Axino-Tech Consulting & Services
14th August, 2011
This is a short article on how to use a spectrum analyser to align a domestic satellite receiving dish, such as for Freeview or SKY, in New Zealand.
SKY (NZ) and Freeview (NZ) both broadcast via the Optus D1 satellite located in geo-stationary orbit at 160 degrees east. The satellite has a large number of transponders which are within the Ku band frequency range from 12250MHz to 12750MHz.
The particular setup in this example uses a Pauxis PX-660S LNB, which has the most common local oscillator frequency of 11300MHz. This means that the I.F output (in L-band) will range from 950MHz to 1450MHz. Some LNB's use different local oscillator frequencies such as 11700MHz or 10750MHz, which will make the I.F frequencies a little different to those in this demo.
Not everyone has a spectrum analyser, but one could be borrowed or rented relatively easily. Installers these days use handheld versions or purpose designed satellite test instruments. While you can make a system work using one of those simple 'satellite finders', it can rely on good luck, because firstly you might point at the wrong satellite and secondly it is fairly difficult to get the 'skew' of the LNB optimised with just those simple devices.
To start, the first important issue is to ensure the mounting pole for your dish is indeed vertical because if not, the elevation marks on the dish mounting kit will be incorrect and so will the marks on the LNB. The very simplest way to align a dish from scratch is to first establish a rough direction to point it and this may be as simple as aiming it in the same direction as everyone else in the vicinity. Then, there are published headings to point a dish from any given location. Freeviewshopnz has such a table and calculator here. I took the angle quoted and used Google Earth to draw a line from my location at the specified angle.(not the magnetic angle on Google Earth). Then I picked a prominent landmark along that line that I knew could be seen from my roof location. I simply aimed the dish at that point. (If you want to use a compass, then the magnetic azimuth must be used, but it is harder than it sounds to get right unless you have a precision compass.)
To get the elevation, using the longitude of your location will be near correct to use as an angle. e.g Wellington at 41 degrees south requires an elevation of about 40 degrees. Marks for elevations will be on the dish mounting bracket. Now we have a starting point and have to get the test equipment out. This is when you would use your satellite finder to make fine adjustments to the azimuth and elevation of the dish.
To use a spectrum analyser, you also need a dc injector. This is a gizmo that puts the necessary dc voltage on the coax in order to power up the LNB. Aerial shops and installers will sell these. I made my own but you do need a bit of understanding of RF construction techniques. Mine takes a 12Vdc input and has a switch selector to supply either 0 volts, 12 volts or 18 volts to the LNB. That way I can easily select the polarisation from horizontal (18V), to vertical (12V), or off, which is also useful. With the injector in line with the coax from the LNB, connect up the analyser and tune to a known L-band frequency. For Freeview I used 1156MHz as my centre frequency and a span of 100MHz. Below is a photo of the screen. Note that I had previously aligned this dish (without the analyser) so it shows that some optimising is still required.
The screenshot shows three traces.
The upper one is what I get on H polarisation, which is the wanted spectrum. The first thing to notice is that we are on the correct satellite because we have carriers at 1156MHz (centre) and 1183MHz (right) which are the two Freeview transponders converted to L-band. The carrier on the left of this upper trace is 1121MHz corresponding to one of SKY's transponders at 12421MHz. If the span of the analyser was set higher, then more carriers would be visible, but we have the ones of interest. Use this trace to maximise the wanted carriers, by fine adjustments of azimuth and elevation. When you get the best levels, carefully lock off all the adjusters.
After peaking the carriers of interest, switch the LNB to vertical polarisation by selecting the dc injector to 12V (range is about 11-14V typically). Now the middle trace shows what we have. First thing here is that the 1156MHz carrier has considerably reduced. However, it could be better. With the LNB on vertical, we now want to minimise reception of 1156MHz. We do this by rotating the LNB in it's holder until we get the least signal at 1156MHz. In this case. I should be able to reduce that carrier by another 2-3dB to bring it down to the level of that small 'valley' just to the left. Having done that, the 'skew' would be optimal. This will be very important in the case of Freeviews 1183MHz (12483MHz transponder). This is because there is a wide carrier on about 1180MHz operating on V polarisation. If you dont get the skew right, this vertical service will contaminate the Freeview 12483MHz transponder when you go back to H polarisation and this contamination might be bad enough to stop that transponder being received error-free. (The Freeview 12456MHz transponder might still work without optimised skew because there is nothing operating with V polarisation on that same frequency.)
The only other important issue to note is that you should get a carrier to noise ratio of around 15-16dB using a 750mm dish; a bit less for a 600mm dish. That is clearly seen in the photo by taking the difference in level between the wanted 1156MHz carrier and the same frequency when the LNB is switched to vertical. In the photo above, that difference is about 16dB, which is as expected. DVB-S using the Freeview parameters, will work with as little as 5dB carrier/noise ratio, but the better the margin, the fewer rain-fading issues occur.
Having done all the above, that is all you need. The system will most likely be as good as it gets. However, if you want or need to actually measure your received levels, this is what to do.
The analyser does show the signal levels and you can read them off the scale. Noting that the reference level is -10dBm (referring to the top of the display) and the scale is 10dB per division, you can see the 1156MHz carrier is about -46dBm in the centre. To get the 'real' carrier level, you must use the analyser 'rms' or 'sample' detector and have to know both the analyser RBW (resolution bandwidth) and the bandwidth of the transponder. The RBW (displayed on the screen) is 1MHz. The Freeview transponders have a bandwidth of 22500kHz. (note not all transponders use this value). So an approximate true level is given by [-46 + 10(log(22.5/1)] or a corrected level of -32.5dBm. I say approximate because there is a little uncertainty around the exact rate of roll-off of the analyser filters. Usually this can create an error of up to 3dB, but usually less.
The HP8591E analyser here also has a 'channel power' function which does all the above calcs and has the roll-off factor built-in. It reports channel power of -30.5dBm.
But there is one more issue to account for and that is the noise floor of the measurement. The analyser is actually reading the signal plus noise. You have to know how much of the reading is noise. That is why the third trace in the screenshot shows the level without the LNB powered. This gives the noise floor of the analyser. In this case it is well below the signal readings, and so is insignificant. Generally if the noise level is more than 10dB below the signal+noise level, the noise floor error is insignificant.
Axino-Tech Consulting & Services August 2011