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N type 50 Ohm or F type 75 Ohm

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Ex Member
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Nov 9th, 2011 at 2:21am  
Hi all,

I am using iDirect modems together with SeaTel 4009/9707 antennas (Codan BUC).

So should I using 50 ohm RF cable with N type connectors or 75 ohm RF cable with F type connectors?

or it doesn't matter, as long as the connector impedance match the cable's?

thanks.

brs
sg
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Ex Member
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Reply #1 - Nov 29th, 2011 at 2:25am  
I would use 50 ohm terminated to SMA-M on the modem side with SMA-F to F-M adaptors.  I have seen N to F adapters around, but the SMA to F ones look more refined to me.
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Reply #2 - Dec 4th, 2011 at 2:08pm  
Check out this link. I know these have been used for your type of situation.

http://www.pasternack.com/product-IMPEDANCE-MATCHING-PAD-50-OHM-SMA-FEMALE-TO-75...
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Reply #3 - Dec 4th, 2011 at 6:47pm  
Do check how any proposed 50/75 ohm adaptor works.  If you are using the 10 MHz reference and DC power supply from the modem, up the BUC cable, then adaptors based on resistors may burn out and ones based on transformers may short circuit or not pass the DC or handle low frequencies, like 10 MHz.

Best regards, Eric.
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Reply #4 - Dec 29th, 2011 at 7:09am  
Quote:
I would use 50 ohm terminated to SMA-M on the modem side with SMA-F to F-M adaptors.  I have seen N to F adapters around, but the SMA to F ones look more refined to me.


Hi onthemove,

so you mean iDirect modem's internal impedance is 50 ohm?

brs,
sg
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Eric Johnston
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Reply #5 - Dec 29th, 2011 at 11:16am  
Personally I would not try to match 50 to 75 ohm impedences. Reasoning: It causes complications/incompatibility with DC the power supply and 10 MHz reference. The amount of ripple created is acceptable for narrow bandwidth carriers, if not use thinner or longer cable to increase the dB loss. Keep any 50/75 discontinuity right at the ends, i.e. at the BUC socket or and/or at the iDirect modem socket. Don't put a 50/75 ohm discontinuity part way along the cable run.

It is important however to use connectors that physically match up properly. Avoid 75 ohm N type plug into 50 ohm N type socket. The centre pin does not touch.

 
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Reply #6 - Jan 25th, 2012 at 9:06pm  
Looking at this post i would suggest using 50ohm Cables

The Seatel antennas are 50ohms throughout the system (rotary joint, mux modems and bulkheads)

on the Rx side the seatel converts the 75ohm idirect modem output to 50ohms going up to the antenna in the 4 channel modem and back from 50ohm to 75ohm at the antenna 4 channel modem back to the LNB

as for the BUC there is always an impedence mismatch 75ohm from the modem up to a 50ohm BUC (unless you use a 75ohm (ftype) BUC but this gives other problems as the rotary joint is 50ohm.

But the big thing for the BUC is that impedence mismatches causes reflection back down the cable to the modem but at the power level you are working on (micro watts) the reflection is so small it does not effect the transmit power from the modem then the double reflection back to the buc will not cause any increase in power out of the buc - also the intermod products/harmonics are so small they cant be seen.

Regards

ScotlandTDMA
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