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Measured comparison of Turnstile and QFHOver the last few weeks I have been trying to correlate the results I have achieved with three different antennas, an outside Turnstile, a loft-mounted QFH, and an outside QFH from Paul Hayes, to the characteristics of the antenna. (See here for more about QFH antennas.) To do this, I have used instrumented versions of both my WXtrack orbit prediction program and SatSignal image decoder program. By combining these two in my NOAAplot program, I am able to plot the signal-to-noise ratio (SNR) versus the satellite observed position. You get a lot of information, and it's really three dimensional, so rather difficult to visualise. By turning the SNR into colour, and plotting versus observed position, the results below are obtained. I found that the outside QFH gives too much pages interference to be directly connected to the RX2 receiver, so I have added a small filter and this is my current working configuration. (The filter acts as much by providing about 1.5dB insertion loss as by removing the pager signals). [Technical discussion: I measured the RMS value of noise in the channel-B (sensor channel 4) "look-at-space" signal compared with the peak white value of the signal. I would expect a maximum value for the video SNR of around (256 / 0.29) or 59dB simply from the quantisation due the 8-bit digitising used for the signal. Note that because of the FM demodulation, there would be a further factor to take into account to get the channel SNR compared to the video SNR. I haven't done that as yet.] In the plots below, white is noise free, yellow and green show slight noise, and the blue has noticeable noise. Thus you should be able to get a quick judgment of an antenna's performance by the radius of total brightness visible, if you see what you mean.... The circles are at 500km intervals, thus the plot is 7000km square.
Note that while the loft-mounted QFH has a more limited coverage, it is more uniform over the area where good signals are received. The outside QFH gives by far the best result, but with the filter gives poorer noise performance. Note that the radial elevation scale is against distance rather than elevation angle to emphasise the performance at low angles. It is not a mistake that some of the tracks appear to cross, as I have included both NOAA-14 and NOAA-15, which have different orbits, in the results. The next step was to try and summarise the performance so that the differences were more clearly visible. The first attempt was to average the SNR versus elevation angle. A weighted average was used to give more prominence to the better SNR values. Part of the justification for this is that on NOAA 14 I do suffer pager interference (at least with the outdoor turnstile), which artificially depress the SNR. You can see the results below.
Finally, I realised that it would mean more in terms of amount of quality picture if the elevation angle was replaced by the distance of the sub-satellite ground point. As you can see, this places more emphasis on the low-angle performance.
My current conclusions are that at certain angles the QFH does indeed perform a little better than the turnstile, but that the loft mounting is detracting from its performance, so better get a QFH outside as soon as possible! The loft-mounting introduces both a flat signal loss, probably worse if the roof is wet, and screening by the other objects in the loft. The outside QFH (at least at this location) requires extra filtering to prevent pagers from completely ruining the pictures. What I had hoped to see, but cannot clearly identify, are if the turnstile has any well-defined nulls. I certainly see these on the pictures I get, but identifying them in 3-D space is not easy. I also compared the averaged SNR against azimuth, hoping to see which directions were good and which bad from this location, but there is nothing definite that I feel I can yet conclude from the data I have so far. See what you think...
It is difficult to see the detail on this graph, and to draw conclusions. However, it is clear that the outside QFH is best, with the QFH/filter combination being perhaps 5dB worse. Towards the north-east, the loft QFH seems as good as anything, so presumably I have a clear take-off in that direction. Everything seems to perform more similarly to the north and south, however this could be because there are no low-angle passes across the north and south, as there are to east and west, so there is much less continuity of data. |
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