Coax Question

Coax Question

Post by Robert1 » Mon, 01 Dec 2003 01:54:51

Hello:

Question on Coax.

I am presently using a 50 foot run of 50 ohm coax to my scanner.

It's the 0.4 inch low-loss diameter stuff (I think LMR 400), and I wanted to
add a short length of
a coax between this cable and the scanner to remove the strain on the
radio's connector due to the stiffness of
the LMR 400.

All I can find pre-assembled with BNC's is 75 ohm RG59.

It's about a 3 ft length.

a.  Think the 3 ft radio-to-75 ohm patch cable, then to the 50 ohm main
cable will cause any "meaningful" loss
     due to the Z mismatch ?

b.  Would I have been better off using a 3 foot RG58 (50 ohm) patch cable,
even though the RG58 is really lossy around 1 GHz ?

Thanks,
Bob

 
 
 

Coax Question

Post by ootman » Mon, 01 Dec 2003 09:30:38


  Radio shack has some solderless bnc connectors for rg59 cable they are
easy to install and seem to work fine


Quote:> Hello:

> Question on Coax.

> I am presently using a 50 foot run of 50 ohm coax to my scanner.

> It's the 0.4 inch low-loss diameter stuff (I think LMR 400), and I wanted
to
> add a short length of
> a coax between this cable and the scanner to remove the strain on the
> radio's connector due to the stiffness of
> the LMR 400.

> All I can find pre-assembled with BNC's is 75 ohm RG59.

> It's about a 3 ft length.

> a.  Think the 3 ft radio-to-75 ohm patch cable, then to the 50 ohm main
> cable will cause any "meaningful" loss
>      due to the Z mismatch ?

> b.  Would I have been better off using a 3 foot RG58 (50 ohm) patch cable,
> even though the RG58 is really lossy around 1 GHz ?

> Thanks,
> Bob

 
 
 

Coax Question

Post by Matt » Mon, 01 Dec 2003 12:00:40

To the purist, you should use the same impedance cable all the way through,
however in practice, you can get away with using the small run of 75 Ohm
cable.  You will incur some additional loss in the system because you are
adding in some additional connectors.
Another option is to redesign the cable run with reagrds to where it
connects to the scanner.  In using cable ties you can also take the strain
away from the connector by tying the cable to the bench or cabinet that the
scanner is on.
Alternatively you could look at using a right angle connector from the cable
to the radio.
Lots of options to consider.

Matt


Quote:> Hello:

> Question on Coax.

> I am presently using a 50 foot run of 50 ohm coax to my scanner.

> It's the 0.4 inch low-loss diameter stuff (I think LMR 400), and I wanted
to
> add a short length of
> a coax between this cable and the scanner to remove the strain on the
> radio's connector due to the stiffness of
> the LMR 400.

> All I can find pre-assembled with BNC's is 75 ohm RG59.

> It's about a 3 ft length.

> a.  Think the 3 ft radio-to-75 ohm patch cable, then to the 50 ohm main
> cable will cause any "meaningful" loss
>      due to the Z mismatch ?

> b.  Would I have been better off using a 3 foot RG58 (50 ohm) patch cable,
> even though the RG58 is really lossy around 1 GHz ?

> Thanks,
> Bob

 
 
 

Coax Question

Post by NetWease » Mon, 01 Dec 2003 15:30:48

Yup, I've used one of these solderless connectors to put a BNC connector
onto the end of the cable of an old school magnet mount cell phone antenna.
Since the 800MHz police/fire stuff that I listen to is pretty close to the
cell phone band, I figured it would work a bit better than the stock ***
duck (and it does seem to help reception in the car).  I've been happy with
the solderless connection.  It was a simple install.
-NW
Quote:>   Radio shack has some solderless bnc connectors for rg59 cable they are
> easy to install and seem to work fine

 
 
 

Coax Question

Post by Fran » Mon, 01 Dec 2003 19:59:02


^ ... you can get away with using the small run of 75 Ohm
^ cable.

I agree.

^ You will incur some additional loss in the system because
^ you are adding in some additional connectors.

Agree, but it will probably be an unnoticeable difference.

Frank

 
 
 

Coax Question

Post by matt webe » Thu, 04 Dec 2003 11:16:02



Quote:>To the purist, you should use the same impedance cable all the way through,
>however in practice, you can get away with using the small run of 75 Ohm
>cable.  You will incur some additional loss in the system because you are
>adding in some additional connectors.
>Another option is to redesign the cable run with reagrds to where it
>connects to the scanner.  In using cable ties you can also take the strain
>away from the connector by tying the cable to the bench or cabinet that the
>scanner is on.
>Alternatively you could look at using a right angle connector from the cable
>to the radio.
>Lots of options to consider.

On a stub cable length  as is being proposed, the losses from the
connectors are likely to be higher then from the cable mismatch.
The loss in the Ghz Range from Coax is huge per 100 feet, but
fortunately 3 feet is only a small fraction of 100 feet.

 Every impedance discontinuity is trouble, and the fewer the better.

>Matt



>> Hello:

>> Question on Coax.

>> I am presently using a 50 foot run of 50 ohm coax to my scanner.

>> It's the 0.4 inch low-loss diameter stuff (I think LMR 400), and I wanted
>to
>> add a short length of
>> a coax between this cable and the scanner to remove the strain on the
>> radio's connector due to the stiffness of
>> the LMR 400.

>> All I can find pre-assembled with BNC's is 75 ohm RG59.

>> It's about a 3 ft length.

>> a.  Think the 3 ft radio-to-75 ohm patch cable, then to the 50 ohm main
>> cable will cause any "meaningful" loss
>>      due to the Z mismatch ?

>> b.  Would I have been better off using a 3 foot RG58 (50 ohm) patch cable,
>> even though the RG58 is really lossy around 1 GHz ?

>> Thanks,
>> Bob