Radio Comm Tech - CB/Antenna/Coax&SWR

Here is the article that the Radio Comm Tech segments for episodes 120, 121 and 122 were based on.

Your CB radio

Having radio communications while on the trail can be useful and enjoyable. However, it is not as easy as throwing the radio in your vehicle and talking to your buddies. You need to decide which radio is best for you. There are many choices and sizes out there, but for inter-vehicular trail communications the compact CBs such as the Cobra 19 or Uniden PRO505/510/520 work fine and are easier to mount as well. They all put out the same power (4 watts) so the amount of "bells and whistles" you want to pay extra for is up to you.



Uniden Pro 505
http://www.c4x4.com/photos/2014//pro505.jpg


Uniden Pro 520
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Cobra 19
http://www.c4x4.com/photos/2014//cobra19.jpg

People may tell you to have your radio peaked and tuned for more power... anything over 4 watts AM is illegal and not necessary for inter-vehicular trail communication within a small group. It doesn't do you any good if you can talk 15 miles but everyone you are trying to talk to can only talk 2-3 miles. If you want to talk across the Country or around the World I would recommend obtaining your General class Amateur Radio license and legally talking on many different bands with up to 1500 watts.

Mounting options may vary slightly from vehicle to vehicle depending on the design of the interior, but in general the passenger side of the transmission tunnel or to the roof/roll bar between the sun visors is popular. With the radio mounted to the roof/roll bar you will end up with the mic cord swinging around in your field of view which can be distracting on the road and on the trail. It is best to have the radio mounted in your field of view for safe operation, however if you are familiar with your radio you should be able to make adjustments to the radio without looking at it. In my early model XJ I have my Cobra 19 mounted to the left of my ashtray putting it with easy reach as my hand is usually on the shifter. Why the CB manufactures have not come out with a remote head CB radio is beyond me. Mobile amateur radios have them and the control heads for amateur radios typically do far more than your average CB so it shouldn't be too hard and would make mounting a CB in any rig so much easier. The Cobra 75 is about as close as they have come, but it has all the controls in the mic rather than just a remote head/face plate which make the controls very small and sometimes difficult to adjust.

Cobra 75
http://www.c4x4.com/photos/2014//cobra75.jpg

Powering the radio can be done a variety of ways as well. You can use a lighter/power port adapter, tap into the fuse block, or the best option....directly to the battery. When powering your radio via the power port or fuse block; you may have a whine that changes pitch with vehicle RPM. This is interference from the vehicle's alternator and/or ignition system. This does not seem to occur in all installations, but when it does; it is typically solved by powering the radio directly from the vehicles battery.



Your Antenna


Now that you have your radio mounted and powered you need to decide on your antenna and its mounting location. Firestik II antennas are popular and I would recommend that whichever antenna you choose make sure it has a tunable tip. Those antennas that do not have a tunable tip require that you trim the end of the antenna when tuning the SWR.

When choosing which antenna you want to run you also need to take into consideration where the antenna will be mounted as the length of the antenna will need to be determined based on this mounting location. Ultimately you want as much of the antenna above the top of the vehicle as possible. Depending on the terrain you off road in it may not be acceptable to have the entire antenna above your vehicle, but the more the better. Unfortunately anything less than a 102" antenna is a compromise and most people can't or won't run that long of an antenna. The shorter the antenna the more of a compromise and the harder to get a reasonable SWR. I typically recommend at least a 4' antenna. I have checked/set SWR on many Jeeps and on most of the rigs running a 2-3' antenna, even when properly mounted could not get a SWR below about 2.5-3.0+. A reading on the higher end of that scale could potentially damage your radio. After checking everything else and making sure the mount is properly grounded and the coax is good, I typically will put my 102" whip on in place of their 2-3' antenna and the SWR will immediately drop to 1.1-1.3. After the owner replaces their antenna with a 4' or longer model the SWR drops to within an acceptable range.

