Extra Coax Coil

Here we go again... I hang out with guys that are antenna engineers and I have yet to figure out where this 18' crap comes from. As far as I have found out and read, you should only need the minimum amount needed to make the run because coaxial cable is SHIELDED. Think about this for a min. If I run 50 feet of cable to a resonant antenna on a tower, or 100 feet, the match will be the same. The longer run will have more loss of course... This is another rumor that comes from CB hack shops, the same goofs that tell guys to tilt their antennas forward. I have actually spoken with the engineer that designed the Wilson 1000 and 5000 antennas about a lot of the nonsense that floats around.

 

This is an excerpt from a page that describes the characteristics of coaxial cable. Most of this jargon is over the average person's head, but you can see that its stated that length has nothing to do with impedance.

Coaxial cables


Introduction to coaxial cables


A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded and the outer conductor is typically a shield that might be braided or a foil.
Coaxial cable is a cable type used to carry radio signals, [COLOR=blue !important][COLOR=blue !important]video [COLOR=blue !important]signals[/COLOR][/COLOR][/COLOR], measurement signals and data signals. Coaxial cables exists because we can't run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Coaxial cable consists of an insulated ceter conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement give quite good shielding agains noise from outside cable, keeps the signal well inside the cable and keeps cable characteristics stable.
Coaxial cables and systems connected to them are not ideal. There is always some signal radiating from coaxial cable. Hence, the outer conductor also functions as a shield to reduce coupling of the signal into adjacent wiring. More shield coverage means less radiation of energy (but it does not necessarily mean less signal attenuation).
Coaxial cable are typically characterized with the impedance and cable loss. The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors. The characteristic impedance of a cable (Zo) is determined by the formula 138 log b/a, where b represents the inside diameter of the outer conductor (read: shield or braid), and a represents the outside diameter of the inner conductor.
Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms. 50 ohms cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet). 75 ohms coaxial cable is used to carry video signals, TV antenna signals and digital audio signals. There are also other impedances in use in some special [COLOR=blue !important][COLOR=blue !important]applications[/COLOR][/COLOR] (for example 93 ohms). It is possible to build cables at other impedances, but those mentioned earlier are the standard ones that are easy to get. It is usually no point in trying to get something very little different for some marginal benefit, because standard cables are easy to get, cheap and generally very good. Different impedances have different characteristics. For maximum power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93 Ohms cable gives low capacitance per foot. It is practically very hard to find any coaxial cables with impedance much higher than that.

Here is a quick overview of common coaxial cable impedances and their main uses:

• 50 ohms: 50 ohms coaxial cable is very widely used with radio transmitter applications. It is used here because it matches nicely to many common transmitter antenna types, can quite easily handle high transmitter power and is traditionally used in this type of applications (transmitters are generally matched to 50 ohms impedance). In addition to this 50 ohm coaxial cable can be found on coaxial Ethernet networks, electronics laboratory interconnection (foe example high frequency oscilloscope probe cables) and high frequency [COLOR=blue !important][COLOR=blue !important]digital[/COLOR][/COLOR] applications (fe example ECL and PECL logic matches nicely to 50 ohms cable). Commonly used 50 Ohm constructions include RG-8 and RG-58.
• 60 Ohms: Europe chose 60 ohms for radio applications around 1950s. It was used in both transmitting applications and antenna networks. The use of this cable has been pretty much phased out, and nowdays RF system in Europe use either 50 ohms or 75 ohms cable depending on the application.
• 75 ohms: The characteristic impedance 75 ohms is an international standard, based on optimizing the design of long distance coaxial cables. 75 ohms video cable is the coaxial cable type widely used in video, audio and telecommunications applications. Generally all baseband video applications that use coaxial cable (both analogue and digital) are matched for 75 ohm impedance cable. Also RF video signal systems like antenna signal distribution networks in houses and cable TV systems are built from 75 ohms coaxial cable (those applications use very low loss cable types). In audio world digital audio (S/PDIF and coaxial AES/EBU) uses 75 ohms coaxial cable, as well as radio receiver connections at home and in car. In addition to this some telecom applications (for example some E1 links) use 75 ohms coaxial cable. 75 Ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 Ohms gives the lowest loss. For 75 Ohm use common cables are RG-6, RG-11 and RG-59.
• 93 Ohms: This is not much used nowadays. 93 ohms was once used for short runs such as the connection between computers and their monitors because of low capacitance per foot which would reduce the loading on circuits and allow longer cable runs. In addition thsi was used in some digital commication systems (IBM 3270 terminal networks) and some early LAN systems.

