Steve Ellingson (Virginia Tech)
This page is http://www.ece.vt.edu/swe/eta/AB_051105/
08 Dec 2005 UT 2130. Improved model for dipole; lead to improved agreement with measurements.
11 Nov 2005 UT 0739. Last plot was for strip (not wire) dipole; also changed to common reference Galactic Background curve.
10 Nov 2005 UT 1800. Initial posting.
Measurement Procedure: The measurement is taken at the indoor side of the egress panel. The signal was pre-amplified using a Mini-Circuits ZJL-3G amplifier, followed by a Mini-Circuits SLP-50 (a mild 50 MHz low pass filter), followed by another ZJL-3G; total gain about 33 dB. That output is input to a Rhode & Schwartz FSH3 handheld spectrum analyzer. The spectrum analyzer is connected to a laptop via optical RS232, and data is acquired using a C-language program written by the author. Each plot below represents the linear average of at least 100 sweeps with the spectrum analyzer set to "sample" (as opposed to "autopeak") detection to facilitate meaningful averaging. Results shown have been calibrated such that they represent the actual levels at the antenna terminals (i.e., between the dipole and the active balun). The 50 MHz low-pass response leads to a gigantic noise ramp above 50 MHz after calibration, and is perfectly normal.
|The Production Antenna Stand: Consists of 2 "inverted V" dipoles mounted at right angles to yield orthogonal polarizations. The dipoles are constructed from 0.75-in x 0.75-in aluminum angle bracket stock, 0.125-in thick, and are resonant at 38 MHz. The production active balun cartridge is used, and is contained inside the 4-in pipe. The installed cable (about 40 m long for this stand) is used and goes through the egress panel (including lightning protection). Everything is measured from the inside of Building 17, and thus this is the "real deal".|
|5-95 MHz in 300 kHz channels. The hump centered on 38 MHz is the Galaxy, showing reasonable agreement with the "predicted" curve. We estimate that performance is Galactic-noise-limited by at least 10 dB from 29 MHz to 47 MHz, based on our estimate that the active balun's noise temperature is about 250K, which was verified in separate benchtop measurements. The Galactic noise spectrum prediction is obtained from Cane's empirical expression for the intensity of the Galactic background radiation, modified to account for the theoretical antenna response and benchtop measurements of the active balun and instrumentation responses. And, of course, there is RFI from the usual sources below 30 MHz and above 50 MHz. At PARI the main linearity challenges come from TV Ch 4 (around 70 MHz), broadcast FM (above 88 MHz), and the ever-present HF-band hash.|
|23-52 MHz in 100 kHz channels. Here we zoom in a bit to see the ETA band of interest (29-47 MHz) with greater resolution. It is clear that the nasty ripple that afflicted previous measurements has been licked, and there is not much strong/persistant RFI.|
|Strip Dipole, for Comparison. Just for laughs, we also look at the aluminum strip (0.75-in by 0.125-in) dipole mounted as the East Outrigger (the longest cable, 158 m), using the same balun design. The Galactic noise prediction is the same as is shown above, in order to facilitate easy comparison. If you compare this with the above plot, you can see why we like the "angle bracket" dipole better.|