Home page of Satellite Internet and Information

Satellite Internet Forum.

Welcome, Guest.
Welcome to this satellite broadband discussion forum. Wherever you are and whatever your problem we are here to help each other. Connecting to the internet via satellite is not always easy but is critically important to those in remote places or with poor terrestrial infrastructure. Both service providers and customers are encouraged to contribute. Register at the bottom of the forum home page if you wish to contribute or ask question. Read the Forum rules.
      Satellite Internet Forum : Home Page          
Pages: 1

LNB cable of different impedance

(Read 6223 times)
Ex Member
Ex Member


May 9th, 2006 at 5:30am  
Hi,

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.
Back to top
 
 
IP Logged
 
Ex Member
Ex Member


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.

//greg//
Back to top
 
 
IP Logged
 
Ex Member
Ex Member


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.

Back to top
 
 
IP Logged
 
Pages: 1