SatSig topic:

Unable to get lock onto W3A

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Mar 22^nd, 2009 at 9:32pm

Hi All,

I am based in Lusaka, Zambia, Central Africa.
I have with me a an iDirect 3000 modem on which i loaded iSite ver 7_1_1 and an option file provided by the NOC.
The following is the equipment: 1.2m Prodelin antenna, iDirect 3000 modem, ViaSat lnb and 2watt BUC.
My location in Lusaka has the following GPS readings:
15deg25'08.13 South
28deg15'47.57 East
I was able to get a lock onto W3A with a reading of 14v on my iSite. This is Ku band.
However, when i went for an installation to the eastern province of Zambia, Chama (this town is almost bordering with Malawi) i was unable to get a lock there with the same equipment. I tried it for two days in that location but to no avail. I could only get 3 - 4 volts on iSite, nothing more than that to establich a lock.I even tried to change the lnb but same. The location in question has the following GPS readings:

11deg 13'030 South
33deg 09'282 East.

I went back to Lusaka and all was well again, i was able to get a lock with the same equipment that failed me in Chama.
Now, it is worth noting that this place, Chama, is in a valley at an altitude of 2620.
Here are my questions:
1. Why was i unable to get a lock there?
2. Do i need a bigger antenna e.g 1.8 or 2.4m antenna?
3. Could it be that i was in a dead zone? But i knowthe W3a footprint covers the whole Zambia including Malawi.

Kindly assist.

Best regards,

Francis

Mar 22^nd, 2009 at 10:57pm

I've worked out the pointing angles for both sites:
Lusaka
Az=316 magnetic compass
El=59.6
Pol=-52.7 (this is nominal, W3a may be tilted a bit, say -56 deg)

Chama
Az=301 magnetic compass
El=57
Pol=-65.8 (this is nominal, W3a may be tilted a bit, say -69 deg)

So when you moved to Chama the polarisation needed turning a further -13 deg anticlockwise, while facing towards the satellite.

I'm guessing that the polarisation is the problem.

I would record now what is your polarisation angle. Then make a deliberate error in the polarisation to see what error causes failure, as perhaps in Chama.

If there are equal power carriers on both polarisations (level 0 dB) then:

No polarisation error, wanted signal level = 0 dB
error of 10 deg will reduce the wanted signal by -0.1 dB
error of 20 deg will reduce the wanted signal by -0.5 dB
error of 30 deg will reduce the wanted signal by -1.2 dB
error of 40 deg will reduce the wanted signal by -2.3 dB
error of 45 deg will reduce the wanted signal by -3 dB

Note that misaligning the polarisation makes little difference to the wanted signal level. Even a 45 deg error only reduces the wanted signal by -3 dB, about the same as a rain fade.

No polarisation error, cross pol interferer signal level = -30 dB
error of 5 deg will make the interference level -21 dB mild interference
error of 10 deg will make the interference level -15.2 dB noticable interference
error of 20 deg will make the interference level -9.3 dB serious interference
error of 30 deg will make the interference level -6 dB very serious interference
error of 40 deg will make the interference level -3.8 dB impossible interference
error of 45 deg will make the interference level -3 dB wanted and interferer same level !!

Note that misaligning the polarisation very rapidly brings up the cross-pol interferer.

I would check where you are now, in Lusaka. Turn the polarisation until it fails. Mark the angle. Turn back until it fails again on the opposite side and mark the angle. Move to the exact centre. Is the angle = about -56 deg? Try an inclinometer sideways across the BUC or LNB. Get your hub to do a CW transmit cross-pol test while you reach up under the feed arm and make microscopic polarisation turning movement.

Using Dish pointing Southern Africa

Lusaka

Chama

Best regards, Eric.

Mar 23^rd, 2009 at 11:14am

I will be the first to say....Incredible post Eric.