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LNB cable of different impedance

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Ex Member
Ex Member

May 9th, 2006 at 5:30am  

I have a Norsat PLL LNB with 75 ohm F-connector.  I plan to use it to connect to SCPC modem (Comtech 570L) for RX.

There are 2 problems that  I am facing:
- The SCPC modem RX port is a 50 ohm N-connector
- Distance from LNB to the modem is about 300 feet

There are 2 choices that I see:
- Use RG-8 cable (50 ohm) with F and N-connector at each side
- Or, instead of using RG-8, use the RG-11 cable

I don't know if there are technical ramifications of mixing impedances at L-band frequency.

Any pointers would be appreciated.
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Posts: 837
Kentucky (USA)
Reply #1 - May 9th, 2006 at 1:11pm  
A 300 foot cable run makes cable loss a critical factor, and mismatched impedance cannot help but add to that loss. I'm not familiar with your specific equipment, but it seems to me that you need not limit yourself to just two choices. Two choices you've not mentioned include:
1. switching to a 50 ohm LNB
2. a 50/75 balun

Once mismatched impedence is eliminated from the equation, cable selection is then dependent upon hardware signal requirements. Compare the LNB rated output level with the modem input level requirement. Armed with that range, you should then be able to go through the cable tech specs. Manufacturers of quality cable are not afraid to publish cable loss specs openly. Once armed with your own "acceptable loss over 300 feet" numbers, you can then select the most reasonably priced cable and connectors for your specific application.

In addition to signal loss, DC loss over 300 feet might be another critical number for you to consider. That's on both conductor AND shield.

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Reply #2 - May 11th, 2006 at 4:26am  
Thanks Greg.

I decide to change the LNB to N-connector and use RG-8 cable instead.

So I don't have to worry about the loss from the impedance mismatch and the RG-8 cable helps reduce the loss compared to RG-11.

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