Totally newb question.

[flyingwolf]

But what exactly is that little black (or sometimes different colored) bulbus thing in the center of CB antennas?

On one I have here it is a standard mag-mount antenna. Has a 9 inch section at the botton, a 4 inch bulbous section with conical shaped silver ends and a black center about 1 inch in diameter. Then there is a (variable lenfgt by setscrew) thin section of 12 inches.

This is a mag-mount CB antenna once again. I am just curious as to what the heck that center piece is.

Thanks for listening to me ramble.

 

[TeRayCodA]

You can unscrew that assy.and look.

It is a loading coil for impedence.(to electrically shorten the antenna)

I've tried those before,and ended up tossing them.I used to love a K-40(54")whip/bottom coil.

 

[flyingwolf]

Got out the vise grips. Damn that thing was stuck together pretty good.

Yep just a plastic core with some coated wire run in a about 100 turns connection the two particular sections.

Now another question. After reading through the definitions of half of your answer I am wondering what the purpose of this loading coil is in a CB antenna.

Any answers for me?

 

[ILbandit]

The length of wire (antenna) that you need for proper impedance to transmitt/recieve on the cb band. (around the 27 MHz frequency range) should be about 102 inches long. So electrically you need a peice of wire that long for it to work properly.
The perfect match/antenna for the most part is a 102" whip. But due to height limitations for most vehichles, you don't want to have anything that long mounted to it because you would be hitting everything overhead all the time.
So to make an antenna shorter in overall length, the put a coil into the antenna, some are at the bottom, some in the middle, some in the top.
I guess to put it in laymans terms, to shorten the length of the antenna, you twist it in the middle. For instance, just try to imagine that if you pulled out the coiled up wired in the center coil of your antenna, and straightened it out between the top and bottom straight parts, it should measure somewhere around 102 inches long from top to bottom.

I know that isn't a real technical explination, but does that help?

Each frequency you transmitt/recive on will work best at a certian length of wire (antenna). The higher the freqency, the shorter the length you will need. You'll notice some antennas on alot of Police cars that are just a few inches long, they are used for the higher frequencies they use on their 800+ MHz trunk system radios. See the difference between the lengths? 27 MHz (CB Band) = 102", 800MHz = just a few inches.

I guess to make it plain and simple, the coil is there to cram 102" of antenna into a 4ft. long antenna.

Make any sense?

 

[flyingwolf]

Makes 100% perfect sense thank you.
I figured that much but now I am sure.
Obviously it is not the best solution to the problem but it works.
Thanks for the help.

 

[ILbandit]

Here's an article that explains it a little more "in depth" the explanation of the coils is about 1/2 way down. http://tinyurl.com/cfj9j

 

[flyingwolf]

I see Im not the only one with the tinyurl plugin for firefox lol.

 

[bassmkenk2508]

wow i learned something new too!

Always wondered about those coils.....



By the way, I have a quarter-wave 144/440 MHz ham antenna. Would a coil near the midlength of the antenna have a part to play in that the antenna is dual band? The antenna is an omnidirectional mobile mag-mount.
Here is a rough tilted "pic": "+++"=coil, [||]=base

------------------------------+++-----------------[||]

 

[Tweekerbob]

Yes, the coil in the middle of your dual bander is what makes it dual banded.

Super short answer:

The coil passes VHF frequencies and those frequencies "see" the whole length of the antenna.

The coil blocks UHF frequencies, so those frequencies "see" only the antenna from the base to the bottom of the coil.

Ryan

 

[N_Jay]

I guess to put it in laymans terms, to shorten the length of the antenna, you twist it in the middle. For instance, just try to imagine that if you pulled out the coiled up wired in the center coil of your antenna, and straightened it out between the top and bottom straight parts, it should measure somewhere around 102 inches long from top to bottom.

Close but not quite.

The length of teh wire in teh coil does not lengthen the antenna to the proper length.

Its purpose is to act as an inductor to tune the antenna to resonence at the desired frequency.

The length has NO relationship top 102 inches.