Terminating Resistor

[guapoharry]

Why (if ever) would you use a terminating resistor on an FTA setup?

Coax on an FTA setup has DC power on it, so that would seem to be a good reason not to use one. Or, am I missing something?

 

[USDownlink]

Coax running between FTA boxes and LNBs are (presumably) terminated properly by the devices they connect to. In general, there shouldn't be a lot of open ends anyway.

Your observation about the DC being present is correct, and a fair part of the reason we don't use things like the common splitter. If you use a splitter in a regular analog cable (without imposed DC) system, you might want the terminating resistor to prevent reflections on open splitter ports.

You are not missing anything. We wouldn't want to draw static power out of a receiver that does nothing, particularly because (if memory serves) that power would be at about the limits of what the receiver can supply over the coax anyway.

 

[McGuyver]

This is something I recently questioned while working on my dishes. So if someone is using only 2 or 3 ports on a 4X1 diseqc switch, can a 75 ohm terminator be used or is it not recommended? I'm just wondering if it makes any difference when used on switches or does it matter? If the resistor be avoided, then whats the best method of sealing off the unused ports without using something like RTV silicone?

I think it would be great if somebody could post a "Do's and Don'ts" concerning 75 ohm terminators. Thanks Guys

 

[Tron]

You should not use a terminating resistor on a DiSEqC switch. To cover an unused port, take an old 'F' fitting, fill it with silicone, and install it onto the port.

 

[B.J.]

I have never used any kind of a terminator, and I can't think of an application where I think one would be beneficial, other than perhaps on the ch3/4 analog I/O ports of a SD receiver. But I sure wouldn't use a terminator on the LNB input side. As mentioned above, if you did use a terminator, I sure wouldn't use one which had a 75 ohm resistive load, because some receivers can only supply around 0.4 to 0.5 amps, and just one 75 ohm terminator would use about half of this, and if you have a motor connected, then you're over the current max of the receiver.
There are some reasons you might want a 75 ohm impedance for the RF, but since these are not weak signal situations we're dealing with here, the problems with the DC current far outweigh any benefit you'd get out of the rf impedance matching. The LNB port is expecting a 75 ohm impedance, but it's not expecting that to be a resistive impedance.
If you're considering putting terminators on a splitter used to connect multiple receivers, I would never use such a splitter without DC blocks anyway, so the 75 ohm resistor isn't really doing much except perhaps sucking up some fraction of the rf signal coming from the LNB.
If you're considering putting terminators on a diseqC switch, again, the DC problem comes up, because if you ever tune in a transponder that isn't on, many/most receivers upon losing signal will attempt to try other diseqC ports and bump the motor position to find signal, and if it hits on one terminated via 75 ohm resistance, you've got the .24 amps from the terminator plus perhaps .35 amps from your motor, and you're up over the current max of your receiver.
If I were going to use a terminator, I think I'd clip off the center conductor, and just use the terminator as a weather cap to keep out moisture. The moisture thing is a real issue with diseqC switches, as the ones I've looked at, are open to the inside via the coax connectors, ie you can stick a wire through all the way to the circuit inside, so it is a good idea to weatherproof the unused ports. But I really can't think of any good reason to use a 75 ohm terminator, particularly not a resistive load.
But this is just my mainly intuitive opinion. I've never really tried one.

 

[Cadsulfide]

I pull the resistor out of the terminator and I have a very nice cap to cover unused ports on switches, ground blocks, etc. It is a good idea to terminate unused CATV splitters used for TV channel RF distribution.

 

[McGuyver]

Excellent advice gentlemen! Thanks for the idea of yanking the center and using as a weather cap. Great!!!

 

[Mr Tony]

I use some of my old twist on connectors and do as Tron does. Works great especially on the switches outside

 

[guapoharry]

This is something I recently questioned while working on my dishes. So if someone is using only 2 or 3 ports on a 4X1 diseqc switch, can a 75 ohm terminator be used or is it not recommended? I'm just wondering if it makes any difference when used on switches or does it matter? If the resistor be avoided, then whats the best method of sealing off the unused ports without using something like RTV silicone?

I think it would be great if somebody could post a "Do's and Don'ts" concerning 75 ohm terminators. Thanks Guys

You should not use a terminating resistor on a DiSEqC switch. To cover an unused port, take an old 'F' fitting, fill it with silicone, and install it onto the port.

Just to be clear, are we talking about filling connectors with silicone grease, or silicone sealant? (the kind that hardens)

As I recall, RTV silicone sealant cures by releasing acetic acid. This is corrosive to electronic components. So, if you wanted to seal the outside of an old F fitting, there's probably no harm there.

Does anyone remember the old commercial where the guy had to fix a wiring problem on his car? He slipped a soda straw over the wires, twisted them together and pumped RTV silicone in to bond the straw to the splice. Maybe people got the idea this is a good practice, but I would avoid it for doing something permanent.

If you want to fill connectors to resist moisture, I have heard to use silicone grease as in the attachment.

Thoughts?

 

[McGuyver]

quapoharry, yes I was referring to RTV the hardening kind. I don't use it except for sealing coax on the outside wall of the house. I don't like it in or on connectors. The silicone grease is a good idea but regarding this topic we still need a apparatus such as a cap to cover an open port and then you could also use a 1/4" rubber O'ring inside the cap for a seal without getting grease all over. Grease would get pretty messy here in the desert heat as it melts and wicks, I know from having to deal with installers using it. I use that stuff on spark plug boots for easy release when doing tune ups on cars, also helps prevent high voltage leaks. There's nothing like leaning over a steel fender and touching a leaky spark plug wire...
you can only guess what part of your body gets zapped at the fender! zzzzzzzzzzappp
((((LOL))))

 

[pedrogarcia]

I use self amalgamating rubber tape, works in desert and Michigan winters.

 

[McGuyver]

I use self amalgamating rubber tape, works in desert and Michigan winters.

yes pedro, that works too and the brand makes a huge diff, 3M tape is the best of all tapes. Other brands of rubber tape dry up out here in the desert and fall off in just a few days. When I used to work at a Oil refinery we always used 3M tape products, nothing less and 3M stands the test of time. BUT, I don't like using it for one simple reason, I'm always changing something in my dish farm and it's a pain to cut the tape off every time I need to remove a coax connector and then retape it. I like using something thats fast and easy like the terminators with the conducter removed as suggested, I don't know why I never thought of that before, maybe I missed one of the McGuyver TV episodes...

 

[Vorg]

The silicone sold at automotive places should be safe since that acid is also hard on aluminum engine parts and when used around fuel/intake system, can it into the cat and damage it.

 

[Ironsides]

LOL

I use heat shrink over my open connections. I just heat, shrink, flip the little open end over and use a zip tie to tie it down and done. Never had a problem with water as I know of...I have a Pansat Switch of all things that has never had anything cover it up, it just hangs around and has worked for several years no problems...The heat Shrink is easy to remove and it's pretty cheap if nothing else is around to use...

 

[Babadem]

Nice suggestions. The search button is a great tool!