Basic Radio Lesson #1 From Turtle - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 09-15-2000, 09:15 AM
Posts: n/a
Basic Radio Lesson #1 From Turtle

I don't know who started the coax length myth, but it was sometime in the 70's during the CB craze that I first started hearing it being passed around as gospel.

Basic Rule: If the radio, the transmission line (coax), and the antenna are all the same impedance, then the length of coax has no effect on the matching to the antenna.

The only effect length has in the above case is attenuation. But for a car at CB frequencies, you would be hard pressed to measure the difference of a few feet of coax, although in theory and reality there is a little more loss for the longer coax. If you were operating at a few hundred megahertz, a few feet more might be a big problem. For practical purposes, at 27 MHz, a few feet is nothing.

A quarter wave whip (which is what 95% of CB antennas are...some are 5/8 wavelength for bases, but we won't go into that now) has an impedance of something around 50 ohms. Some whips are physically 1/4 wavelength (around 108" for CB if I recall correctly), and some are physically shorter but have inductors or coils inserted to make them appear electrically 1/4 wavelength long. The CB can't tell the difference. CB radios have an output and input (for transmit and receive) impedance of around 50 ohms. Coax cables have a characteristic impedance of around 50 ohms. The length of the coax does not matter and is not a factor in fixing an SWR problem. I say impedances are "around" because nothing is exact, especially with commercial CB products. But they are pretty close, and close is good enough.

There are a couple of cases where coax length does matter. First is a special case where the antenna and radio are substancially different impedances (never the case in CB). A quarter wave length of yet another impedance of coax can be used to match the antenna to the radio. This calculation is not too difficult for an electrical engineer, but you would never accomplish this with a rule of thumb. It is a complex calculation that is exact. And now, not only is the antenna frequency sensitive, but also the transmission line impedance transformer.

Also in the case where multiple antennas are fed by a single radio to provide directionality, the length of the coax line matters because it sets the phase between the two antenna elements, and hence antenna pattern. By the way, two CB antennas mounted on the mirrors of the big rigs never work any better than a single antenna properly tuned. In order to achieve any noticable gain over a single element, the two antennas must be separated by a distance that is large compared with the wavelength of the operating frequency. Mirror to mirror is not even close at CB frequencies.

So the next time someone tells you to adjust the length of coax to fix a CB problem, unless you have a hundred feet coiled up in the trunk, don't listen. It is not going to make any difference. Go look somewhere else.

If anyone wants a similar explanation of what SWR is and what affects SWR, and what is adequate SWR in the real world of antennas mounted on vehicles, I would be glad to provide that info in the future.

If I were not sure, I would not post. Or at least there would be a caution that I was not sure. Meanwhile, I stand by my position that I am not trying to flame anyone, I simply am providing technically correct info on the subject.

Have a great day, and may your Jeeps stay upright.


"Pleased to meet you, hope you guess my name."
Sponsored Links
post #2 of (permalink) Old 09-15-2000, 09:48 AM
Posts: n/a
Re: Basic Radio Lesson #1 From Turtle


When you say that a "few feet" doesn't matter in cable length, when DOES it matter? How much can I cut off before I'll have a problem?

Also, who's making good CB's nowadays? Where's the best place to mount the wip?


post #3 of (permalink) Old 09-15-2000, 09:56 AM
Posts: n/a
Re: Basic Radio Lesson #1 From Turtle

Is it true that you are never supposed to cut the antenna cable on a magnetic mount antenna and then re-splice? I have heard this and it sounds like a myth. Can you shed any light on this?


I know what you're thinking. Did he fire six shots or only five?
Sponsored Links
post #4 of (permalink) Old 09-15-2000, 10:57 AM
Posts: n/a
Re: Basic Radio Lesson #1 From Turtle

While I wont disagree with Turtle, I will offer up a little different view. I borrowed this from Firestiks website:

When it comes to coax selection
it is often "Buyer Beware"

The coaxial cable running from your radio to the antenna is unbelievably important. Everything you transmit and receive must travel along its length. All too often the coax is ignored and performance suffers because of the lack of attention it receives. The fact remains, when it comes to buying coaxial cable, bulk or in assemblies, quality is everything!

Although there are many types of communications grade coaxial cables on the market, this discussion will primarily focus on those used in mobile communications applications. Before starting, a few terms and/or phrases need to be clarified.

Coaxial: two conductors sharing the same center
Center Conductor: the wire at the very center of the cable
Insulator: the material surrounding the center conductor
Shield: the outer conductor surrounding the insulator
Jacket: the outer covering of the cable
Propagation velocity: speed of signal traveling in coax

For mobile installations, there are three (3) primary types of coaxial cable used to build the assemblies.

