Solving mobile antenna problems in winter

Tuesday, November 19, 2013

Now that winter has set upon us here in the midwest, lots of problems with radio systems in vehicles involve antennas.

Certainly wind (and low overhangs) do their damage, but water and ice are the most common culprits. There are a few other kinks that find their way into the mobile antenna system and cause problems, and we'll look at those too.

Water is Unique

Water is unique, from the standpoint that it expands both when it is heated into steam, and then when it is cooled into ice. The surface temperature of your outdoor antenna would never get hot enough to make steam, but in winter it sure will get cold enough to make ice.

When checking the mobile antenna system because of poor performance, the last I start myself is the antenna itself. Unscrew the antenna nut or base coil and look inside. Unless you live in a dry climate I'll bet you'll find some water or moisture inside.

Use a clean paper towel to wipe off the top of the mount, and the inside of the nut or coil. If there is ice present in there I'll take the assembly inside and use a heat gun on low power. Remember, do this gently, the idea is to get rid of the ice and water, not to get the plastic so hot it melts! The water usually gets in there due to rain, snow, condensation or carwashes - and most times it is because the antenna was installed incorrectly or it was disassembled and put back together and a gasket fell out.

Before reassembling the antenna, look at the mount. Is it tarnished or corroded? If so, you will need to buff it up using a piece of fine sandpaper or crocus cloth. Make sure you wipe and blow all the particles away so they do not become trapped inside. Do NOT use emery cloth, as it contains metal particles that could short out the assembly.

Before doing the reassembly, put a thin layer of silicone grease on the threads of the coil or nut. Do not use petroleum grease or other products here, as it would cause any rubber parts or gasket to swell, and not fit properly. Check to make sure any set screws are present and tight. Replace any missing O rings or gaskets and put the antenna back together.

The Connector

The connector on the cable going to the back of the radio can cause a lot of problems, or no problems, depending on who installed it. I have seen factory installed mini UHF connectors that fail due to flexing, pulling and heating of the cable. Using a wattmeter placed in series with the radio and cable I key up the radio with one hand and while watching the REFLECTED power, wiggle the cable nearest the connector. If there is a problem, the needle on the power meter will jump around quite a bit. In that case there is nothing to do but cut that connector off and install a new one.

You did leave plenty of cable at the radio end to allow this procedure, didn't you? I always love the installer who puts the radio in, then tightens up all the cables and doesn't even leave you enough to pull the radio out of the console or mount, so you can't even get to the connector. Why do I love them? Because I get paid by the hour!

If the connector is a regular UHF PL 259 type, I always unscrew the sleeve and inspect the soldering, many times the barrel of the connector was not heated up enough to adequately melt the solder applied and does not adhere to the barrel or braid. Gently twisting the coax while observing through the solder holes of these connectors will tell you if this was done properly or not.

Due to the vast amount of heat required, with a small diameter cable such as RG58, I almost always use a crimp-on type as a replacement. Just follow the instructions on the package, and use the proper crimping tool. For good measure I make sure the center lead comes all the way through, so I can see it. Then I solder the center pin, rather than crimp it, and cut off the excess lead. I have not had any problems with this type of repair.

If it's not the Connector

The problem is not always with the connector, though I automatically check them both so I know the install was done right. If I have a high reflected power on my meter, the only thing left is the coax cable. Most times the problem is caused by a poor routing of the cable. I have seen (and replaced) cable that was run directly in front of the air vent.

In the summer this does not cause a problem, but in winter with the hot air cranked up, many types of coaxial cable will "MELT" internally, and cause the center conductor to shift - sometimes even touching the shield.

You can easly check for a short by removing the antenna and radio end connector. Use an ohmmeter to check at both ends of the free cable for a short. You might wonder why I check both ends - well that was caused by a cable that had two problems. It was open at one end and shorted at the other!

In some cases you can trace the cable and feel a "stiffness" in the section that has melted and re-solidified. In othe rcases I have seen cable run under the floor mat right above the catalytic converter. A catalytic converter gets hot, and a malfunctioning one gets even hotter - hot enough to cause the same problem mentioned above.

I have also seen the cable run neatly down the side of the car, under the molding, and when the trim is put back into place a screw is drivin right through a portion of the coax cable. This is a time bomb of the worst kind. In most all cases replacing the cable is the only thing that will prvide an acceptable and problem free solution.

So now that you have read this article, get out there and check your system and correct any little problems, before they become bigger problems.

73

By Jim Arcaro WD8PFK

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