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Why is it bad to coil up your coax?

Help with installing an Antenna, or just choosing the right antenna to go with your radio, or your mobile.

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Why is it bad to coil up your coax?

Post by Turbo-T » Wednesday, 14 October 2009, 5:48 AM

I seem to recall I was told why but now for the life of me cannot remember.

But I've been told it's a bad idea to coil up the excess coax inside the vehicle coming off the radio and going to the antenna....and instead you're supposed to "uncoil" it and sort of spread it around the interior, under a seat or so.

Can anyone please help me remember why?
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Re: Why is it bad to coil up your coax?

Post by alhefner » Wednesday, 14 October 2009, 6:06 AM

It isn't "always" bad to coil the coax but it can be bad. What happens when you coil coax is that you create an RF choke. This does nothing bad to your ability to transmit. I does tend to prevent "common mode" currents from traveling back down the shield of the coax into your radio and CAN make your receive a bit better. I have never actually found that an RF choke helps my CB operation at all.

What I do with extra length of coax is cut most of it off and install a new connector.



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Re: Why is it bad to coil up your coax?

Post by 'Doc » Wednesday, 14 October 2009, 9:46 AM

Just some general reinforcement.
What happens on the outside of coax doesn't really affect what happens on the inside of coax, UNLESS you deform the inside of that coax past a certain point. So, don't coil it up too tight.
If coiling up coax means a problem shows up, then you've done one of two things. Wound the coax too tightly, or just discovered a problem with something somewhere else. The best solution is to find that problem and fix it where it is occurring. That coiled up coax is just an indicator of a problem, not necessarily -the- problem.
Seeing a problem after coiling up coax is fairly common. Not because of the coiled up coax, but because 'problems' are very, very common with antenna systems because -something- else wasn't done right. Sometimes, the guy you see coil up coax then straightening it out, really isn't a big an idiot as you might think. He's just doing some checking, sort of. (Then again, some of us really are that idiotic at times. Whatever.)
If it won't cause future problems, just cut the stuff off. If you don't think you should for whatever reason, coil it up.
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 14 October 2009, 10:32 AM

Hi Turbo-T

I always recommend coiling up a coax no smaller than 12" in diameter. At 27 MHz it does not make that much difference. If you can hide the 12" coiled coax then I would leave it. Coiling around the mirror bracket very tightly has always caused problems. The higher the frequency the more the affect of coiling the coax up has on the antenna systems performance.

Doc makes a good point about the coiling effect showing you a problem that you probably did not know you had. There are some instances that wrapping a coax too tightly will affect the radio/antenna system performance.

Never do it when you are working with Gigahertz, even bending a wire will affect inductance.

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Re: Why is it bad to coil up your coax?

Post by Turbo-T » Wednesday, 14 October 2009, 14:19 PM

Thanks, please tell me.....

What I had in mind was coiling some of the RG58U off my mag mount under the seat of the p/u truck. I don't want to have to cut any of the coax since this is a mag mount; it will allow me to use it on other vehicles.

I would have it coiled up in about an 8 to 10 inch circle. There's probably a better part of 10 feet inside the cab that will be coiled up.
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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 14 October 2009, 14:25 PM

Hi Turbo-T

Do not cut the coax off the magnet mount, leave it the length it is. A 10 inch coil will not affect the antenna system. It is better to hide it now and have the length you need if you go to another vehicle.

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Re: Why is it bad to coil up your coax?

Post by wifi » Wednesday, 14 October 2009, 15:04 PM

This post brings to mind the idea of coax length and the effect that wavelength has on need a specific length. The idea floating around on the CB FAQ's online is that you need to either have a certain amount of length of coax from the transmitter for example "9 foot". I believe this is bogus? I never hear amateur radio guys talk about this. I do hear the fact that the longer the coax obviously dB loss. Coiling coax brings to mind the idea of coiling a specific length of coax (to the point that you dont comprimise the integrity of course) creating a homebrew Balun when working with dipoles.
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Re: Why is it bad to coil up your coax?

