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Use of Active/Passive Combiners with HUB RFT

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Ex Member

May 15th, 2013 at 11:16am  
Dear Mr. Eric
For the HUB installation we use combiners & splitters with HUB RFT can you explain how we select the types of combiners required active or passive  with a HUB RFT and HUB chassis.

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Eric Johnston
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Reply #1 - May 15th, 2013 at 4:56pm  
A passive combiner or splitter is more reliable, so use passive device if the losses involved allow it.

A separate L band booster amplifier is a possibility.  Pay special attention to cooling, reliability, spares and to the quality of the DC power supplies.

Terminate all unused ports with 50 or 75 ohm terminations as appropriate. On the transmit side use well screened cables to avoid broadcasting local interference (FM, TV, cell phone etc) to the satellite.

On the receive side, an L band feed from the LNB may arrive in the building and need splitting for several receiver devices.

Consider how you will supply a reliable DC supply up the cable if required.  Splitters may come with diodes so that DC power may be fed from alternative sources.  A DC injection point is also a possibility.

On the transmit side, if you need a combiner be very aware of the possibility of causing unacceptable intermodulation interference, due to transmitting more than one carrier in the BUC/HPA.

If a transmit modem accidentally set to full output power can saturate the BUC/HPA or even wipe out the satellite transponder, then make this impossible by installing a fixed in-line attenuator after the transmit combiner. This means that modems will all be normally outputting at nearer to their maximum levels and 'finger trouble' with transmit modem power settings is less likely to be catastrophic.  

A splitter (e.g. a -10 dB tap) is also a possibility on the transmit side if you want to monitor your composite transmit signal prior to its final amplification in the BUC/HPA.

Anticipate growth in services and make it possible without interrupting existing operations.

If you are talking about high power microwave combiners to enable the operation of several BUCs or HPAs together then please say.  It is much more complicated.  Both passive (lossy, e.g. -3dB, -6dB) and low loss frequency selective filter multiplexers are possible.  The high cost may be justified however if it allows each BUC/HPA to transmit just one carrier (at up to full rated power). Consider the cost of the HPAs, the AC mains power and air conditioning if you are planning to waste several hundred watts of RF power into dummy loads.

Document your IF distribution system and the signal levels.

Best regards, Eric.
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