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Exposing the 18' Coax Myth
 

As many of you know, I am rather outspoken when it comes to mobile communications, especially when it concerns those found in off-road vehicles....and more specifically, a Jeep.  I've been associated with RF electronics & communications since 1971 as both a hobby and a means to make a living.  For almost 15 years, it put food on the table and a roof over my family's head.  Today, I push a keyboard and pound out code for a living, but I still keep myself entertained with my ham radio hobby.  I still maintain an assortment of beam and vertical antennas at my house that are fed from a variety of HF, VHF, and UHF radios.  In my TJ, I can cover any legitimate ham frequency from 7.0 Mhz to 450 Mhz, using CW, SSB, AM, or FM modes of operation.  By no means do I claim to be an RF engineer or an expert in the field.  I do know something about it though, as I taught electronic communication students for a college for almost 5 years. So....what is this all leading up to.....its about the urban legend and wives tales that surrounds the process of adjusting (or tuning) a vertical antenna, specifically, a 1/4 wavelength CB vertical antenna designed for mobile use.

About once a week, I'll read a thread on the off-road related forums that boldly state that you need 18' of coaxial cable between your CB radio and the antenna so it will tune up properly.  Every time I read that, I respond that the proper length of coaxial cable needed is that length that is just long enough to connect the radio and antenna together.  In other words, there is no special length needed, no magic measurement that you buy so your radio will work better.

When I respond with my comments about an 18' cable being unnecessary, I usually get remarks such as "Well, my friend has an 18' cable on his radio and it works great.  Before he replaced the old cable with the new 18' long piece, he couldn't get his radio to work worth beans."  (or something similar to that....I think you get the point).  Of course, what the person may not be realizing is that the piece of cable that was replaced was more than likely damaged or the connector(s) was bad or loose and any new piece of cable would be a great improvement over what it replaced.

The response I like any better is "Well, the XXXXX  website FAQ says you are suppose to use an 18' long cable.  They should know what they are talking about...they are, after all, the experts."  To that I usually reply that about 50% of the information contained in the FAQ is BS and that which isn't is often times a combination of half truths and misinformation.  Now before everyone gets their underwear in a bunch, let me say that XXXXX makes a good antenna, in my opinion.  There is nothing wrong with their product.  I just have a problem with the bulk of their "facts"...since it would appear that they are presented in such a manner as to support the sale of their products.  As for me, you should understand I am not here to sell you anything and the only thing I have to gain from this is the satisfaction that you are not dumb enough to fall for their sweet sounding line of dribble.

So....how am I going to convince you that the "Experts" are wrong and I am right?  Well, like I said before....most of the data contained in the FAQ is either wrong or mostly incorrect.  I trust that you will be wise enough to realize that when a company is caught up in their own misinformation, it doesn't lend much to their creditability and you should be able to see who is blowing smoke.   I'll also state now that all of the information and formulas that I will use are taken directly from the ARRL Antenna Handbook, 17th Edition.  (It has been around for a few years, about 55 of them when the 17th edition was printed in 1994, and is an excellent reference.)

Before I show you the proof, you need to understand a couple of very basic RF facts. They have to do with measuring the length of a radio signal as it travels through space, which is referred to as wavelength.  You don't have to understand how to calculate wavelength.  It is important for you to remember the wavelength values since we will need a couple of these numbers later on.  Here are the facts that we will use:

1.  The frequency of our CB signal is 27.4 MHz (somewhere around the middle of the CB band).

2.  The wavelength (in feet) of our CB signal is 36 feet.

3.  A half wavelength is 18' and a quarter wavelength is 9' (or 108").

4.  The CB signal is an RF sine wave, which rises above and drops below a zero reference point once each full cycle (or a complete wavelength).

You should recognize most of these numbers.....especially the 108" value, which just happens to the the length of the ever popular 108" steel whip antenna, made very popular during the CB craze of the 70's.  And don't forget that 18' is a half wavelength for our CB signal.

The other concept that must be understood is that when radio signals travel through coaxial cable, they slow down a bit.  They no longer can travel at the speed of light as they do when traveling through the air.  How much they slow down is determined by the type of coaxial cable being used (yes, there are dozens of different types of coax cable....luckily we only use a couple of common ones for CB radios).  You don't have to know why or how this all works....just the fact that it does happen.  So that RF engineers and technicians can accurately determine cable lengths when doing radio related work, the manufacturers provide a value that is know as velocity factor, or VF for short.  The VF is used as a "correction factor" and is applied to the cable measurement so that the person building the cable assembly can physically determine the length based on the unique properties of the coaxial cable being used.  This is important because wavelength is dependent on the frequency of our radio signal so it changes as does the frequency.  VF for a perfect cable is a 1.0, and since we have no perfect cables in the world, the VF is always less than 1.  The cables commonly used for CB applications have a VF range from about .6 to .8 or there abouts.

The following can be found on the XXXXX  FAQ concerning coaxial cable length.   It says:

"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."