Post by 'Doc » Wednesday, 14 October 2009, 16:02 PM

What it brings to mind for me is the obvious lack of knowledge which always results in 'myths' like specific coax lengths. The only time a specific coax length will make a difference is when the load on the end of that coax isn't close to the impedance of the rest of the system. Fix the problem where the problem ~IS~, that's the solution to that. Then it'll make no difference what length of coax to use.
That coiled up coax is never a balun, it's a 'choke'. A balun does something completely different than coiled up coax. Sort of like calling a fuel line a fuel filter. It may have fuel running through it, but a fuel line doesn't filter anything. Well, maybe the really big chunks... then it's usually called "stopped up"...
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 14 October 2009, 16:59 PM

Hi wifi

The point at which the SWR is to be taken is at the antenna. If you take an SWR reading from the CB then the length of the coax makes a difference. Try taking an SWR reading from the radio using different lengths of coax. The reading will be different at different lengths. At 18 feet of RG58 the reading at the radio will be the same as the reading at the antenna.

That is why every antenna I see sold recommends 18 feet. It is not that the SWR reading will change at the antenna it is because you can read what is at the antenna from the the radio using 18 feet at 27 MHz. What driver have you ever seen measure the SWR at the antenna? Technically once the SWR at the antenna is established the coax can be cut. I don't do it because at that length the loss is insignificant, besides what if I want to measure the SWR again.

Okay Doc I am ready :icon_e_smile:

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Re: Why is it bad to coil up your coax?

Post by 'Doc » Wednesday, 14 October 2009, 17:56 PM

IF that antenna is tuned correctly, meaning that it's resonant and properly impedance matched, the length of coax will make no difference at all. The SWR would be the same no matter where it's read. The 'biggy', the important part of that, is that the antenna is well tuned, which always means both resonant and impedance matched. If the antenna is NOT well tuned, the SWR can be all over the place depending only on where it was measured.
- 'Doc

So do the antenna right and quit worrying over feed line length.

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Re: Why is it bad to coil up your coax?

Post by wifi » Wednesday, 14 October 2009, 19:35 PM

Hmm I am seeing a trend here. I think this post ties in with another in regards to impedance and resonance. Learning curves here. I guess my basis of my post is this: First take care of my equipment and not burn it out. How can I accomplish this and then secondly resonant and broadcast as efficient as possible. You veterans in this forum are crazy. You have a lot of stamina to coach others along. Not to mention knowledge base....
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Re: Why is it bad to coil up your coax?

Post by Turbo-T » Thursday, 15 October 2009, 5:01 AM

So someone please explain to me what this "choke" is that I keep hearing about with a coiled up coax....and how does it affect the radio?

Thanks!
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Re: Why is it bad to coil up your coax?

Post by alhefner » Thursday, 15 October 2009, 8:31 AM

Very short and untechnical answer coming up:

The "choke" can be useful in stopping what is called "common mode currents" from traveling back down your coax into your radio. Common mode currents can greatly affect the noise level you hear through your radio in some instances. Common mode currents run back to the radio along the shield braid of the coax. A choke should not have any affect, good or bad, on your transmit.

I have tried my antennas on CB with and without a choke and found no real difference in performance. Your results may vary.



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Re: Why is it bad to coil up your coax?

Post by 'Doc » Thursday, 15 October 2009, 10:26 AM

And just a touch more of that simple explanation so it'll make a little more sense.

A coil, or an inductance (same thing) slows or stops the flow of alternating current which is what RF and noise is (noise can also be a 'pulse' type thing but it responds like AC).
What goes on inside coax cable is not the same thing as goes on on the outside. The desired signal is on the inside of that coax. In most cases, noise travels on the outside of that coax. (Unless the desired signal is 'dirty', and you 'cure' that at the transmitter.) What affects the outside of that coax doesn't affect the inside of that coax. That's why you can bend coax, run it next to metal and other thingys without affecting the signal inside.
So! If you wind that coax into a coil (an inductance) that inductance DOES affect what's on the outside of the coax, but it has no affect on what's inside that coax. That inductance (coil) chokes off what's travelling on the outside of that coax (or most of it). Whatever id traveling on the outside of that coax stops at the 'choke' point, doesn't get past it. See where all this is going??
That's where the name 'choke' came from, it really does explain what's happening once you see how this stuff works, right?
It just makes sense to put that choke as close to the antenna as possible. Means there is less feed line that stuff will be traveling on the outside of, right? And since a choke doesn't really affect what's on the inside of the coax it's made from, having more than one isn't going to hurt anything until the amount of coax you have rolled up into a coil gets absolutely ridiculous.
[Do you think that if we changed the name from a 'choke' to a 'stripper', guys would be more willing to have one or two around? I do! But then, that "significant other" would probably get the wrong idea and raise @#$$. Oh well.]
There are always 'catches' to anything good, and chokes have them too. You can only bend coax so far without damaging it, so, different size/stiffness coaxs have different degrees of 'bend' before getting messed up. Bigger bends are usually safer than smaller bends, so larger coils of coax are usually better than smaller ones.
There isn't anything -perfect- in this world (except me) so a choke will probably never stop everything you want it to stop.
All coils/inductances are frequency dependent. That means lower frequencies usually require more coil/inductance than higher frequencies. There's a formula for figuring that (inductive reactance) if you feel like it, or you can just 'ball-park' it like most people do. Ain't stopping enough junk? Add another coil or two, sort of, till it gets really ridiculous, and that's when you start looking for the source of the problem and kill it there.
There's more stuff that's handy to know but it doesn't come to mind right now, so figure it out for yourself.
None of this is 'magic' -IF- you know what's going on. If you make enough mistakes, and if you ask questions, there's not much you can't figure out... eventually.
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by wifi » Thursday, 15 October 2009, 13:44 PM

