Coax Question

[bsavery]

OK... it's been a few years since I learned basic electronics but this just don't seem right to me....

In searching around the internet looking at base antennas for my scanner.... I ran across several sites that sell kits that include an antenna complete with F connectors and RG-6 cable.

Ummm.. do scanners not care that it's 75 ohm cable??

And if you can use CATV cable and connectors... could you use a regular CATV splitter and run the feed to 2 scanners?

It would seem to me the impedence mis-match would kill the signal.

:? :?
Bob

 

[RolnCode3]

There are many threads dealing with both of your questions. I am not an expert, but have read many of those threads.

 

[K5MAR]

OK... it's been a few years since I learned basic electronics but this just don't seem right to me....

In searching around the internet looking at base antennas for my scanner.... I ran across several sites that sell kits that include an antenna complete with F connectors and RG-6 cable.

Ummm.. do scanners not care that it's 75 ohm cable??

And if you can use CATV cable and connectors... could you use a regular CATV splitter and run the feed to 2 scanners?

It would seem to me the impedence mis-match would kill the signal.

:? :?
Bob

OK, short answer. This isn't a big deal on a receive-only system; and a wide-band antenna doesn't have a flat impedance. The slight mismatch is compensated for by the improved signal strength due to the lower loss of quality RG-6 (use RG-6QS) compared with standard RG-8 or RG-58 at the higher freqs.

CATV splitters generally go to around 500 MHz, you want something that goes to 900 MHz of you monitor 800 MHz systems. Splitters designed for sat. use will work. But passive splitter do cost you a bit in signal strength.

For more details, search the forums, as RolnCode3 said, this has been widely covered before.

Mark S.

 

[bsavery]

There are many threads dealing with both of your questions. I am not an expert, but have read many of those threads.

To quote another charactor your avatar brings to mind.....

Search...... DOH!



Bob

 

[bsavery]

OK, short answer. This isn't a big deal on a receive-only system; and a wide-band antenna doesn't have a flat impedance. The slight mismatch is compensated for by the improved signal strength due to the lower loss of quality RG-6 (use RG-6QS) compared with standard RG-8 or RG-58 at the higher freqs.

CATV splitters generally go to around 500 MHz, you want something that goes to 900 MHz of you monitor 800 MHz systems. Splitters designed for sat. use will work. But passive splitter do cost you a bit in signal strength.

For more details, search the forums, as RolnCode3 said, this has been widely covered before.

Mark S.

Well... after a quickie search of this forum... I see it'll work, but it's not good, impedence doesn't matter, but it does.... almost as many answers as a politician at election time! Widely covered indeed!! :lol:

CATV splitters here, in order to work with the new digital stuff they use, are rated at 5-1000 mhz. They do average around 3 db loss though. I can't imagine that'll be good.

Hmmm... I think I'll go see what the LAN/WAN guys did with the LMR-400 cable segments after they switched over to Cat 6 fed access points. Since I'm in charge of wiring for the Health System, it is my job to make sure abandoned cable is removed! 8)

I owe... I owe... it's off to work I go.....
Bob

 

[N_Jay]

You want the truth?

You can't handle the truth!

http://www.arrl.org/catalog/index.php3?category=Antennas,+Transmission+Lines+&+Propagation

 

[K5MAR]

I think you'll find, that except for a couple of somewhat retentive people :lol: , it is generally agreed that the impedence difference isn't a factor (in receive-only systems). This is also the way it is presented in the books. Example: The Practical Antenna Handbook - Third Edition by Joseph J. Carr has the scanner antennas fed using 75 ohm coax (actually RG-59, but RG-6QS is now preferred due to it's lower loss). He also states, when discussing matching coax to antenna impedance "But the actual impedance of a real dipole may vary from a few ohms to more than 100 ohms.". So you can see how matching coax to a broadband antenna is more than just matching the numbers. This is why many vendors offer RG-6QS for scanner antenna systems, it's low loss, affordably priced, and quite usable in receive-only systems.
Of course, if you've got LMR-400, USE THAT!

As for the insertion loss of the splitter, 3db is approx. 1 S-unit. I believe I mentioned that there was slight cost in signal strength. If you've got a few bucks to spare, and need every bit of signal, then check out Stridsburg multicouplers: http://www.stridsberg.com/

N-Jay recommends some good books put out by the ARRL, they are worth the cost.

Mark S.

 

[bsavery]

You want the truth?

You can't handle the truth!

http://www.arrl.org/catalog/index.php3?category=Antennas,+Transmission+Lines+&+Propagation

Books??!! I was really hoping you'd offer to drop by with 3 or 4 samples and install them for me so I can see what works best!!



Thanks! Time to see if my library card is still good!

Bob

 

[bsavery]

I think you'll find, that except for a couple of somewhat retentive people :lol: , it is generally agreed that the impedence difference isn't a factor (in receive-only systems). This is also the way it is presented in the books. Example: The Practical Antenna Handbook - Third Edition by Joseph J. Carr has the scanner antennas fed using 75 ohm coax (actually RG-59, but RG-6QS is now preferred due to it's lower loss). He also states, when discussing matching coax to antenna impedance "But the actual impedance of a real dipole may vary from a few ohms to more than 100 ohms.". So you can see how matching coax to a broadband antenna is more than just matching the numbers. This is why many vendors offer RG-6QS for scanner antenna systems, it's low loss, affordably priced, and quite usable in receive-only systems.
Of course, if you've got LMR-400, USE THAT!

As for the insertion loss of the splitter, 3db is approx. 1 S-unit. I believe I mentioned that there was slight cost in signal strength. If you've got a few bucks to spare, and need every bit of signal, then check out Stridsburg multicouplers: http://www.stridsberg.com/

N-Jay recommends some good books put out by the ARRL, they are worth the cost.

Mark S.

Retentive people... lol ... naw... no one like that around here!! :lol: Well.. not when I take my meds!! :twisted:

For the bulk of my listening, I could prolly run speaker wire outside and wrap it around a coat hanger and get plenty of signal. It's the "hmmm.. what else can I hear on this critter?" factor that has me wondering.

That and a desire to do things the right way. When I learned RF, it was "RG59 is great for channel 9, but RG58 is used to communicate!" We didn't spend a lot of time on the little stuff though. All our transmitters used wave guides and 20,000v power supplies!

Now... since N-Jay won't come install a new antenna and feed line for me... I guess I'll have to pull out the ladder and go recover some LMR-400! :wink:

Thanks for the info!
Bob

 

[K5MAR]

Bob, if you were a little closer, I'd come help you for a share of that LMR-400! That's definately the stuff you want to use. Enjoy!

Mark S.