RG-58 type: This type of coax is used for single antenna installations and for jumper wires that go between the radio and a test meter (SWR meter). The RF resistance of this type of coax is 50-ohms. Within this group you will find coax labeled with RG-58 or RG-58/U. These cables have a solid center conductor. The second type is RG-58A/U and they have a center conductor made up of many thin wire strands (normally about 17) that are twisted together to form the center conductor. The common outer diameter for this type of cable is about 0.20".

RG-8X: This type of coax is also used for single antenna installations or jumpers between pieces of equipment. Like the RG-58 type coax, RG-8X also provides 50-ohms of resistance. In short, this cable could be called hi-performance 50-ohm cable. It always has a stranded center conductor and a high shielding percentage. It will also handle higher power (wattage) and has a higher propagation velocity. For the general user, it is more than what is required. However, if you are using amplifiers or just like to get the absolute most from your set-up, it will deliver. The outer diameter of RG-8X is typically about 0.24".

RG-59 type: This type of coax is used for dual antenna installations only. The RF resistance of this type of coax is 72-ohms. Within this group you will find coax labeled with RG-59 or RG-59/U. These cables have a solid center conductor. The second type is RG-59A/U and they have a center conductor made up of many thin wire strands (normally about 19) that are twisted together to form the center conductor. The common outer diameter for this type of cable is about 0.22".

About the center conductor: We strongly believe that mobile installation should always use stranded center conductors. The reason behind this is due to potential breakage of solid conductors due to vibration and/or repetitive flexing. If you have ever picked up a piece of wire and bent it repeatedly until it broke, you would fully understand our reason for recommending stranded center conductors on mobile installations.

About insulation: There are two common types of insulating material used in coax. First there are the foam (polyfoam) types. Although most specifications sheets show that polyfoam insulated coax has a faster propagation velocity, we do not recommend it for mobile installations. We prefer the plastic types (polyvinyl, polypropelene, etc.) because they are just plain tougher. The small loss in velocity, for all intents and purposes, is insignificant insofar as low power, low frequency communications are concerned. The properties of coax cable changes if the center conductor is not in the physical center. Polyfoam insulation deteriorates faster than the plastic types and also tends to collapse easier than the plastics when pinched or sharply bent

About shielding: The shield surrounds the center conductor and prevents internal leakage and external interference. The typical shield used on two-way radio communication cables is a woven braid. For the most part, it is formed with either bare or tinned copper wire and is a very important consideration when trying to determine the quality of the coax. Low percent coax has a loose braid and exposes more of the center conductor to leakage. Unfortunately, the cable industry did not invent a coding system that designates the type and percentage of shielding used to build the coax. Shielding percentage is the most abused part of the coaxial cable manufacturing process. There was a time when 70% coverage was considered the absolute minimum, but we have seen cheap cable with as little as 58% shielding being sold in recent years. Because it is readily available, we do not recommend coax cables with shielding less than 90%. Using anything less is the equivalent of watering your lawn with a hose that is full of holes and has a restricted opening. You must exercise caution when it comes to shielding.

About the length: This is a testy subject with many engineering types. They have argued with us on many occasions regarding this matter. They say that if your system is set-up properly that the length of the coax is irrelevant. We agree! However, mobile installations have so many variables that a perfect set-up is the exception, not the rule. One guy has a pick-up and another has a fiberglass motorhome. One wants the antenna on the bumper, another on the hood and a third on the roof. Few people want to drill holes in their vehicle so quality grounds are always a consideration. Because of the imperfect world, we almost always recommend 18' (5.5m) when our products are used. We do so with good reason too! At 18' the voltage curve has dropped back to the zero voltage point where the cable meets the antenna which reduces the reactance within the cable itself (a null cable if you would). It has been our experience that if the antenna location makes it somewhat out of sync with its surroundings, cable lengths that are not multiples of our 18' suggestion adds to the problem. To that, our complaining experts say, "Then the antenna should be moved!" to which we say, "You tell the guy with the $30,000 vehicle that he must drill a hole in his roof so he can use a 9' cable". We solve problems in the best way we can given the boundaries that the customer establishes.

On that note, when you have 18' of coax going to a radio that is only 8' away, what should you do with the other 10'? We recommend that you serpentine it like a skein of yarn so that it is 10-14" long and tie it in the center with a wire tie then tuck it away. Do not roll it up in a tight circle as this can cause it to act like an RF choke, which often times will cause system problems.

Other ways to wreck your coax: Wear holes through it, slam it in the door a few dozen times, attempt to splice it as you would a wire going to your taillight, tie it in knots or make real sharp bends in it.

For your information, all of our Fire-Flex coaxial cables have stranded center conductors, polyvinyl insulation and bare copper shielding in the 95% range. Even though we offer some cables that are not 18', we do so for the knowledgeable installer, who knows that if a problem shows up, he must exercise the 18' recommendation. All of our mini-kits and complete kits are packed with 18' cables.