Good stuff! I think this forum has now turned my ARRL Antenna handbook into a supplement and not a source haha
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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Thursday, 15 October 2009, 15:47 PM

Hi 'Doc

There is a reason that every mobile antenna recommends 18' of coax. You may notice that a full wavelength is 36 feet. All coax jumpers that are made can be divided into 36 evenly. (3,6,9,12. 18)

You are right coax length does not matter. It is 18 feet so one may take a measurement at the radio to see what the SWR is a the antenna. I got some good news, my shop is reopening this month.....again. I will take some measurements and get back to the forum with the results.

There is a reason every manufacturer suggests 18 feet of coax for their antenna.

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Re: Why is it bad to coil up your coax?

Post by Century21 » Friday, 16 October 2009, 7:49 AM

Now we can get into all that garble on VF... blah blah. 18 ft is nice, but if there is some type of common mode going on, and in a mobile your options to correct it are not as easy, then guess what, your talking off your shield of the coax as well as the antenna,. and 18 ft of braid makes a nice antenna. lol. I know in my mobile, i was making nmy defrost go on and off, tearing the radio up and stuff. I couldnt get it to stop. I made 6nturns of the coax just beyond the point of entry of the vehicle about 6 inches round ( CHOKE ) and quit all the interference. Why is 18 ft important. Well gives you enough length in most common vehicles to put the antenna where you want. If changing your coax lengths changes SWR you have other problems. And only time coax lengths is critical is when co-phasing. I always try to use a non-resonant length of coax just to help eliminate common mode. But think about it, you got 36 ft tower 72 feet coax, rf likes a 50 ohm load best, ( your antenna ) If its not right, then its going to seek the best load, which is why you can run, multiple wire antenna's off 1 coax. Such as my 75,40,20 and 10 meter wire, fed all from the same point. Also how other multiband antennas work.
Dont know if everything i stated is 100% true in theory or not, but in real world examples that i have had it is. And after all its what works best for each individual situation. What works best on my vehicle or base, may not work at all for anyone else.
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Re: Why is it bad to coil up your coax?

Post by 'Doc » Friday, 16 October 2009, 11:53 AM

TheCBDoctor,
You're right, there is a reason most manufacturers use 18 feet of feed line on mobile antennas. That reason has no electrical basis, it's a matter of a general length that will reach from here to there with most mobiles. There is no significant electrical reason for 18 feet of feed line. It is NOT an electrical 1/2 wave length with any commonly available coaxial cable. That get's into that nasty VF thingy no one seems to think makes any difference. If I'm making the jumper, I unroll what looks like a length that will 'fit' the situation and use that. Sometimes it's a multiple of 3 feet, most times it isn't. If I have to buy a jumper, I take the length that's going to fit, to be long enough, and just don't worry about what that length happens to be. Why? Cuz it just doesn't matter in most cases, and that's cuz I do try to tune antenna/whatever to what they should be.
Coaxial cable can be used for impedance matching. It is not the best way of doing things. But if that's your only alternative, do it. But if you're smart, you'll look for those other alternatives cuz coax was never made to do that sort of thing. It's up to you...
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Re: Why is it bad to coil up your coax?

