Splicing/extending Cb cable

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
On a CB, is the cable running to teh antenna just regular RG59 or RG6 coax cable?
I need to cut a section and add in a connector, if the above is true, any reason not to use a regular old coax-style thread-down connector (like w/ TV cable)?
 
After soem Googling, I see recomendations for 18' lengths.
Jeez, that's rediculous for a truck. The cable on the little shorty magnet antenna I have seems to be about 10' (def > 9).
Seems it is commonly RG58, I'm not clear what the difference is btwn that and RG59 or RG6.

I guess what I'm wondeirng is, if you needed to cut/reconnect it, could a different type of connector work (like BNC - have a million of those @ work and the toosl to crimp on)
 
Its not good to splice coax it will give less trouble if it is one solid length. you would need to have the lenth in 3' incraments
Also check your standing wave when your done.
 
Never mind the length, it just needs to be long euff to reach the radio on one end and the ant. on the other.

No you can't use the TV stuff.
It's probably RG58, unless it says otherwise on the coax.
But 58 and 59 are close enuff to be ok. ( only 1 difference) :lol:
If it already has a connector, just buy a barrel connector. ( 2 male ends) Then add a ready made jumper to fit.
Then tape over the connectors and barrel connector.

That said....you probably could use the cable TV stuff it you used it all the same ...don't mix and match....but 58 and 59 are ok.


Matt
 
According to this
http://en.wikipedia.org/wiki/RG-58
RG58 is 50 Ohm, RG59 is 75 Ohm. I guess that matters huh.

What's up w/ the 3' rule? I still have a lot to learn in the RF world.
I'd think that as far as the "standing wave" goes, the external antenna above the wbase would be more important?

The problem/case here is that I'm installing a CB overheadin front of the rearview mirror (seems a popular place). To keep it clean, I'm running the wires up and through the ceiling trim, down the A-pillar and under the dash on the pass. side. I haven't commited to a final fixed locale antenna, so for the time being I wanted to have it end in a connector there. Then when I need it (just for wheelin getc) I can slap the magnetic antenna on the hood cowl, run the cable in through the window, plug it in and go.

After thinking baout it, for the connector I'd probably use BNC, it makes a nice strong connect and is easy to disconnect too.
 
Quarry Trash said:
standing wave feq for a cb radio opperates in the foot wave lenghts. The best possible lenght it 18' coax check this thread. This thread is actually talking about antennas but they get into coax lenghts http://www.rednektv.com/smf/index.php?topic=8318.0
Click to expand...

Thread requires member access, can't get in sorry

I suppose I could get 18' total by adding 8' to the 10' already on my antenna - but that means I'll have to have a lot of it all bunched up on either end of the connection.
 
I've got a Dodge Ram Mega-Cab. I mounted a K40 on the roof and I used every single inch of the 18' cable.

Down the rear window, between the bed and cab, through the floor board under the driver's seat and to the console. I could use another 3' to route it perfectly.

I'd over-estimate!
 
The 18' is a long standing myth.
It is total bunk. But believed like a lot of people.

The cable carries the signal to the radio from the antenna, it can not create or modify wave factor.

RG-59 and RG-58 are very different. Ive always heard rg59 will not work (btw diff between 59 and 6 is .01...the decimal is often left out. For video, quad shielded RG6 only (assuming you cant go comp.,dvi,hdmi, etc.) for radio 59 is the cats pajamas) For what its worth a wird up a CB in an old jeep with 59 and it did in fact work. We neer tried to talk on the interstate but for short trail comms it worked great.
 
RG 58 is a standard 50-52 ohms impedance coax. This is what the Power Amplifier in your radio looks for. (This also has the same affect on receive quality) Your CB antenna is also looking for 50-52 ohms. Cable TV type wire is normally 75 ohms. Yes this "work" but you will be creating a mismatch in impedance. This will result in excessive heat in the output of your radio. Heat is what kills transistors. Same effect as not correctly matching the VSWR on you antenna to the cable/radio. A higher VSWR means the antenna is not resonating at the correct freqeucy and your radio will not see a 50 ohm load.
Final word, use the same IMPEDANCE cable when you do a splice and yes a BNC is acceptable at 27MHZ. Just use a barrel between the 2 BNC's and a standard PL259 to the radio. BEST advice, buy a cable near the length you need with connectors factory installed. And check the VSWR when you are done.
RATLABGUY, I've seen you do math on here before, I know you can see the logic in this :)

One more thing. You can tune an antenna by adjusting the length of the coax, but there are a few variables in doing this. Best way to tune is by adjusting the length of the radiating element on the antenna. Point being, dont buy into the 18 ft cable theory.
 
