Coax CAble question (An old timer once told me...)

[pblaze725]

That my cable from my CB to my Firestik antenna needs to be 18'-0".

He says a radio wave is about 31'-0" long & the cable is actually part of the antenna and it needs to be at least 1/2 the length of the wave to receive it.

Is this true?

If so, then why do they sell 9'-0" antenna cable?





I have a bumper mounted antenna, with a 'fire-flex' coax. I am troubleshooting a poor signal. We changed the antenna and got the same results. Now I suspect the cable, or connection at the bumper.




EDIT- Question- Do any of you run a 9'-0" cable with no problems?

 

[Deuling]

It makes sense if it is indeed true. Idk anything about them though

 

[pblaze725]

I have a 9 ft firestik cable in the garage. I'll give it a try.

I could have damaged the 18 footer too.

We'll see tomorrow...

 

[pblaze725]

I went & checked things out.
First of all, a 9 foot cable is too short to reach from my CB to the bumper so that's out.

Second I opened the package and read the installation instructions. It appears that I am missing a nylon washer that goes between the mounting tab and coax connection.
Without the nylon I am grounding out my signal.

-END...?

 

[Deuling]

U try the washer?

 

[TerryD]

That's an old wives tail of CB radio. Just use what it takes to get where you want to put your antenna.

 

[pblaze725]

I think I fixed it

U try the washer?

I went to the auto parts store & got a tephlon washer in the oil drain plug section. I installed it and had a signal for just a second but when I tightened the antenna down I lost the signal.

Upon further examination of installation instructions I see the tephlon washer has a lip on it and the coax connector also has a lip. This prevent grounding of the signal where the bolt goes thru the tab.

I could see the lip on my coax connector was gone, and the washer I bought didn't have a lip on it either so I sat & thought about it...

Then I see my O ring kit.
I put an O ring on the bolt and rolled it down into the tab so they don't make contact. Put the tephlon washer in per the instructions and all of the sudden I can hear the trucks on the highway!

I don't have anybody around to give a radio check, so every time I hear chatter I jump in with a "BREAKER 1-9 GOCHA YA EARS ON?" We'll see when I get with the gang and hit the trail.

That's an old wives tail of CB radio. Just use what it takes to get where you want to put your antenna.

Thanks. By coincidence 18"-0" is just the right length anyway.

 

[4xcrazy]

It's not just a washer, it has a little lip inside the hole to prevent the side of the antenna from touching the side of the mounting bolts/stuff
I'm sure Radio Shack would have something for that too.

 

[pblaze725]

Yes sir, The proper term is 'nylon insulator' and 'raised nylon lip' according to the Firestik installation instructions.

I put an O ring in it's place and it seems to be working...

Thanks...

 

[imiceman44]

That's an old wives tail of CB radio. Just use what it takes to get where you want to put your antenna.

Not true, the length of the cable is actually important, but it needs to be a multiple of 3'
I have a 9' on my big rig because it's the best length to get from the unit to the antenna on my door.
You don't want it to be too long because any excess has to be losely laid out, not coiled.
When you coil it you lose the signal.
If 18 feet is the right length for you that is great

 

[Smokinthehippies]

Not true, the length of the cable is actually important, but it needs to be a multiple of 3'
I have a 9' on my big rig because it's the best length to get from the unit to the antenna on my door.
You don't want it to be too long because any excess has to be losely laid out, not coiled.
When you coil it you lose the signal.
If 18 feet is the right length for you that is great

The coiling part I was always told by my grandpa (lifelong trucker). The other thing was make sure your antenna has plenty of room above the top of rig.
And also if you run dual whips they have to be like 6 or 9 feet apart or some such.

 

[TerryD]

Not true, the length of the cable is actually important, but it needs to be a multiple of 3'

Why multiples of 3?

 

[pblaze725]

The way the old timer described it to me was radio waves are like ocean waves. The radio wave has a swell of 32'.
By using a cable that is 1/2 the length of the wave, you will pick up the best signal.
He also said think of your truck as a satellite, your body is the dish, and the antenna needs to be at least a ft above the top of the body/ dish.

I have never heard of the 3 ft increment rule.
This is interesting...

 

[Blue85]

The 18' thing is silly. I think the idea is to have a "null" at the base of the antenna. This could help with some mismatch or grounding issues. But 18' of cable can't do that.

The most obvious issue is bandwidth. The CB band ranges from 26.965MHz to 27.405MHz. So you would really only be "dead on" for one channel (33?) with this approach.

The one nobody talks about is the REAL WAVELENGTH. Just because a 27MHz is about 36' long in free air, has nothing to do with the wavelength inside the coaxial cable. It has a drastically different velocity factor than air, so applying the 18' rule to it is nonsense. You could look up specs for your coax and calculate, but I've never heard of anybody doing it.

Why do the manufacturers sell them in lengths of 9', 18'? I don't know, other than it keeps their inventory managable. Now for dual antennas, their lengths relative to each other is very important.

Keep in mind that every foot of coax in the system loses some portion of your transmitter power.

 

[Smokinthehippies]

I always thought the biggest issue was to get the srw guage and just tune your setup based on antenna location.

 

[sreidmx]

The reason why the length of the coax is so important is because it loads the tx circuit just like an antenna, swr is a load based calc so if you have a coiled mess of wire it add impedance to the circuit..

 

[Jagged]

The reason why the length of the coax is so important is because it loads the tx circuit just like an antenna, swr is a load based calc so if you have a coiled mess of wire it add impedance to the circuit..

Almost.

You just run circles around the smith chart if the antenna isn't matched to the coax.