I find it interesting that they actually admit that the "engineering types" argue that they are wrong and that a properly installed system can use any length of coax desired.  Well, at least that part is 100% true.

The part of the quote that I highlighted in green is one of those half truths and misinformation parts that I mentioned at the beginning of this article.  One thing is very true....at 18' the voltage curve is in fact zero (remember that 18' is a 1/2 wavelength) in a perfect cable cut for our CB frequency.  The typical vertical antenna installation, such as we use on our Jeeps, have a zero RF voltage point at the base of the antenna.  This zero voltage point will be repeated again one half wavelength down the cable.  If you remember RF fact #4 from above, I said that our signal is a sinewave and that it goes positive and negative once during each full cycle of travel.  Since our signal is positive for 1/2 of the cycle and negative for the other 1/2 of the cycle, we can see that the it crosses the zero point (changing from positive to negative or vice versa) half way through the cycle.  Another way to say this is that the zero voltage is at the half wavelength point.

So, the FAQ is correct when they say "At 18' the voltage curve has dropped back to the zero voltage point".  After that comes some interesting stuff, "which reduces the reactance within the cable itself (a null cable if you would)".  Sorry folks, there is no such thing.  The reactance is constant along the length of an RF transmission line (ie., a coaxial cable).  The cable can be diagrammed using repeating values of series inductance and parallel capacitance.  My god....if the reactance within the cable were reduced, you would no longer have the same kind of a cable that you started with. 

But wait, I am getting off track here,  it gets a lot better real fast.....it's that 18' part.  Now...do we remember that little old concept called VF (velocity factor).  That is the property that all coaxial cables have because we don't have any perfect cables in the world.  VF causes the RF signal to move through the cable slower, which means the physical length of cable that equals a wavelength is actually less than the length if it were traveling through a perfect conductor. 

On the FAQ, they recommend using either RG-58 or RG-8X coax cable (which I agree is great for the job).   If we look at chapter 24, page 17 of the Antenna Handbook that I referred to above, we find that RG-58 cable has a VF of .66 and that RG-8X has a VF of .75 (yes, the 8X is better cable, it is closer to the perfect conductor).  The FAQ (same page as above) also recommends that we use RG-58 coax cable. 

So....how much cable do we need in order to get our "important" zero voltage point that they say is needed....well, let's figure it out.  We know that out 27.4 Mhz CB signal has a wavelength of 36'.  So, a half wavelength for our frequency is 18'.  Hey, that is just what the FAQ says we should be using!

OOPS......it seems they forgot a rather important fact....it's that pesky velocity factor.  So we multiply 18' times a VF of .66 (per the formula on page 24-3 of the Antenna Handbook) to determine the physical length of our coaxial cable, now corrected for the velocity factor.  The result is 11.9'.

WOOPS.....wait a second, we are suppose to be using 18' of coax because this will give us the zero reference point that will make everything work better.

The FAQ said:  "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." 

Remember, they say it is that 18' length that gives the zero voltage point (that makes that null cable, so to speak).  So...by their own measurement, they are actually adding to the problem because you and I both know that we really need 11.9' of RG-58 cable in order to have the zero voltage point that will makes things work better.

OK....now, they can't have it both ways....either they admit that they are using a cable that is 50% too long (18' vs. 12'), or they admit that this zero voltage point has nothing to do with tuning a CB antenna for minimum SWR.  No matter how you compute the numbers, their 18' cable will not produce a zero voltage point at any CB radio frequency.

Honestly speaking, they are wrong all the way around....first, you don't need a coax cable to be 1/2 wavelength long in order to have your radio and antenna work properly together.  Secondly, an 18' long cable is not even close to being  a 1/2 wavelength at our CB frequency.

But hey....you have to admit that all of that info in their FAQ is pretty darn impressive....it makes for great reading and these guys are EXPERTS....except that their own data conflicts with their own information (or should that be misinformation).  Why could this be?  Well, when you mix bogus info and half truths and misinformation together, you get a bunch of gibberish that the average person sucks up like a cold drink on a really hot day.

Now, I will say it again.....they are in the business to sell their antennas and coaxial cable.....and if you think they are here for anything else, you are more of a guppy than I thought you were.  I know that some of you will not believe what I have written, and that is fine.  I challenge you to pick up any recognized RF text or industry RF handbook and prove me wrong.   I hope the rest of you will stop spreading this bogus information about needing 18' of cable.  The average CB install on a short wheel base Jeep needs about 10' or so of cable.  I wish I had a business where I could sell you about twice as much of what you really needed....and make you feel good about me taking your hard earned money....so good in fact that you actually will tell other people that they too need to buy my magic cables when they come to my store.  Yes sir....they have a really slick pitch for selling you something that you don't need.

I hope you have learned something from this article.  It was fun digging out the old reference books and cranking the numbers through the calculator again.  

There are other myths in mobile communications, as well as myths in the computer industry. A common misconception about CMMS is that generic software will be effective in any company or industry. Implementation of CMMS software should be specialized for the particular company or industry to achieve the best results.

 

 

 

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