Testing SWR?

[mtnbike28]

OK I am in the middle of the CB install and went to Radio Shack and bought a $50 SWR Meter... but not the darn cable between the meter and CB (why can't they include that?) I ran power to the CB with RA 58 shield cable directly to the battery (read that tip on the internet). So I spliced together the ends from the the cut up RA58 cable I used for power to make a short antenna cable to connect the meter and CB - is this bad, ie messing up my SWR readings? I just want to use it long enough to test SWR. I tested the continuity and it buzzed...
Thanks

 

[Nevets10]

I would get yourself a short coax cable for a few bucks and do it right. Otherwise you'll never be sure your SWR reading is correct.

 

[mtnbike28]

what is short?

The shortest Radio Shack had was 10 feet. That seems like it would throw off the SWR too... Thanks for confirming what I was thinking

 

[Scenic WonderRunner]

http://www.walcottcb.com/coax-coax-jumpers-c-28_355_443.html




.

 

[mtnbike28]

Thanks

The Radio Shack in the next town had a 2 foot cable (the RS web site allows you to search store inventory)
so it was an excuse to get the 40 out for a drive : )

Thanks again for the help

 

[DaveInDenver]

I would get yourself a short coax cable for a few bucks and do it right. Otherwise you'll never be sure your SWR reading is correct.

As you correctly note, don't skimp on the cable between the radio and SWR meter. That section of feedline will not be measured in the reflected energy, so you don't know the SWR of it. The amount of energy lost will be reflected in the forward power and so if you know your radio's transmit power, you can infer if that piece of patch coax is bad or not. Short, good connectors, no pinches in the cable, it's important that it's in good shape.

 

[mtnbike28]

Cool stuff

Thanks I just metered my radio out.. all under 1.5 and I hit a trucker on the freeway 6-8 miles away... It's pouring rain so I quit tuning the antenna when I got that close. After re-reading the internet install info, I see I used the wrong cable for powering the CB. I used RG-58 directly wired to the battery, I need to find RG-58a - it is a braided center wire, I used a solid wire (RG-58)
I figure it will last at least a week till I find the replacement. Thanks again for everyone's help.

 

[gary in ohio]

Thanks I just metered my radio out.. all under 1.5 and I hit a trucker on the freeway 6-8 miles away... It's pouring rain so I quit tuning the antenna when I got that close. After re-reading the internet install info, I see I used the wrong cable for powering the CB. I used RG-58 directly wired to the battery, I need to find RG-58a - it is a braided center wire, I used a solid wire (RG-58)
I figure it will last at least a week till I find the replacement. Thanks again for everyone's help.

while it will work for CB since the current is pretty low, RG-58 is NOT the cable of choice when running power to a radio. RG58 is for cable from the radio to antenna.

 

[DaveInDenver]

while it will work for CB since the current is pretty low, RG-58 is NOT the cable of choice when running power to a radio. RG58 is for cable from the radio to antenna.

I guess their theory is that it's shielded? I dunno. More important that the ground and supply are the same length and don't come near anything noisy. It would be better to run proper power cables inside a foil sleeve than to use 20AWG RG58 IMHO.

 

[gary in ohio]

I guess their theory is that it's shielded? I dunno. More important that the ground and supply are the same length and don't come near anything noisy. It would be better to run proper power cables inside a foil sleeve than to use 20AWG RG58 IMHO.

While power lines noise can be a problem, the noise that enters the power lines are usually coming in from the source and NOT radiated in the lines so shielded or non shielded power lines will not make a difference. The noise on the power lines is often injected from the alternator onto the DC line directly. Take a look at all the police/fire setups or even the megawatt car stereo's. None of those are feeding shielded lines.

Much of the noise from an auto is radiated into the antenna.

 

[mtnbike28]

what to use?

Thanks guys, yes the idea was shielded power to reduce noise. I still want to correct the solid wire problem, what do you suggest? I did run equal ground/power and added another ground close to the firewall since the braided ground wires around the shielded power were so fine, just an insurance.
SO - what to use.

Thanks

PS Gary what software/GPS were you using in your Jeep in WV?

 

[DaveInDenver]

what to use.

For a CB, 14AWG should be sufficient. I like good quality copper stranded with 125C GXL or higher insulation, but TXL is fine as long as it's 125C rated. Just me.

Matching power and ground lengths helps reduce common mode noise, whatever is coupled onto the wire run will be coupled equally on both power and return. Direct to battery if possible.

For around 10A or higher radios (like a 50W ham, which is around 8 to 10 amps), I'd go with 12AWG or 10AWG if it's convenient.

Like Gary says, shielding isn't terribly helpful on the power supply as long as you make solid connections. What does help, particularly with AM (like CB) is shielding the ignition and spark plug wires. This is not a trivial thing to do, but can reduce noise. Alternator noise can be reduced with equal length power cables and a filter, but short of enclosing the thing in a cage there's gonna be some brush noise.

Far and away best noise reduction on the DC supply is using big enough wire and making good connections, everything beyond that is extremely incremental and really sorta compulsive unless you're a dedicated ham going for small signal and DX.