The characteristic impedance of a coaxial cable is determined by the relation of outer conductor diameter to inner conductor diameter and by the dielectric constant of the insulation. The impednage of the coaxial cable chanes soemwhat with the frequency. Impedance changes with frequency until resitance is a minor effect and until dielectric dielectric constant is table. Where it levels out is the "characteristic impedance". The freqnency where the impedance matches to the characteristic impedance varies somwehat between different cables, but this generally happens at frequency range of around 100 kHz (can vary).
Essential properties of coaxial cables are their characteristic impedance and its regularity, their attenuation as well as their behaviour concerning the electrical separation of cable and environment, i.e. their screening efficiency. In applications where the cable is used to supply voltage for active components in the cabling system, the DC resistance has significance. Also the cable velocity information is needed on some applications. The coaxial cable velocity of propagation is defined by the velocity of the dielectric. It is expressed in percents of speed of light. Here is some data of come common coaxial cable insulation materials and their velocities: Polyethylene (PE) 66%Teflon 70%Foam 78..86% Return loss is one number which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and [COLOR=blue !important][COLOR=blue !important]installation[/COLOR][/COLOR] defects (cable damaged on [COLOR=blue !important][COLOR=blue !important]installation[/COLOR][/COLOR]). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB. Return loss is same thing as VSWR term used in radio world, only expressed differently (15 dB return loss = 1.43:1 VSWR, 23 dB return loss = 1.15:1 VSWR etc.).

http://www.epanorama.net/documents/wiring/coaxcable.html

 

Located at: http://www.stu-offroad.com/misc/myth-1.htm is a good article about the myth of the 18' coax length.

I understand Firestik in their writings claim the 18' foot coax is correct. But as it is clearly shown they are wrong. One has wonder if they don't understand something as simple as coax length what else don't they understand.

One of the important facts in this article is the velocity factor of coax. As you see this changes the cable length. So the 18' myth can not be correct.

Now there is parts of the article (both this article and the quoted material) about the "null cable" and it's description that I question it doesn't affect the 18' myth.

Leon
(kc0iv)

 

Zachary,
I thought this had been covered in a previous thread. What Leon and Delta5 said are correct. As long as the impedance of the line matches the impedance of the load and the impedance that the radio device expects to "see" (as "output impedance" is somewhat of a misnomer) then any length of coax is fine. The 18-foot thing came about from specific antenna arrays (more than one antenna element, all fed together) that need impedance matching transformers of one kind or another. The most common form is the 18-foot chunks of 75-ohm coax that's used on co-phased antenna pairs. In that case, the two 18-foot lines are length-dependent to do their job, but at their radio end, they present close to a 50-ohm load. Truth is, you could put any random length of 50-ohm coax on at that point and still be matched, as long as the co-phase harness is not disturbed.
As has been pointed out, the coax is shielded, so nothing you can do, short of bending it so sharply that the distance between the inner and outer conductors changes, will harm the signal going in either direction through it. The only reason to leave it at its original length is so you can change your mind later about the position of the radio or antenna, or if you aren't comfortable putting on a new connector after you cut the coax. I always leave a foot or two of spare to allow for ease of inserting an inline wattmeter after installation, or to be able to cut off and re-attach new connectors if people wear them out by poor installation or frequent coupling and uncoupling.
Cut it or shape it how you like; just don't kink or crush it.
-- Handlebar --

 

Zachary140, you have pretty much the right idea but not quite correct. Since coiling the excess cable into a loop will act like a choke with its own impedence, I suggest that you gather the excess coax in the shape of a "Figure 8". You are correct about making the excess cable into a length of about 12 inches. I prefer to secure it with any kind of tape.
.
The urban myth about requiring 18 feet of coax cable really needs to be put to rest. I believe it started with the idea that 18 feet of coax will be enough to connect the radio to the antenna no matter where you put the antenna on a full sized car.
.
Let me compare RG 8 to RG 58. The advantage to RG 8 is better shielding and it gets a higher percentage of the signal to the antenna. At 100 feet, RG 8 gets 78% of the signal to the antenna. It loses 0.22% of the signal per foot of length.
.
RG 58 is less expensive and more flexible than RG 8. At 100 feet RG 58 gets 66% of the signal to the antenna. It loses 0.34% of the signal per foot of length.
.
.18 feet of RG 8 gets 96.04 % of the signal from the radio to the antenna.
.12 feet of RG 58 gets 95.92% of the signal from the radio to the antenna.
The difference between 18 feet of RG 8 and 12 feet of RG 58 is 0.12%.
By using a shorter length of RG 58, you get equivalent performance to RG 8 coax at a cheaper price. This is an easy way to save some money.

 

Listen to "Handlebar", "Delta" and others that tell you that "coax length" is a CB myth and a bunch of hooey. This comes from CB, LIVES on CB, and seems to forever gain traction from those of us who simply do not know what they are talking about! It-is-not-necessary to cut, fold, coil coax for ANY CB radio to work. There ARE some antenna systems who require the coax to be a "certain" length by their design, OR because there is not sufficient ground, or counterpoise, for the antenna to work. So long as you have sufficient metal (or ground) to work against, simply CUT the coax to whatever length you need to get to the antenna, and TUNE/TRIM the antenna itself to get proper SWR.

