PERFORMANCE TESTS OF THE 432 MHz FEEDHORN
Return loss measurments and port-to-port isolation measurements were made using an HP8753C network analyzer.
Photo above: Horizontal port return loss measurement shows -46 dB return loss at 432 MHz. (May 19, 2010)
Photo above: Vertical port return loss measurement shows -44 dB return loss at 432 MHz. (May 19, 2010)
Photo above: S21 measurement between the H and V ports shows -20 dB of isolation between the ports at 432 MHz. (May 19, 2010)
Photo above: 432MHz feedhorn is mounted into position, complete with T/R relay and LNAs. (May 19, 2010)
The effectiveness of the feed horn can be determined from a noise temperature measurement of the complete receive system with the horn installed. Receive system noise temperature is straightforwardly obtained by measuring the ratio of ground noise to cold sky noise power levels. The ratio of ground noise to cold sky noise is commonly referred to as the Y-factor ratio, defined as
where Tg is the noise temperature of the ground, Tcs is the noise temperature of cold sky at the observing frequency, and Trx is the noise temperature of the receiving system. From the equation above it follows that the noise temperature of the receiving system is
The figure below shows the results of a ground noise to cold sky noise power measurement to obtain the Y-factor ratio. The ground noise power level was obtained by pointing the dish toward the ground at an elevation angle of –3 degrees. The cold sky noise power level was obtained by pointing the dish toward the constellation Leo, which is a region of comparatively low radio emissions. Noise levels were measured by sampling IF noise power at the output of the 432 MHz to 28 MHz converter using a wide band log detector. Power levels were recorded every 100 mSec for a period of approximately 150 seconds with the dish pointing at ground then, while continuing to make measurements, the dish was moved from ground to cold sky for an additional time. Note the high levels of pulse noise interference that are present in the measurement. It is an unfortunate fact that in many areas of the United States the radio spectrum at and near 432 MHz is crowded with emissions from a variety of sources that interfere with precision measurements. Interfering signals include pulses from domestic remote reading weather monitors, pipeline telemetry signals, over-the-horizon radar and digital television, to name only a few and these vary with locale. Nevertheless, useful noise measurements can be obtained by making rapid noise level measurements over reasonably long periods of time, as shown below, from which base line noise levels are apparent even in the presence of the pulse noise interference.
Plot above: Ground to cold sky measurements at 432 MHz show 6.7 dB difference between ground and cold sky. (May 22, 2010)
From the plot above the ground to cold sky power differential is
The Y-factor ratio is then
On the day of the measurement the ground temperature, Tg, was 294 K (70 F) and cold sky, Tcs, at 432 MHz is approximately 45 K. The receive system noise temperature using this horn is then approximately
The very low noise temperature measured for the receive system while using this horn shows that the horn is an appropriate high-performance component for even the most demanding weak signal work, such as EME communications and radio astronomy.
Photo above: The physical blockage of the aperture of the dish by the new feedhorn can be viewed by pointing the dish at the sun and observing the shadow of the feedhorn on the reflecting surface of the dish. As you can see, the blockage is minor relative to the size of the reflecting area. In fact, the physical blockage by the feedhorn is only 1.5% of the aperture of this dish. (May 19, 2010)