Ok, i decided to try and explain where the 18 feet of cable theory comes from. CB radio is on a rough set of frequencies around 27.5MHZ. 27.5 MHZ has a rough wavelength of 11 meters for 1 full wave. (wavelength = 299792458 divided by frequency). 11 meters is roughly 36 feet. The best point on an antenna line to feed the antenna is at one half wavelength because this is the point where voltage on the line is at a null and current is at a peak ( Ohms law: P(power)=I(current) times E (voltage). Anyway without going further, you can see that 1/2 wavelength at 11 meters (CB Band) is roughly 18 feet.
 
dbguync said:
Ok, i decided to try and explain where the 18 feet of cable theory comes from. CB radio is on a rough set of frequencies around 27.5MHZ. 27.5 MHZ has a rough wavelength of 11 meters for 1 full wave. (wavelength = 299792458 divided by frequency). 11 meters is roughly 36 feet. The best point on an antenna line to feed the antenna is at one half wavelength because this is the point where voltage on the line is at a null and current is at a peak ( Ohms law: P(power)=I(current) times E (voltage). Anyway without going further, you can see that 1/2 wavelength at 11 meters (CB Band) is roughly 18 feet.
Click to expand...
Nice try, but wrong. Yes, that is the reason frequently given to justify the 18' myth, but it doesn't pan out in reality. Why? It appears to work out when you graph it out, but what you forget is that the graph you made is just a snapshot in time. In reality the voltage and current relationship is constantly flowing back and forth. Remember, ultimately we're dealing with alternating current here. It is entirely incorrect to say that at any given spot on the cable the voltage is zero for more that just an instant in time, because it's constantly changing. Since the voltage and current are both constantly changing along the entire length of the cable, it is wrong to say that there is a "null" at any point along that cable.

The upshot of this is that you can use any length of cable as long as it will reach from the antenna to the radio. Just use the shortest length practical, because that avoids unnecessary line loss. Don't splice a cable to add extra length except as a temporary band-aid. If more length is needed just replace the entire cable length. Use high quality RG-58 when possible, double shielded if you can find it. The truck stop quality cable is just inexcusably bad from a loss perspective. Also, RG-58 is nominally 50 ohm cable, and is the proper cable to use for a CB. RG-59 is pretty much the same size, but is 75 ohm and is generally used for video purposes. I'm not saying that RG-59 won't work, but it is not the right cable. The radio and the antenna are designed around a 50 ohm impedance feedline, and it is best to stay within the design parameters.
 
dbguync said:
Ok, i decided to try and explain where the 18 feet of cable theory comes from. CB radio is on a rough set of frequencies around 27.5MHZ. 27.5 MHZ has a rough wavelength of 11 meters for 1 full wave. (wavelength = 299792458 divided by frequency). 11 meters is roughly 36 feet. The best point on an antenna line to feed the antenna is at one half wavelength because this is the point where voltage on the line is at a null and current is at a peak ( Ohms law: P(power)=I(current) times E (voltage). Anyway without going further, you can see that 1/2 wavelength at 11 meters (CB Band) is roughly 18 feet.
Click to expand...


The problem is that you are mixing transmission line ( coax) theory in with antenna theory.

Your formulas are for building antennas.... not coax.

Do like i said in my first post and you will be fine.

dbguync said:
One more thing. You can tune an antenna by adjusting the length of the coax, but there are a few variables in doing this. Best way to tune is by adjusting the length of the radiating element on the antenna. Point being, dont buy into the 18 ft cable theory.
Click to expand...

Ummm ... no you can't.

Matt
 
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