Visit the ham websites like QRZ and see that there is little concern over "coax length". That's because it is not a real issue.

Yes, there IS more to the field of antenna tuning/resonating, but at 27 MHZ, much of the fiddling, impedance matching, gammas, coils, and stubs aren't necessary. Don't get all tangled up in figuring out stuff that simply does not need figuring out! It ain't worth it!


GF

 

JJD shared this calculator before, but I like it.

Coax length calculator.

I've got to say though, soldering PL259s isn't all that easy for someone that's not good at soldering. I've melted the insulator on a couple, so I would avoid cutting the coax and soldering on a new one unless it's just absolutely necessary.

 

If one wants to cut coax, by all means, do it. It is a "placebo" that makes one's meter see what they want it to see. I can disprove this misplaced, misguided "theory" with the following.

RF transmission theory is the same for ALL types of radio---with a few exceptions. CB is not some special case that requires us to do anything different from these other radio services. CB deals with ONE band, one set of frequencies, and one single-band antenna to cover all of it. Let's take ham radio for a disparate example since it must, by its nature, cover a huge range of frequencies, particularly where it concerns HF (which includes CB). One can set up ONE antenna to cover ONE band, or he can set up an antenna for MULTIPLE bands of frequencies. OR he can use what is commonly called a "tuner" that forces a single-band antenna to operate (sorta) on a wide range of frequencies. It is, however, a compromise solution that sometimes works well.....sometimes not because there IS no device that "tunes" an antenna unless it can physically change the length and matching of same; you'll still have a mismatched antenna hanging out there with varying degrees of results.

Around 1991, a Mr Don Johnson, a ham, invented a mobile HF antenna that used a handheld screwdriver, motor-driven, to move a loading coil up or down inside a hollow mast surrounded by "fingerstock", springy, metal "fingers" of metal that contacted the coil at all times. This revolutionized High Frequency mobile almost overnight and made it very popular again. That was because one could press a button on the dash inside the car, move this coil UP or down as needed, and "set" the antenna to be fully resonant, have a great SWR, and be ready to operate at VERY high efficiency on ANY HF frequency in seconds--all while cruising at 65 per down the road. Shortly thereafter, people came up with auto-positioners for their antennas, and all they had to do was set the frequency in the radio, and the antenna would move to the right frequency with a 1.2 SWR with NO intervention from the owner! This frequency range was 3.5 thru 30 MHZ-----and some of them would work 52 MHZ (6 Meters).

Now let's go back to this CB "coax length" myth. There is NO magic formula that applies to CB radio to the exclusion of all other, so this means that if the fake antenna "gurus" are right, I now have to have 120 feet of coax in my car to work 3.5-3.9 megs, right? Now, I want to work 7.255 MHZ, so now I've gotta have 64 feet for that............. Then, for 14.300, I've got to have 32 feet.............. Then's there 25 feet for 18 MHZ................... And 22 feet for 21 MHZ. And 18 feet (according to the CB "experts"-----and actually, it would be 17.3333333, if you want to be "technical" about it, and then I'd have to have another 16 feet for 29 MHZ!!!!!!

SO! If all this hooey were true, I'd have to have over 10 lengths of coax, ALL of different cuts, for this multiband antenna to WORK!!!!! Where would I COIL 120 feet, 60 feet, 30 feet, 17 feet and 15-16 feet of coax in a small pickup??????????? Yet there THOUSANDS of ham, military, and commercial mobiles operating with ONE antenna, covering THOUSANDS of HF frequencies, doing it at reasonably good efficiency, and doing it with ONE coax cable cut to NO particular length except that which is required to REACH to the radio!!!!!!!!!!! They did NOT use "coax length calculators" or CB radio "experts" to arrive at their "right length". I know I'm being a bit of a smart-### here, but it is simply to make people see it for what it is! HOOEY and PHOOEY!!!! If all this hocus pocus were true, I could not have worked around the entire WORLD with ONE coax and one multiband antenna without having to trip over all that coax!!!!!!!!


The bottom line is: If you WANT to cut coax to a "certain" length. Do it. If you THINK you are getting some sort of advantage out of it, CUT it. Truth is, you are wasting your time unless there is a specific application that requires it. All that CB theory stuff is mostly HORSE-S### put out there to make some CBers feel like they are more knowledgeable than others. You rarely hear this stuff anywhere else except on CB forums.

GF

PS. How did this myth get started? It appears that in the old days, most cars were around 20 feet long. It took 18 feet to get from the radio to the antenna. People began to believe that, since that was what was in the package, it was the right length of coax and was required. This really got started in the 70's when CB really took off. Before that, you didn't hear that bull---- too much. Once it got started, it, like so many other myths and legends in CB, it was a fire that won't ever be quenched.

 

Baw-Haw-Haw....worked for me!