dBm (P=k*T*BW). This initial -95 dBm needs to be increased to approximately -22 dBm, which is the input power that the analog-to-digital converters (ADC) need to output 16 counts rms into the correlator. This is the optimum value we want going into the correlator.

The antennas are set up with an East-West baseline of 259 inches (approx. 16.5 wavelengths DOUBLE CHECK THIS NUMBER) . Each antenna is propped up onto a ground screen, which has a circle cut-out in the middle to allow the signal-path cables to go through. The ground screen is used to reflect away anything coming up from the ground, as well as the sidelobes characteristic to dipole antennas.

The signal is first run through a balun, which turns a differential signal into a single-ended signal. Next comes a series of amplifiers. The first is a 12.5 dB gain amp, with a noise figure of 2.5, connected directly to the balun. Following this first amp are two 18.5 dB gain amps (NF = 3.5 each), which are connected by a 40-foot LMR cable to a chassy which contains a 530-730 MHz filter, and three 13.5 dB gain amps (NF = 4.5 each). The three amps and the filter are connected in series by hand conformable SMA cables.


The output of the chassis is connected to the iADCs inside the ROACH board. Lastly, the iADC is also connected to a signal generator that outputs a 800 MHz sine wave. This is needed to sample the signal coming in at 800MHz, which corresponds to a bandwidth of 400 MHz. Note that since our bandpass filters go from 530-730 MHz, we are looking at the second Nyquist band (400 - 800MHz).


\subsection*{First Trial-HFA} The first trial was run at 6pm on 10/11/2011. Unfortunately, we did not observe any fringes. After double-checking all connections, the auto-correlations did not show any convincing passbands.

\subsection*{Secong Trial-HFA} The second trial was run at approximately 2pm on 10/12/2011. The signal path had to be changed from that described above, since we found that the sky signal power had significantly increased from the standard -95dB. This was likely due to more RFI present during the day, and possibly the Sun(??). We removed all but one 13.5 dB gain amplifier (I THINK WE TRIED LEAVING THE 1309 BY ITSELF TOO?), for each antenna. Although this increased the power to an appropriate level for our iADCs, we still did not detect any fringes, and the auto-correlations for each antenna again did not show convincing passbands.

\subsection*{Third Trial-Leuschner}