by Ian Drummond VE6IXDThe method I used is described in two separate articles by David Shaffer W8MIF and Bob Atkins KA1GT in Chapter 7 of the ARRL UHF/Microwave Experimenter's Manual, published in 1990 by the American Radio Relay League, Newington, CT.
The basic concept is to measure the change in power output of the receiving system when pointed towards and away from the sun. As the solar flux at microwave frequencies arriving at the Earth is known, the sensitivity of the receiving system can be calculated.
The equipment I used:
I also monitored the AGC (automatic gain control) voltage of the receiver via the jack on the back of the unit. The method assumes that the gain of the system is constant. I did not know how to disable the ICOM AGC, so I contented myself by checking that the AGC voltage remained constant within the 10 mV resolution of my voltmeter.
One last requirement is a radio (or a friend with a radio) which can receive WWV on 10 MHz at 18 minutes past each hour. A daily report of the solar flux at 10.7 cm wavelength is given.
I started by getting all the electronics turned on and running for 30 minutes or so to make sure it was settled down. I then positioned the dish so the feed cast its shadow on the centre of the dish. The audio volume of the radio was turned up until a reading of about 20 mV was obtained at the datalogger. As the feed was positioned only approximately at the focus of the dish, I moved it along the axis of the dish until the audio output reading peaked.
I learned later that this position may not represent the best signal to noise ratio, as what I did was to maximize the signal plus noise, and this is not quite the same thing.
I then rotated the dish so the sun's shadow was just before the exact centre of the dish. The audio gain was raised so the DC output was about 2/3 full-scale, and I simply waited about one hour while the sun moved across the dish.
The graph of the output of the datalogger looks like a half of a bell-curve. From this output I measured the following values:
While not necessary for evaluating the system temperature, I also measured the time for the voltage to drop from the peak value to the 3 dB voltage of 43.7 mV (0.71*(52.4-22.4)+22.4). This was 8.5 minutes and it enabled me to measure the 3dB beam width of the dish and compare it to the calculated value. Just a little confidence booster that everything was working right.
WWV informed me that the solar flux on that day was 74 Solar Flux units (10^-22 watts. metres^-2. Hertz^-1) at 10.7 cm.
First I calculated the antenna temperature when the dish was pointed at the sun.
Ta = F.G.L^2 / 3.468 (Page 7-58 ARRL Manual) where
G = 4 pi.f^2.A / c^2, where
The ratio of these two temperatures has been measured,
as the (power) ratio of Vp and Vb
So Tsun / Tsky = Vp^2 / Vb^2 = (52.4/22.4)^2
So in this case the equation
Tsun / Tsky = (Trcvr + 780) / (Trcvr +30) = 5.47
can be solved to give
Trcvr = 140 K
I was able to do the sun traverse at 1296 MHz as my helical feed has a wide bandwidth, but this may not be possible for many antenna types. I would have liked to do the traverse at 1420 MHz, indeed at several other frequencies. The only difficulty in doing the traverse at another frequency was knowing what the solar flux was at that frequency. The reference in the ARRL manual gave convertions of the WWV information only to amateur bands. Some way of calculating the flux at SETI frequencies would be very useful.
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this page last updated 28 December 2002
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