Adapting an old CB antenna to Comm hand held

Kiddo's Driver

Kiddo's Driver

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Jim
I would like to use one of my old CB antennas as a base station antenna at my house. I have a Yaesu hand held and two roof mount CB antennas to choose from. The antennas are ¼ wave length (about 4.25 ft).
(Edit: It turns out that is not how long it is, see later in the thread.)
They have the standard PL-259 screw on connectors. I’ve found a SO-239 to BNC connector on Amazon that will allow me to connect the antenna to my hand held.
http://www.amazon.com/gp/product/B001JT3J7W/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A2CK5NG72UVSTT
Now I just need to figure out how far to trim down the CB antenna to make it work. ¼ wave length for 123.70 MHz is 1.89 ft. ½ wave length is about 3.6 feet.
The question I have is can I cut the CB antenna to 3.6 feet and then mount it on a 3.8 foot (3.6 * 1.05) disc of metal for the base plane. Will this give me a ½ wave ground plane antenna?
Should I just trim the thing down to ¼ wave length (1.89 feet) and not worry about it?
-Jim
 
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22.7 inches works out to be the right quarter wave antenna length. Just doubling it to make a half wavelength is NOT going to work. The thing isn't going to match no matter what you do.
 
flyingron said:
22.7 inches works out to be the right quarter wave antenna length. Just doubling it to make a half wavelength is NOT going to work. The thing isn't going to match no matter what you do.
Click to expand...

Doubling it and drooping the four horizontal legs will make it appear as a 1/2 wave dipole. The 1/4 wave ground plane antenna has an input impedance of 36 ohms. The cable is 50 ohms. A half wave dipole is 76ish ohms. A half wave grounded plane with dropped horizontal legs gets it pretty close to 50 ohms. Much closer to 1:1 on the SWR. It does not have to match. Just be ball park.

Jim
 
It sounds like your "CB" antennas may have loading coils in them. 4.25 feet is quite a bit short of a quarter wave at 27MHZ. If thats the case then cutting the whip to 22.7" isnt going to do it.
 
No coils.
492/27=18.2 ft is 1/2 wave
9.1 ft is 1/4 wave
Hummmmm....I'm going to go measure them. I was going from memory. They are definitely shorter than me and I am 6 foot.
Could that sucker be 1/8 wave? 4.6 ft?
 
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Unless you're going to be transmitting, which you are probably not licensed to do, this is not something to obsess over. The difference between a perfect antenna and one that is cut way off will probably not be detectable when you're just listening.

Kiddo's Driver's suggestion of a simple vertical antenna with four ground plane legs at 45 deg. is probably the easiest. You can do this with some copper wire, a hefty soldering iron, and a SO-239. Just solder the radials to the body of the connector. The design equations for this are easy to find on the 'net and if you're off by 50% it will still probably not matter for receiving. Sell the unmutilated CB antennas to someone for a few bucks and you'll make a profit on the deal.
 
So 3.6 feet for the main antenna, 3.8 feet for the four legs, and angle the legs down at 45 degrees? I can use a torch to do the soldering.

I'll figure out which connector to use to make it work with my hand held.

On the Little Wil, the whip is held in place by a set screw. I can buy the right diameter whip at Home Depot and cut it to the right length. That way I can put the old one back in and sell it if I want. (I already have the adapter on the way so there won't be any permanent changes to the Little Wil.)

Now to figure out what the coil does.

Jim
 
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To heck with it. I'll just make a 1/2 wave dipole antenna. It only needs 3.6 ft up and 3.6 ft down. Solder the Coax split with conductor on one and shielding on the other, and a BNC on the other end of the wire.

Jim
 
Kiddo's Driver said:
To heck with it. I'll just make a 1/2 wave dipole antenna. It only needs 3.6 ft up and 3.6 ft down. Solder the Coax split with conductor on one and shielding on the other, and a BNC on the other end of the wire.

Doubling it and drooping the four horizontal legs will make it appear as a 1/2 wave dipole. The 1/4 wave ground plane antenna has an input impedance of 36 ohms. The cable is 50 ohms. A half wave dipole is 76ish ohms. A half wave grounded plane with dropped horizontal legs gets it pretty close to 50 ohms. Much closer to 1:1 on the SWR. It does not have to match. Just be ball park.
Click to expand...

You seem to have made the same mistake twice. A halfwave dipole is not a half wavelength on both sides. It's a half wavelength total. That is, each side of the dipole is 1.89 feet (for 123.7 MHz) for a total of 3.78 feet.

Neither design you mention above is going to match to anything without a lot of effort.
 
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airdale said:
Unless you're going to be transmitting, which you are probably not licensed to do, this is not something to obsess over. The difference between a perfect antenna and one that is cut way off will probably not be detectable when you're just listening.
Click to expand...

Then he might as well just plug in the CB antenna unmodified and be done with it. All of his proposed solutions are going to be as bad of a match as the 27MHz tuned antenna.
 
flyingron said:
You seem to have made the same mistake twice. A halfwave dipole is not a half wavelength on both sides. It's a half wavelength total. That is, each side of the dipole is 1.89 feet (for 123.7 MHz) for a total of 3.78 feet.

Neither design you mention above is going to match to anything without a lot of effort.
Click to expand...

Good catch! The funny part is, the hand written note that I put in wallet yesterday has the correct numbers on it!

(EDIT: See my apology farther down in this thread.)

BTW Ron, has anyone ever told you that you have caustic way to speaking to people? It makes you sound like you are too busy talking down your nose to take the time and point errors out without being snippy about it. Just an observation. It's not meant to start a wizzing contest.

-Jim
 

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Kiddo's Driver said:
Good catch! The funny part is, the hand written note that I put in wallet yesterday has the correct numbers on it!

BTW Ron, has anyone ever told you that you have caustic way to speaking to people? It makes you sound like you are too busy talking down your nose to take the time and point errors out without being snippy about it. Just an observation. It's not meant to start a wizzing contest.

-Jim
Click to expand...

Sorry, I wasn't intending to be snippy or caustic. I thought the post was just succint. I could go into details as to why it's not going to match, but I figured it was a brain fart what makes a halfwave dipole.
 
flyingron said:
Sorry, I wasn't intending to be snippy or caustic. I thought the post was just succint. I could go into details as to why it's not going to match, but I figured it was a brain fart what makes a halfwave dipole.
Click to expand...

Thanks, and I do appreciate you pointing it out. I would hate to put the effort into building something exactly twice the size it is supposed to be!

Jim
 
Kiddo's Driver said:
Good catch! The funny part is, the hand written note that I put in wallet yesterday has the correct numbers on it!

(EDIT: See my apology farther down in this thread.)

BTW Ron, has anyone ever told you that you have caustic way to speaking to people? It makes you sound like you are too busy talking down your nose to take the time and point errors out without being snippy about it. Just an observation. It's not meant to start a wizzing contest.

-Jim
Click to expand...

I've stewed on this for a few days and I can't get over the fact that I was caustic right back to him! I could and should have handled this much better. Maybe a PM. Just something other than what I did.

Ron, my apologies for snapping back at you. I should have PM'd you and asked/settled this quietly instead of in the thread.

-Jim
 
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