No Network Coverage

"Full coverage area" is a pretty nebulous term when dealing with mobile carriers. Most of them advertise their coverage area, but signal strength can vary widely within that footprint. It can also vary widely within the day, depending on the load on the network and your distance from the tower. I would recommend contacting your carrier with the problem and getting their read on it. It may not be a problem with your device, but with the tower you are talking to. Be prepared to tell the carrier WHERE the problem occurs and WHEN it occurs. These are key elements in tracing RF problems. If it happens ar multiple times in multiple locations, it may indeed be your device. Usually, the only repair you can make is to replace the device.

First, I am going to assume that we are talking cellular phone. If you are using a hand-held cell, inside the car, communications will be lousy. A metal car body will block a large percentage of the RF signal.

Now lets assume you are using an external antenna. The typical cell antenna I see is a straight piece of wire, a coil, and another straight piece of wire. The number of straight pieces of wire in the antenna determines the "gain" of the antenna, or how well it transmits and receives the signal. An antenna with two coils (and 3 straight pieces) will work better than one with only one coil. If your car has a metal top, optimum placement of the antenna would be in the center of the top. With a convertible, the optimum place would be center of the trunk lid. These antennae are designed to work with a flat plane of metal underneath them for optimum performance.

You may have replaced your antenna, but what about the cable from the antenna to the radio? Assuming that you have coaxial cable as a feed line to the radio, is there any possibility that it might have been crimped? Like being slammed in a car door? An analogy to this is crimping (bending) a garden hose to stop the flow of water temporarily. With coax, a crimp might not recover, and the electrical characteristics of the cable are destroyed, thus it would not work efficiently. I would look at the feedline for places that are flattened (rather than round) and cuts or abrasions in the outer jacket of the cable. If so, replace the cable, with preferably NEW cable of the same type. Coax ages over the years, so coax coming from your grandfathers stash in his attic will as likely as not be bad. Plus the new cable has to have the same characteristic impedance as the old cable. I would guess you would need 50 ohm cable, although it also comes in 75 ohm (used in cable TV), 91 ohms, and others. Using the wrong cable results in signal reflections going up and down the feedline rather than being transmitted (or received).

The only next option I can think of would be to replace or have the phone serviced. It is remotely possible that it might need an alignment (or tuned up if it were a car engine).

Anyhow I hope this helps.

Bill