another CB thread

[loquito]

I have an CB coming in the mail. I wanted to mount the antenia on my future ARB Bumper ( passenger side) . Can you guys tell me the length of the coaxial from the CB to the bumper? What did you guys use?
I will put the CB in the spot below the radio when I put in the 1 din radio.
Thanks You Guys.
Steven

 

[MoJ]

Check out the FAQ at www.firestik.com

They, along with Wilson, recommend using 18 ft. of coax for best performance. However, it's important not to tightly coil the excess.

hth

 

[loquito]

Check out the FAQ at www.firestik.com

They, along with Wilson, recommend using 18 ft. of coax for best performance. However, it's important not to tightly coil the excess.

hth

Thanks Alot.
I just came from there site and ordered the kit I need. What do you do with the extra cable?

 

[MoJ]

This is from their FAQ

Q. I only needed 9 feet of coax to go from my radio to my antenna. How should I handle the excess?
A. What ever you do, do not roll it into a small convenient coil. It will become an RF choke. If you cannot let it lie loose under a seat or in a headliner, wrap it into a yarn-like skein of about 12 to 16 inches, put a wire tie in the center and tuck it under your dash, seat, etc.


I routed mine the long way around to use up as much slack as possible but my antenna is in the rear. I'm sure folks are using shorter lengths of coax with OK results, heck for that matter if you're just using it on the trail to talk to other rigs you don't need to get too fancy. The 18 ft is usually listed for the max performance. Going from 12 to 18 cut my SWR in half and my broadcast is pretty nice for a $35 CB.

 

[loquito]

Thanks Again
I'll follow your advise. It is mainly for trail use, one of the first questions on my first off road trip was if I had a CB or not. I then got handed one of the little hand held radio's. It worked good, but I need something to fill the space below my new stereo.

 

[Bob_Garrett]

If you're using a typical single cb antenna, there's no benefit to using more coax than necessary to reach from the antenna to the radio. Additional cable just increases the signal loss between the radio and the antenna. Just use good quality cable and connectors.

Bob

 

[Pskhaat]

Agree with Bob, the reason 18 feet is suggested is that 18 feet is approximately half a wavelength on 11 meter and the coax can be used as a conterpoise (ground) for the antenna if the installer doesn't RF ground it very well.

This is why MoJ probably saw a decrease in his SWR. Going to 9 feet instead of 12 would most likely have had the same affect as 9' being ~1/4 wavelength multiple.

On a properly installed antenna with good RF counterpoise, the more coax will lead to greater loss.

 

[Bob_Garrett]

This is why MoJ probably saw a decrease in his SWR. Going to 9 feet instead of 12 would most likely have had the same affect as 9' being ~1/4 wavelength multiple.

That's certainly possible. However, if nothing else changes, adding more coax will *appear* to reduce the SWR. But actually, you're just increasing the loss between the antenna and the meter, so while SWR may appear to be lower, it's actually the same. In fact if you use enough coax, the SWR will be good even with no antenna at all.

Best way to measure SWR is to first measure it at the antenna and tune as necessary, then check it at the radio to ensure that coax and the connection to the antenna are ok.

Bob

 

[Pskhaat]

Good points.

SWR is just a ratio of impedence mismatch that never drops below 1.0 and doesn't tell the whole story. The mismatch may be in any side of the equation, either in the antenna or the coax feed line (or connections thereto/from).

 

[CDN_Cruiser]

Bob and 3FJ raise some really good points that often seem to be missed by people installing radios.

The coax should be viewed as a feedline, not the antenna. If you properly ground the antenna - from a RF perspective (ie grounded to the roof, metal-on-metal antenna mount, etc) - the feedline shouldn't enter the equation as being part of the antenna. Ideally the feedline is as short as possible to minimize the loss before the signal gets to the antenna. No different than the loss you see sending 12V down a wire - the further from the battery (and the crappier the wire) the greater the loss at the end where it matters!

If it's not grounded, the feedline starts to act as a ground and it should be a 1/4, 1/2 etc wave of the freq., but you will start to experience loss along the line, even if you seem to have a great 'match' (low SWR). This is especially true with the poorer quality coax feedlines that are sold with most antennas.

So, what's the point? If you're only trying to talk down the trail, slap it on and don't worry - someone will hear you. If you want to be a little bit clearer and heard a bit further, take the time to properly ground the antenna and check the SWR.

Cheers, Hugh

 

[RavenTai]

Got a question along these lines wile we are at ground points, If I make a bracket for an antenna does the material matter? Would steel be a better choice than Aluminum or stainless? Does the higher permeability of steel/iron to flux still matter at RF frequencies?

 

[Riley]

I don't think the material matters that much except as it pertains to a good electrical contact. Keeping the resistance and inductance low is what counts. What's probably more important is how the different materials will react with the material you are attaching to.

 

[Gumby]

Doesn't look like much is 8- specific in here. Off to Outfitting it goes.

'cause there ain't enough CB radio threads over there.

 

[alia176]

had a question on grounding the antenna on an ARB front bumper: should I run a ground lead from the base of the antenna mount to, say the horn ground (through the grill)? My radio sounds fine but it wouldn't hurt to make it more efficient.

Thanks fellas.

 

[Pskhaat]

I don't want to repeat myself and I know y'all get bored of hearing this, but ARB mounting isn't the best. If you DO mount there, a ground strap to the BODY (side fenders) which is where your ground `plane' will be.

 

[alia176]

Thanks for the info. I agree, front bumper for a CB antenna isn't the best. For my short convoy needs, this setup works marvelously. I'll run a ground wire to the fender somewhere.

Ali

 

[Kofoed]

Agree with Bob, the reason 18 feet is suggested is that 18 feet is approximately half a wavelength on 11 meter and the coax can be used as a conterpoise (ground) for the antenna if the installer doesn't RF ground it very well.

.

Yes, in a vacuum. You need to add in the velocity factor of the ocax, which for crappy RG-58 is 0.66. So you need more like 12 feet.

The other guy saying that the coax length doesn't matter is right. The SWR should be measuredat the antenna feed point, which nobody does. Then you need a meter which reads forward and relected power, to tune the antenna.

If you are mounting the antenna on the front, I'd get as tall and antenna as possible. You can ground to the mounting point. You will not have a ground plane with this set-up no matter where your ground is.

P.S. A wilson 1000 mag mount on the roof would be sooooooooooo much better.