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The mount you use needs to have a good ground with the body of the vehicle as the body of the vehicle acts as half of the antenna or it can be looked at as a "reflector" to maybe make it easier to understand. There are mounts on the market that are made to mount to the tailgate/hatch or spare tire carrier. This is not the best option as the tailgate/hatch and spare tire carriers are isolated from the body of the vehicle and require additional grounding straps to work properly. Hoods and fenders are slightly better options but are still likely to require grounding straps. A lot of the aftermarket mounts are painted or coated, this prevents a good ground to the body and though it looks good it requires you to scrap the paint/coating off before mounting. Grounding of the mount to the body of the vehicle is required to have a properly tuned and operational radio system. Typical good mounting locations are on the rear of the vehicle are near/above the taillight as the mount is high enough to allow the antenna to clear the top of the vehicle, make a good ground with the body, and the mount to be protected from trail damage. It is also recommended to use a spring to allow the antenna to absorb impacts it may receive on the trail. It may also allow the antenna to be pulled over so that it may clear low entrances such as your home garage or parking garage. They also make quick-disconnects for the antenna and mounts to allow the antenna to lay over.

Now that the radio is mounted and the antenna is mounted we need to connect them and check the SWR.



Coax and SWR

Your radio and antenna are now mounted, but they need to be connected. This is done with RG 58 coax with PL 259 connectors. You may have been told that you need 18' (½ wavelength) of coax or your SWR will be off, this is nothing more than a myth. You only need enough coax to go from the radio to the antenna; there is no magic number for the coax needed in CB (11 meter) radio. Also if you wanted to get technical the velocity of the radio signal is slowed as it passes through the coax. RG 58 has a VF (velocity factor) of .66. This means that the signal would actually complete its ½ wavelength in 11.88' rather than 18', so even 18' of coax is too long if you follow the half wavelength requirement for the coax. Again, you only need enough coax to connect the radio to the antenna, whether that is 2'....10'.....or 18' it won't make any difference. . If a ½ wave length of coax was required and I wanted to operate my amateur radio on 40 meters while mobile I would have to have about 70' of coax in my Jeep.

SWR Meter
http://www.c4x4.com/photos/2014//302-PDC2_medlrg.jpg

You may also be told that if you do have excess coax to not coil it up as it will also hurt your SWR. This is also just a myth. In amateur radio a coil of coax is called a "RF choke" and is used to keep stray RF (Radio Frequency) from traveling down the shield of the coax back to the radio. This will not hurt your signal or your SWR. What will hurt your set up is if the coil is too tight causing damage to the coax outer sheath, inner conductor, or isolator. Generally no less than a 10" diameter coil is fine.


Now that you have your radio and antenna mounted and coax run it is time to check your SWR. I have mentioned SWR many times over the last couple of articles. SWR is Standing Wave Ratio and is basically a measurement of how efficient your antenna set up is. You use a SWR meter which measures the amount of signal being returned from the antenna versus the signal being sent to the antenna. The less signal being reflected the better. Optimum measurement is 1.0:1 but really anything less than 2.0:1 is ok. The closer you get to 3.0:1 the more you risk damaging your radio because too much signal is being sent back to the radio. You will take measurements on channel 1 and channel 40. If your reading is higher on 40 than 1, your antenna is too long and you will need to adjust the tip in. If your reading is higher on 1 than 40 your antenna is too short and your tip will need to be adjusted out. This is why I recommend the tunable tip antenna. If you get a high reading (near 3.0:1) on channel 1 and 40 you likely do not have a good ground connection for your antenna mount or you are running a very short antenna or in some cases you could have a short in your coax. If your antenna is next to the side of your vehicle, depending on what type of vehicle you have the upper side of the vehicle (metal) could cause the signal to be reflected back at the antenna causing a high SWR. This is why I mentioned running the longest antenna you can to get as much of the antenna above the vehicle as possible. A cheap compact CB radio with a great antenna setup will work just as good if not better than a high dollar radio with a crappy antenna setup. For example, my Cobra 19 with a 102" whip on the back of my XJ can talk around 10 miles in the flat land. It could do better if the antenna was mounted to the top of my XJ rather than above the taillight. I understand not everyone can or wants to run an antenna that long, but realize that as the antenna gets shorter it is a compromise in performance.

Terrain has a lot to do with the distance you are able to talk. Flat land or hill top to hill top is the best scenario. Objects such as foliage, buildings, hills and mountains will also minimize your signal. With a typically off road vehicle setup, running a shorter antenna partially blocked by the vehicle don't be surprised if you can only communicate a couple of miles, possibly up to 5 in the best conditions. CB is a good short distance band but the FCC did not intend for it to be a long distance band, hence the limited radios. As I have said before, there are better options for long distance communication both in the woods and around town (Amateur Radio)

I hope you have enjoyed this series and it has been somewhat educational for those wanting to put a CB in their vehicle to keep in contact with their buddies on the trail.