SatSig topic:

Sudden Tx power reduction

(Read 3756 times)

Oct 24^th, 2012 at 7:24am

I recently did C-Band installation. Its a 2.4m Skyware global circular antenna on a idirect x3 SCPC. The problem is that Tx power goes from -22 to -17. this happens everyday at specific times. It remains at -17 for around 5hours, then drops down to -22. The power supply to the modem is stable. Snr is always stable.

I was asking, can this be a problem at the hub side?

Oct 24^th, 2012 at 4:19pm

It sounds as though the hub is telling your remote to increase its transmit power from -22 to -17 dBm. This would be due to your signal being received with poor quality (i.e. low c/n or distorted) at the hub.

A sudden increase of 5 dB suggests to me the start of some interference at the hub. The associated decrease several hours later perhaps occurs when the interference stops. Ask the hub to watch their receive spectrum over 24 hours and diagnose the cause.

More gradual changes might be due to antenna mispointing coupled with satellite movement, rain at either end etc. Sudden, instantaneous, changes are associated with poor connectors and the start/stop of anything nearby and interference.

It would be worth checking your -1dB gain compression level. The hub must never tell your modem to exceed this modem output level otherwise your BUC will go into saturation, cause interference to adjacent carriers, cause distortion of your transmit carrier and high error rate, and possibly damage your BUC.

Oct 26^th, 2012 at 12:49pm

Thank you Eric for your advise.

I'll check with NOC on hub side to see if there is interference. I'll also check for interference on remote side. connectors were changed and antenna had earlier been re-pointed, but, still there is no change,

In any case there is interference on my Tx at remote side, Other than relocating the antenna, is there any other way of sorting it out?

Oct 26^th, 2012 at 2:55pm

If the problem is due to your remote site it could be one of the following, affecting your transmit signal:
Your antenna swings about in the wind.
A vehicle comes and partially blocks your beam.
Your BUC power supply is unstable.
It rains at your site.
Your antenna is mis-pointed and due to slight satellite movement your signal varies gradually over 24 hours.
Your BUC temperature changes.

The problem could be due to interference at the satellite degrading the quality of the carrier from your remote site, either a cross-pol carrier coming up, adjacent carrier interference starting up, co-pol interference carrier starting, intermodulation due to other carriers starting or adjacent satellite interference where a new carrier has started. The large hub dish is best placed to observe and investigate. Measure your carrier for 24 hrs, plot the co-pol and x-pol and adjacent satellite spectrums.

The problem could be at the hub site, either local interference or fault in their LNB system.

Where a problem suddenly comes and goes it helps to look for any events that occur simultaneously. Are heaters/lights etc turned on/off at the same time.

Interference on your TX at the remote site is improbable. Do you have two transmit modems or anything else connected into the BUC cable? Does someone put a transmitter device next to your BUC cable ? How can interference get into your BUC cable ? Is the BUC power supply and 10 MHz reference the same all of the time ?