C-Band Digital TVRO Interference Problem

[assyrian]

I have an operational C-band TVRO providing a stereo audio feed to an FM stereo transmitter operating with a maximum output of 10 kW. We have noted that at random periods (1-2 seconds) at random times the digital audio drops out.

We have used an SA to determine that no TI is the culprit. WE have determined that other locations in the area are receiving the signal with no audio interruptions or drop-outs.

An adjacent Ku-band TVRO downlinking the same program material is not affected in anyway. Audio has never dropped.

Naturally our first thought was that the FM transmitter is cause. It is operating at 98.1 MHz. We have noted that when the power is throttled back to 5 kW, the problem clears.

Therefore we surmised that the transmitted signal is causing a signal or voltage to be induced into the system. More than likely via the RG-11 coax. When the operational L-band frequency is checked with a SA, no hash or interference can be see.

Another suspect is the C-Band LNB L.O. This is the one difference between the KU and C-band systems (not counting the operating DL frequency). Otherwise the systems are the same.

A real head scratcher.

Any thoughts would be welcomed.

 

[brutus]

Sounds similar to another posted problem. Read the associated threads under the post C-Band downlink interference.

 

[nbucska]

Looks like you need at least 6 dB improvement
which should be possible.

I would suggest a few things to try:
a.) shield the Rx from the Tx
b.) Try to develop a >> 6dB null in the TX antenna's
radiation pattern toward the Rx antenna
c.) put ferrit around the coax

<nbucska@pcperipherals.com>

 

[tonkawa]

There may be another reason for your problem.
Here's what I have learned by trial and error. The output of a c band digital receiver can also input signals to the receiver. This is either an echo of the original and out of phase or interference picked up through other sources. This reverse signal will cancel or reduce the originating incomming signal from the LNB causing a quick, short drop in the digital and sometimes analog signals. I fought this for three months and discovered the answer by accident.

If you connect other components to your digital receiver, you need a unidirectional filter or unidirectional amplifier to correct the problem. I was surprised to find most amplifiers are bidirectional allowing signals to pass in both directions.