Post by jwalker48 » Saturday, 17 October 2009, 10:17 AM

I saw a post on several other CB forums about this subject which will be debated until the last CB transmission on the last antenna ever to exist. Not to make a huge post about it, but the CB "gurus" should go to their search engine and type in "screwdriver" antenna which is a motorized mobile ham antenna that covers ALL frequencies (in various models and mfrs, etc) from 1.8 MHZ (a WAAAAY low frequency) to 29 MHZ (at the top end of 10 Meters). This antenna is resonant and matched (when properly installed) at ANY and ALL frequencies from this 1.8 to 29 MHZ. Now IF what the CB folks say is true, then a fella would have to have at least TEN coaxes cut to a "certain" length to cover all the specified bands ranging from 240 feet (WOW!) down to about 8 feet in order to cover everything. Now RF and antenna theory are the SAME for CB, commercial, HAM, and anything else; there's NO special rules for CB. If it applies to CB, then it applies to ham, too. YET, HAMS OPERATE THIS MOBILE ANTENNA BY THE THOUSANDS, OPERATE ON THOUSANDS OF HF FREQUENCIES USING *ONE* COAX CUT TO NO PARTICULAR LENGTH OTHER THAN WHATEVER LENGTH IT TAKES TO GET FROM ANTENNA TO RADIO :lol: :lol: Their SWR will range from 1.2 to 1.5 THRU OUT this HUGE range of frequencies with-OUT trimming coax or running "bookoos" of coaxes in their cars!! Using the CB "doctrine" of coax length, it cannot be explained. Sure, you can do this if you want to---I'm not making fun-----but it just shows that coax length is faulty theory. Me, I run out enough feedline to reach, tune the ANTENNA to resonance, set impedance by standard-taught ARRL means, get an SWR of 1.5 or so and forget it. I used to have a regular QSO with a guy in Barcelona, Spain on 14.203 MHZ every day coming home from work, and worked Moscow almost as often!


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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Saturday, 17 October 2009, 17:39 PM

Hi Doc

Here's my take on the length of a coax for mobile use. A driver comes in and says he can not talk further than the edge of the parking lot. I go out and see that the driver is using a 9 foot coax on his Wilson 5000. The SWR is high and as he says his distance is poor.

I put on an 18 foot coax as per the manufacturer's instructions and measure his SWR. It is now below 1.5 to 1 and I try a radio check. I now talk 5 miles instead of 5 feet. I understand that the coax length does not matter, I may just as well have cut his whip in half and get the same results; who knows?

What I do know is that it took me less than 20 minutes to fix his problem and I now have a happy customer. Everything works except he needs to stow the excess coax somewhere. Time is money, I have to move on to the next radio or antenna system. The easiest way is to add the recommended coax length and be done with it.

I would bet that if I put an I-Max 2000 on the 18 wheeler, I could use any length of coax, and get the same SWR. Of course this is not practical. There is something about mobile antennas that makes the coax length critical. Could it be eddy currents, skin effect or maybe a poor ground plane? What I do know is that using the recommend length suggested by the manufacturer takes care of the problem and gives me a happy customer. One that will come back again.

My shop will be reopened soon. I had to be closed during my busiest time of the year and now must work in the cold. I am going to take two meters and place one at the antenna and one at the coax end. I will use different length coax and see what I get for a result. I am sure that will make Marconi happy, but in the end it is about talking a good distance, and if that means 18 feet of coax that is what I will use.

I have noticed that different length coax will effect the distance I can talk. This is a direct correlation to the SWR I measure. I do not care what the ARRL hand book says, in theory the coax length does not matter, in the real world the coax length does matter. It provides me with happy customers and some spare cash.

What would you do in my situation?
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Re: Why is it bad to coil up your coax?