Now I also know that you must consider the coax as part of the antenna (no matter how good the shield is, it still radiates), and in that respect it does directly affect the swr. You can tune a "102" by doing this, or by adding a spring (making the antenna a "108"). So coax length can be used for tuning (I do not reccomend this) but is not practical for the novice installer.

<font color=blue> "Sometimes one pays most for the things one gets for nothing. "
-- AlbertEinstein
</font color=blue>

post #5 of (permalink) Old 09-15-2000, 11:07 AM
Join Date: Mar 2000
Posts: 4,447
Thanks: 0
Thanked 0 Times in 0 Posts
Re: Basic Radio Lesson #1 From Turtle

Most of the guys I run with have at least 250 watts of amplified power, and even with that amount of power we get noticeable difference is transmision problems when we wander from the standard practices of using 18' lengths. I notice huge losses of power if the the line between the amp and the antenna are coiled, so yes we are getting at least enough inductance to cause impedance, this is also very hard on the finals. I have never hooked up the unit to an scope and signal genarator, but I do no what works, and 10,000 truckers cant all be wrong. Ill stick with the 18' length and 3'. I would say that was not the problem though in the last post, I would think its a grounding prob. Coax length will lead to reduction in performance, of a weel tuned unit, not the almost non exhistance of out put signal. As far as buildings causing problems, it should be noticed more as a lack of reception if you are in an area where the incoming signal has been disturbed and hasnt had a chance to recover. A radio signal is like a ripple in the water, if you place your finger in the path of the ripple you will notice your disturbance of the wave, but given a little distance the wave will reastablish itself.

89 Wrangler
I take my Jeep "On the Rocks",usually "With a Splash of Addrenalin"
H8monday is offline  
post #6 of (permalink) Old 09-18-2000, 10:29 AM
Posts: n/a
Re: Basic Radio Lesson #1 From Turtle

Thanks for the input Joey, mycj5, Dan, and Jeff.

What a great demonstration of how a little info can be misused! Let's hit the 18' of coax in the head. The article provided brought up a point I omitted in my piece, although I considered including it at one time. If you keep coax cut to multiples of 1/2 wavelength, the impedance will be the same at each end, regardless of the characteristic impedance of the cable itself. For instance, if you have a 50 ohm radio, a 50 ohm antenna, theoretically you could have 300 ohm (or whatever) impedance coax and everything would be happy if cut to multiples of 1/2 wavelength. This is what the article is trying to talk about with the voltage curve. However, this won't do a thing for you if you have an antenna problem, or any situation where the radio and antenna aren't the same impedance, and hence high SWR. That is, keeping the coax to some magic length will not mask or help any other problems at all.

This would seem to make sense with the 18 foot rule for CB since 18 feet is about 1/2 wavelength in free space (468 divided by frequency in MHz). But there is a problem here, and it is called velocity of propogation. In free space, RF moves at the speed of light. But in the coax with a plastic insulator between the inner and outer conductor, the RF moves at about 70% the speed of light. Now, electrically, it only takes about 12' to equal a half wavelength. Ooops, some CB expert missed that one, or it would have been called the 12 foot rule instead.

Sorry, but it is a myth that is being passed around by people that don't totally understand what they are talking about. If the length of coax seems to affect performance, then you have a larger radio or antenna problem that needs to be solved. Cut the coax to any length that makes sense and solve the problems without regard to the magic 18', regardless of how many people say it is so. You will be better off. You will also be operating with the benefit of many years of scientifically proven understanding that some people possess, and one is trying to share with you on this forum. This is not a matter of opinion, it is proven fact.

Joey, you asked about attenuation: how much is too much. The attenuation of typical CB coax is less than 3 dB per hundred feet at CB frequencies. You lose 1/2 power with 3 dB. So if you have 5 watts transmit power, with 100 feet of coax, you would be down to 2.5 watts (if you had a full 3 dB). Or if you look at it another way, with this example, you would lose <.025 watts per foot. An extra ten feet would cost you less than a quarter of a watt. There is also a penalty on the receiver sensitivity with longer coax, and you can't fix that with a linear amp.

mycj5, you asked about mag antennas and the need to not shorten the coax. It could be possible that some manufacturer has designed a special antenna/coax combo for matching. And they would certainly be needing to warn you not to adjust coax length if that were the case. I'm not saying it isn't out there, but I haven't seen it if it is. I have adjusted mag antenna mount coax lengths with no problem at all. Logic tends to discard the idea and likelyhood of anything special. A 1/4 wave whip is easy. Why would they do anything different? It is more likely that they have used a cheap coax that is difficult to splice with normal hardware, and they are discouraging you for that reason. Like I said, it might be out there, but I haven't seen it. There is nothing magic about mag maounts technically that would lead you to that conclusion.

Any more questions? Any more myths? I hope this helps clear up a few more points. I'll be glad to go further if anything is still unclear.


"Pleased to meet you, hope you guess my name."
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Off-Road Forums & Discussion Groups forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome