Suppose your car radio brings in mainly static, or plays only sometimes, or has quit altogether. Before you trade in your car, or take the radio to a shop for repairs (both expensive), try checking the antenna with an ohmmeter.
An ohmmeter, a handy, inexpensive instrument you can use to check many other electrical components and wires besides the antenna. Basically, it measures resistance to current flow in an electrical circuit. Too much resistance in a circuit prevents adequate current flow; too little, and too much current flows.
An ohmmeter has its own power supply, a small low-voltage battery. When attached to a wire or electrical component, it sends this small, known voltage through the item being tested and shows the resistance to this current in ohms. It should never be used in a live circuit -- one in which current is flowing -- because this can give you a false reading or damage the meter.
When you buy an ohmmeter, get one that has at least three ranges, or scales -- for example, X1, X100 and X1000, that you select by turning a knob. If you were using scale X1, you would multiply the meter reading by 1; if scale X100, you would multiply the reading by 100, and the same with scale X1000. In other words, a reading of 10 means 10 ohms if you had the knob set on scale X1, but 1,000 ohms if you're using scale X100 and 10,000 ohms if you're on X1000.
Use the scale that's appropriate for the item you're testing. For example, if you're testing a spark plug wire that's supposed to have a resistance of 30,000 ohms, you'd use scale X1000; but X1 would be more appropriate for, say, a ballast resistor supposed to have a resistance of 5 ohms.
Any time you use an ohmmeter, before you begin a test, turn the power on and connect the meter's two leads together. The needle on the face should read zero. If it doesn't, adjust the calibrator knob until it does; and if you can't get a zero reading, check the battery.
Now for the antenna. Disconnect the antenna cable from the radio. TEST 1. Connect one lead from the meter to the tip of the antenna, and the other to the radio pin -- the piece at the end of the antenna cable that fits into the radio. The reading should be low, usually under 5 ohms; that means a good electrical connection between the tip of the antenna and the radio pin. TEST 2. Connect one meter lead to the cable and the other to the antenna base. A reading below 5 ohms indicates a good ground connection. TEST 3. Connect one lead to the cable end and the other to the radio pin. The meter should read infinite resistance -- the needle will travel as far as it can go, resting near a symbol that looks like the number 8 tilted on its side, the sign for infinity. This indicates no electrical connection between the radio pin and ground.
When making these checks, wiggle the antenna. The readings shouldn't change when the antenna is wiggled.
If you make these tests, and the readings you get don't match the readings given here, the antenna is bad, and you ought to replace it. If, on the other hand, everything checks out okay, the antenna is good; then you can start thinking about taking the radio into the shop or just trading in the car.