Post by 'Doc » Saturday, 17 October 2009, 20:13 PM

TheCBDoctor,
What you described can certainly work! But I can tell you exactly what the 'problem' was, and that it wasn't corrected, just made to 'work'.
Anytime the length of feed line makes a difference in SWR (except in cases dealing with phasing/timing) is when the impedance match between antenna and feed line is not good. That means that is the feedline is 50 ohms, and the transmitter's output is 50 ohms, then the antenna's input impedance isn't 50 ohms. Simple as that. The absolute mostest best 'cure' for that condition is to make the antenna's input impedance 50 ohms. Then, the length of the feed line will make no difference. Actually, the only difference will be the resistive losses for that particular length of feed line, whatever that is.
There are always two things/steps to tuning any antenna. Making them resonant, and matching impedances to the feed line (except for those special cases of phasing/timing). Resonance deals with length and loading of the antenna to get rid of, or neutralize reactances. that's not that bigg of a deal, it's done all the time. The other part, matching impedances, is almost never done except with antennas having impedance matching devices like gamma/beta/hair pin/whatever devices. It's almost -never- done with mobile antennas unless a matching device is put at the input of that antenna. One of the least efficient means of doing that impedance matching is by using the feed line length. Since coax was never designed to do that sort of thing, it will always result in harm of some kind. It may not be much, or it may not be very quickly done, but it always happens.
Why does the ->'electrical'<- 1/4 and 1/2 wave thingys work? It's just a characteristic of those two particular fractional wave lengths. Certainly comes in handy at times too! Another 'catch'. 9 feet of any coax is not a 1/4 wave length. 18 feet of any coax is not a 1/2 wave lenth. Huge difference between a physical 1/4 or 1/2 wave and an 'electrical' 1/4 and 1/2 wave length of feed line. It deals with how fast an electron travels in a particular material, called... get ready for that nasty phrase... velocity factor. Yeah, I know, it's a PITA to contend with, but it's a fact.
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by scottm » Sunday, 18 October 2009, 11:40 AM

alhefner you hit the nail on the head.As long as you dont have gobs of extra coax theres nothing wrong with wraping some in a loop.Like alhefner said you can make a coaxial balun which will help block rf from coming down your coax.To make a coaxial balun just wrap your coax atleast 4 or 5 turns 8 inch diameter and have it as close to the antenna as possible.As far as inductance changing whe you bend your coax i guess that could be true but when i bend my coax i dont see any difference in my swr reading.Its never a good idea to exceed the manufacturers maximum bend radius if you do you will damage the coax.
As long as you dont put kinks in the coax you will be fine if you do you could damage the insulation between the center conductor and the sheild and end up with a short.Maybe not right away but maybe after you run some juice.
Heliax cable with a coragated out sheild is very delicate if you put a kink in that stuff you might as well throw it out.

If you have an extra 5 ft or so wrap it up you never know you may re use the coax someday or the antenna on another vehicle.But if you have alot left just buy another connector.

cb coax like 8u or 8x is very flexable

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Re: Why is it bad to coil up your coax?

Post by Century21 » Sunday, 18 October 2009, 22:57 PM

Hmmm.... And thats all im saying folks. lol
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Re: Why is it bad to coil up your coax?

Post by scottm » Monday, 19 October 2009, 5:56 AM

it is possible to acheive a perfect swr flat 1.1 in a car with the right antenna.Its a little more difficult with magnet mount types I think it has to do with grounding and havng to go through paint.

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Monday, 19 October 2009, 9:58 AM

Hi 'Doc

I understand the velocity factor concept. It is no different than looking into a fish tank and "seeing" the fish that is actually slightly in a different place. Just like current running through wire; light will slow down going through water.

I guess my point is that as long as I am not sending a space probe into the outer reaches of the solar system, then making a radio system work by using the recommended coax length and disregarding Marconi's work, will not be that big of a deal. The customer is happy, I am happy, other drivers now can hear the radio because I put on the 18 feet of coax that is recommended by the manufacturer. Hiding the excess coax is the drawback. I have found that coiling the coax in a 12" loop has no interference on the performance.

I think sometimes many of the theories behind the physics can be overly thought out. I just want a happy customer, a working antenna system. Making some spare change in the process doesn't hurt either. I know that Motorola "fixed" a problem locally for a Police Dept by taking a vertical antenna and putting it at a 15 degree angle. I know that is not professional but it cured the symptom even though the problem still existed. The city was happy, the Motorola dealership did not have to rip the system down and spend a week fixing the problem, and the system worked. Time is money.

I am not a COB yet but I am gaining on you. :icon_e_wink:

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Re: Why is it bad to coil up your coax?

Post by 721HACKSAW » Monday, 19 October 2009, 14:26 PM

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This is a "choke" that I use on both of my base antennas, probably not practicle for a mobile though.

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Re: Why is it bad to coil up your coax?

Post by jwalker48 » Monday, 19 October 2009, 15:17 PM

TheCBDoctor wrote:Hi 'Doc

I understand the velocity factor concept. It is no different than looking into a fish tank and "seeing" the fish that is actually slightly in a different place. Just like current running through wire; light will slow down going through water.

