i have read some posts about matching the anteanna to the cb... could someone break this down barney style for me?
thanks

 

Your CB sends a signal to your antenna. But if your antenna sucks at forwarding the signal on, then no one will hear you very well. Also, that will cause heat to build up which could possibly hurt your system.

SO, you want to go to Radio Shack or some CB store, and buy a SWR meter. Will cost 20 bucks or less. Instructions for using it should be in the box. The SWR meter will allow you to make sure your antenna isn't loosing too much of the signal it gets from the CB.

 

ahhh ok

 

Yeah, you want the antenna to "forward" the signal that IT recieves. If it forwards eveything that it is sent than you have a 1:1 "match". If it don't then you get feedback to the radio which causes problems which is the "heat" that WX is talking about and eventially (if it's bad enough) it will destroy your radio. Worst of all it robs your output power.

(sing that to the tune of "this old man" if that helps you)

 

This stuff can get really involved, but it's not magic. Once you can, "see," how it works in your head, it's not so bad. I have a little time, so if you get bored feel free to do something else. I'll try to keep it simple.

The RG58 cable that you hook to your CB has a 50 ohm impedance. This 50 ohms is pretty standard for radio stuff and this number is used by designers when they build the radio. (CATV guys, on the other hand, use 75 ohm impedance cable for video signals. You CAN use 75 ohm when you're building duel mirror mount, "trucker," antennas....this gets deep quick - you'll only be tested on 50 ohms.) It's mostly just a standards thing.

Impedance is the term used when you're talking AC. Resistance is used in DC. When you see the 50 ohm figure on your coax, it refers to an AC impedance. If you hook up your digital volt meter, set on ohm's, to the coax it won't read 50 ohms. The meter's designed for DC.

Ok now, your radio is 50 ohms and your coax is 50 ohms. You need to make your antenna act like it's 50 ohms, too. This is where they get the term, "antenna matching."

Try to think of your antenna as a load, like a heating coil, (which it is, just for rf energy instead of heat energy.) One way to picture it is to think of the starter on your truck. If you were to put a 14 gauge wire from the battery to the starter relay and tried to start the truck, that wire would get hot and burn up. Thinking in DC terms, the solution would be go to a bigger wire - one that could handle the the extra current. With your CB that's not an option. In radio terms you'ld have to change your starter.

The whip antenna on your truck is only half the antenna. It's the positive side. Your truck is the negative side. (When you use a handheld, your BODY is the negative side.)

When you're matching the antenna, what you're trying to do is make the positive and negative look like 50 ohms at the positive side. (It's a bit more involved than this. If you can apply Ohm's law to DC circuits; half way down your whip the current is highest and voltage is lowest, at the ends voltage is highest and current is lowest, the ratio of current and voltage anywhere along the whip should be 50 ohms.)

When you trim the whip to tune it, you're just changing the impedance, trying to get that 50 ohms. When it's matched, the rf energy just flows along, with nothing to hold it back. Goes out to who you're talking to. Just as important, the incoming signal reaching your radio will be large enough, without all the restrictions of mismatch, that your radio will be able to amplify. A good transmit antenna makes a good receive antenna. It doesn't do you much good talking out at someone if you can't hear them well. You can't read the 50 ohms with a DVM, because it's not DC. You use a SWR meter to see how much energy is coming back down the line.

Once you start getting into antennas, you'll soon find that 4 watts is a lot of power. It's the gain of the antenna and propagation that makes the difference. High power is a way seperate you from your money.

A mismatched antenna shouldn't hurt your CB unless you were to just key the mike for a long time. It's a duty cycle thing. So unless you were to rubber band the switch down and go to bed, I wouldn't worry about it, (I'm talking 4 watts legal here.)

Isn't theory fun? I like it, because no one can really know it all, just pretend like they do.

 

SWR= Standing Wave Ratio This the ratio of power that your radio is putting out to the ratio that the antenna is "feeding back" to the radio. The lower the ratio the better your antenna and radio are matched. The better the match the better your ability to talk to people and recieve their signals.

The antenna and its match to your radio are more important than how much your radio costs.

Also experiment with where you mount your antenna. You will find that an antenna mounted on the rear and center will normally transmit a stronger signal to the front of your vehicle.
Mobile antenna's are omnidirectional, or transmit equally well in all directions. That isn't always exactly true. Most SWR meters also act as "Field Strength Meters", meaning you can plot out where the strongest signal is being transmitted in relation to your vehicle.

 

Best way to avoid haveing to match your antenna.
is to go buy a k40, you dont have to have it adjusted.

Otherwise. Go buy the antenna you want. and take it to a place that will adjust the frequency of it for you

if you key the mic before you get it adjusted properly. It will blow the finals out in yoru radio.

i fyou ahve the proper equipment, you can adjust it yourself. its just a wire inside the antenna that you cut to adjust it.

 

You still have to tune a K40. They are just a bit easier to tune, because they have a deeper hole in the coil base, so you don't have to cut the whip as much.

You can tune your own, simply by buying a SWR/Watt meter down at RS and following the directions. They don't cost much and will pay for themselves. You can use the meter on the radio, but it's not as accurate and sometimes hard to read.

In most cases, it's really hard to blow your finals. When you're tuning the antenna, momentarily keying the mic long enough to look at the SWR meter won't hurt them. Just like the processor chip in your pc, heat is the enemy. Try not to heat the finals up too much by long key times when you know you have a short in the antenna system.

A kind of neat little gizmo to make: Take a PL-259 connector, (the one that's on the coax that screws into the back of the radio), and a 1156 or 1157 light bulb, (standard automotive turn signal light). Solder a short length of wire from the center pin of the connector to the contact(s) of the bulb. Then solder a wire to connect the outer part of the connector to the out part of the bulb. What you've made is a very simple watt meter. Screw it into the radio and talk into the mic for a second, (not too long!), and watch it light up. It's cheap and very handy to take to yard sales. You can know in a sec. if that radio is worth $5.

 

Howdy, from what i have read. The K40's are self adjusting.

I would immagine you can tune them manually though. *shrugs*

nice idea with the light bulb!

 

Unfortunately, you still have to tune the K40. They are a nice antenna, but are really nothing more than the standard LC base loaded whip, like a Larson, Antenex, or Wilson.

There are self tuning antennas. Do a search on, "screwdriver antenna", or, " ATAS-120". The ATAS-120 is an all ham band mobile antenna. Basically they took a battery powered screwdriver motor, mounted ATR on it with a whip that moves through a huge coil, (yes, it's that simple and it's $300!). The automatic tuner inside the radio, "reads," the SWR and sends a DC signal to the speed controller, which will move the rod up or down till it see's the proper voltage. It's very mechanical.

The CB band is very narrow and it's quite easy to obtain a good SWR over the whole band with a simple tune on any commercial CB antenna, one time on a given vehicle. But if you change locations or change vehicles, tuning will need to be done again. Remember, your whip is only half the antenna, the steel in your truck is the other half. And like Highpockets658 was saying, you can tweek the gain of it, (sometimes unfavorately), by changing mounting points on the truck.


Some people tune by matching the SWR on ch. 1 and ch.40. Others go for the lowest SWR on 19, because it's in the middle of the band. If you have a favorite channel and don't hop around much, by all means, tune your antenna to that channel. That way, you'll get the best Rx, Tx right where you like to be.