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Mobile Radio Install Tips

 

Maintaining radio communications between vehicles, while on the trail, enhances the enjoyment of any trail ride.  The ability to summon assistance in times of emergency can not be stressed enough.  Various types of radios are available to help fulfill these communication needs.  Although most of the methods and techniques covered here apply to all radio installations, I'll use the CB radio as the focus of this article.  It is the most popular radio in use today on the trail.  (That does not mean it is the best type of radio to use for trail communications....it just means it is popular.)  My experience in this area dates back to my first days of being a ham radio operator, more than 25 years ago.  After high school, I enlisted in the US Navy and served 8 years as an electronics technician.  The government spent well over a year on my education in electronics and radios....that is 40 hours per week, over 52 weeks of training.  After my military tour, I taught electronics and communications theory for almost 6 years, employed by a college.  I no longer work in the electronics field, but I still have my ham radio license and help a lot of my friends with their radio installs.  I hope you will find some of this information helpful in assisting you with your radio installation project.

This article will not address the dozens of makes and models available on today's market.  New models emerge weekly while older ones disappear.  This article will cover the basics of a good CB install and provide you with some tips and hints to help make your install a success.

For some anecdotal information on CB RF noise suppression, check this out.

 

DC Power

Your installation will be off to a good start when you connect your CB to a clean power source.  What is clean power?  Clean power simply means that the radio's 12 volt power cable is NOT tapping into some other part of the Jeep's wiring harness or a "borrowed" power point.  By borrowed, I mean something like putting one of those "adapt a fuse" devices on the fuse panel so you can piggy-back your radio's power cord on an existing fuse.

Dirty power can cause interference in your radio.  Today's vehicles come with a multitude of RF (radio frequency) noise generating devices.  They were not designed to make RF noise, but do so as a byproduct of their normal operation.  When operated in the near vicinity of the radio, or often times when sharing a common power circuit, these devices can reduce the ability of the receiver to pick up weak signals.  They can also cause a hum or buzz to appear in your transmitted signal too.  A few of the components that have been found to cause interference to radios include the engine computer, wiper delay module, electric fuel pump, dash panel gauges, and even the digital clock in the AM/FM radio.

If you tap into the wiring that directly supplies these components, you increase chance of getting RF noise on the power leads going to your radio.  

By connecting your radio power leads directly to the vehicle battery, or as close as possible to the battery terminals, you reduce the chance of getting interference in your radio.  Many off-roaders mount an aux power distribution block (i.e.., an extra fuse panel assembly) close to the battery and run their radio equipment from it.  They usually hook their extra light systems into this block also.  The lighting system is not a common source of RF noise and therefore can share the power connection without creating problems.

One thing to remember is that not all radios are as susceptible to RF interference as are others.  Some vehicles do not generate as much RF noise as others.  The mixing and matching of radios and vehicles can make it difficult to determine the source of the noise and likewise, a good fix to eliminate the problem.   

 

Coax Cable

Good quality coaxial cable gets you one step closer to having a good reliable radio system.  Stick with the name brands and you won't regret it.  Belden has made good quality cable for many years and continues to do so.  Radio Shack's coax cable has gotten better over the past few years.  Their early offerings were pretty bad.

When routing the cable in your vehicle, try not to run it next to other vehicular wiring.  By doing so, you run the possibility of picking up RF interference in your coax, and more importantly, you run the risk of causing RF interference with the many electronic systems in your vehicle.  I've seen radio installations where transmitting on the radio results in a change in the vehicle's cruise control setting.  That is not a good thing to have happen.  Many manufacturers, including Chrysler, publish recommendations on how to install two-way radio systems in their vehicles.  I would recommend contacting your dealer or customer service office for more information on this topic.  It is also possible to permanently damage vehicle electronics with RF energy from a radio transmitter.  This is another good reason to stay legal and NOT run those illegal CB radio amplifiers.  Your warrantee will be null and void when the component damage results from your radio transmissions.

Top quality cable will help ensure that RF energy does not leak into sensitive electronic components on your vehicle.  Once your transmitted signal gets to the antenna, it will hopefully not cause any problems.  If it does, you may have to decrease your power output to stop the interference.

When routing the coax, be careful not to crush or deform the cable.  If the cable is squeezed or pinched going under the gasket on the trunk or tailgate, the impedance of the cable will change and it will make for a mismatched system.  This mismatch cuts down on the efficiency of your antenna system and gives reduced performance.  Considering how inexpensive coaxial cable is, spend a couple of bucks and replace it.

For a short how-to on installing a CB coax connector, check this out.

 

Antenna

The antenna used for mobile operations probably has the biggest influence on how well your signal gets out to others and how well you can hear your friends on the trail.  There is more voodoo half-truths and plain bogus information about antennas floating around than any other part of your radio system.  Part of the reason for this is that few people understand why and how an antenna works.  Granted, you don't need to know anything about antenna theory to use a CB radio properly.  However, the less you know, the easier it is for a dishonest company to sell you a worthless antenna and and specially cut to length cables for way to much money.  To make it worse, these company's put up FAQs on their websites and fill them with a little bit of truth and the remainder with urban legend and old wives tales. 

The shortest antenna that can electrically radiate RF energy is 1/2 wavelength long, or about 18' in length for a CB radio.  The 1/2 wavelength antenna can be constructed by using a 1/4 wavelength vertical element and a 1/4 wavelength ground plane.  The result is a vertical (or whip) antenna that stands about 9' tall (or 108") and requires an RF ground plane underneath it.  

For CB users, the most popular type of antenna is the 1/4 wave vertical.  A quarter wave vertical is a ground dependent antenna.  It uses the vehicle's metal body as half of the antenna.  This works out well for mobile operation because of the ease in which the upright radiating portion of the antenna can be mounted.  Most of these antennas are not physically a 1/4 wavelength long.  The old 108" steel whip antennas that were so popular back in the '70s CB craze were just that, a 1/4 wavelength long.  A common CB antenna today measures about 4' tall.  One type uses a fiberglass rod and winds the wire around the rod, with the bulk of the wire being closely spaced at the top of the antenna.  This is called a top loaded whip.  Another antenna uses a 4' long steel whip which protrudes from a large base, about the size of a small coffee cup.  In this base is a coil of wire which makes up for the rest of the wire that is not sticking up into the air.  This is called a base loaded antenna.  Although there are other designs (most of which are totally bogus in what they promise for improved performance), the top and base loaded types are the most commonly used styles by people that know something about antennas.

Because your vehicle acts as half of your antenna, it is important to get a very good electrical (and RF) connection between the antenna mount and the car body.  Many CB antennas have a 3/8" x 24 studded base that screws into the antenna mount.  The mount is usually located towards the rear of the vehicle. It is important to make sure that the bolts securing the mount to the vehicle body are making a good, clean contact.  It is possible for the mount and vehicle body to be partially insulated from each other because of paint, rust, or powder coating.  Usually, the mounting bolts (or screws) make a good enough connection by touching bare metal on both the mount and the body.  High SWR problems are usually the result of a poor connection at the mounting point.  Many folks refer to this as "having a bad ground at the antenna".  This is technically very wrong because the antenna is NOT grounded.  The mount for the antenna is grounded.  There is a big difference between the two and applying a ground to the antenna will cause infinite SWR and the possibility of a burned out RF power amplifier transistor in your radio.

 

Antenna Preference

I get asked time and time again which antenna I recommend.  Up until recently, I didn't have a particular favorite.  I had found most name brand antennas to be pretty much the same as far as results are concerned.

I ran a 36" long K-40 antenna for better than 5 years with very good results.  It is extremely flexible and can be bent into a "U" shape without causing any damage to the antenna.  For me, that is important as our trails can get rather overgrown with brush and desert plants.  Snapping off your antenna in the middle of a trail run is a big bummer for sure.  This antenna routinely ran a SWR of about 1.5:1 using a variety of mounts and several different mounting locations on my TJ.  I would recommend it to anyone.

I had an old Radio Shack 4' long antenna sitting in the garage that I kept as a spare.  As it so happened, I was getting ready to run a trail with a friend of mine (BradW) and he had recently broken his antenna and asked if I had a spare.  I brought it to the trail head and he installed it for the run.  Wouldn't you know it....part way through the trail, it broke right off at the threaded base.  I told BradW not to worry as it was my old spare and its loss was not an issue for me.

BradW being BradW, he shows up a week or two later at my place with a new antenna.  It was a 4' long Wilson Silver Load (Flex-4, 5/8 wave).  He had bought one for himself and one for me.  BradW said that if I didn't keep it, then neither of us would have a spare when we needed it.  He wouldn't take no for an answer and so I put it in the garage so a spare antenna would be available if needed. 

A month later, I was remounting my radios in the TJ and so decided to give this new Wilson a try.  I removed the K-40 and installed the Wilson on the quick disconnect mount I use for my antenna.  A check and just a bit of adjustment at the antenna's tuning tip yielded an SWR that was less than 1.1:1.  I couldn't believe how nicely the antenna adjusted and how low the SWR came in.  I am impressed.  I have mine optimized for the low end of the band (channel 4 in my case).  At channel 40, it is about 1.3:1.  Very good indeed. 

The Wilson is also very flexible.  It too can be easily bent in a "U" without any damage.  BradW said his held up on the trail for many years before it required replacement.  At just $25, that is pretty good considering the number of trail miles BradW racks up each year (ie., LOTS of them). 

So.....if antenna height is your limiting factor, then try the K-40 antenna.  If you can handle a 4' antenna, then give the Wilson a spin.  I feel confident that you will be very satisfied with either one.  But like they say, length does matter.....so go for the longer one if you can. 

 

SWR

SWR stands for Standing Wave Ratio.  There is but one standing wave ratio that can be measured on the antenna coax cable, so please don't say something like "My SWRs are pretty high."  By using the plural of SWR, you are only showing how stupid you really are....period, end of discussion.

There, with that out of the way, lets briefly discuss just what SWR indicates.  Basically, the SWR measured in an antenna system indicates how well the antenna is matched, or adjusted in the case of most CB antennas.  When all is well, the output impedance of the radio (50 ohms) matches the coax cable's characteristic impedance (50 ohms) which in turn matches the antenna, which is hopefully 50 ohms.  If the antenna is a bit too electrically long or short for the frequency it is operating at, some of the power being sent to the antenna will be reflected back to the transmitter.  The more the antenna is "off frequency", the higher the amount of reflected power.  If the reflected power gets too high, it can cause damage to the transmitter and it will also limit your range and the ability of your radio to communicate properly with others.

When everything in your antenna system (coaxial cable and antenna) are doing fine, the standing wave ratio is 1:1 (pronounced 1 to 1).  I always try to get things properly adjusted so that the SWR is at or below 2:1.  It is not too difficult to get 1.5:1 or something close to that.  With an SWR of 2:1, you have about 10% of the power being reflected back from the antenna.  In the professional mobile communications install world, this is considered acceptable.  I do not recommend operating a radio where the SWR is 3:1 or higher.  At this point, you are getting quite a bit more reflected power (sorry, I don't remember the exact mount) and you will certainly notice how poorly the radio performs (can't hear many others, they can't hear you very far away, etc.).

How to:  CB Antenna Installation and Adjustment 

 

 

 

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