I guess my point is that as long as I am not sending a space probe into the outer reaches of the solar system, then making a radio system work by using the recommended coax length and disregarding Marconi's work, will not be that big of a deal. The customer is happy, I am happy, other drivers now can hear the radio because I put on the 18 feet of coax that is recommended by the manufacturer. Hiding the excess coax is the drawback. I have found that coiling the coax in a 12" loop has no interference on the performance.

I think sometimes many of the theories behind the physics can be overly thought out. I just want a happy customer, a working antenna system. Making some spare change in the process doesn't hurt either. I know that Motorola "fixed" a problem locally for a Police Dept by taking a vertical antenna and putting it at a 15 degree angle. I know that is not professional but it cured the symptom even though the problem still existed. The city was happy, the Motorola dealership did not have to rip the system down and spend a week fixing the problem, and the system worked. Time is money.

I am not a COB yet but I am gaining on you. :icon_e_wink:

Respectfully,

Now bear in mind I AM playing with this subject. I mean NO harm and don't want to make anybody mad that holds hard fast to the "coax length, coiling coax thing". But is there some special rule that applies ONLY to CB Radio to the exclusion of all other forms of radio? What about the guy who installs a radio that works on 1.895 MHZ in his mobile? Using the usual CB formulas for determining length (however I DO use a slightly different that achieves the same result for the LENGTH of the antenna) I would have to have 247 FEET of coax in a Malibu for the system to work IF the usual CB theories are used. WHERE in the WORLD would one PUT all that coax like the poster on another board said? NObody has answered my question about the multiband screwdriver antenna that is actually A RESONANT antenna at ANY frequency within its range. It doesn't use a "tuner", there's no gadgets to tune it ( except a required feedpoint impedance device for matching). It is ready to go and will tune ANY frequency from 1.8 thru 29+ MHZ! On the ham forums there is RARELY any discussion of "coax length", and the hams who operate the screwdriver antenna don't worry about how long the coax is; they just run whatever it takes to get from the radio to the bumper.

I can't get anybody to tackle this "mystery"!!! LOL! Now if you get good results with this coax length thing, hey, it's OK! I just wonder why CBers "must" have 18 feet, and everybody ELSE just runs what it takes to reach!

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Re: Why is it bad to coil up your coax?

Post by 'Doc » Monday, 19 October 2009, 16:53 PM

jwalker,
I can answer that!
But people aren't gonna like it.
There are only two things you really need to worry about with using something as an antenna (no matter what it is). One of those things is getting rid of any un-neutralized reactances in that antenna which makes it resonant by definition. The other thing is matching impedances of that antenna to the rest of the antenna system (feed line and whatever else is there till you get to the back end of that radio). When you do that, the length of the feed line only makes a difference in resistive losses, nothing to do with SWR or frequency.
So why do people do the 'feed line length' thingy? Typically because they don't know any better and that's how they were told it works. And it's easier to measure feed line with a yard stick than it is figuring out how to do it right. What's so hard about figuring it out? Can't do that figuring in your head? I don't either, I use a calculator. I even remember how to do it with a pencil and paper (without too many mistakes most of the time).
[HINT: Most cell phones have a calculator and a place to make notes. Cheat! Use that stupid cell phone.]
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Tuesday, 20 October 2009, 7:40 AM

Hi jwalker48

I would like to address the part about CBers needing 18 feet. A you stated "I just wonder why CBers "must" have 18 feet, and everybody ELSE just runs what it takes to reach!"

I worked for a radio shop that dealt with CB, Ham, commercial, and police radios. The cost to wire an end to a Mic for a CB was $10 total, the cost to wire a police radio or a Taxi Cab radio was $35 labor and $10 for a mic plug. Why? It is no harder to do a Police wiring than a CB. The factor was the CBer would not pay $45 to wire a mic for a radio that is not worth $100.

The taxi service or the police paid the money because the cost of their radios ran as high as $800 or more and the mics cost over a $100 to replace. The same theory applies for coax length. The CB antenna manufacturer recommends 18 feet because it shows a low SWR and works. The cost is low and the time to do the work is quick.

Police and other mobile services will spend hundreds of dollars on an antenna system. Since they are paying it is worth the time for a radio shop to do it right or by the book. That is my point it may not make sense but if I can sell an antenna, installed for a CBer for $65 it is not worth the time to do it by the book because I have customers waiting, so I follow the instructions; I use 18 feet of coax. It works and I am in and out in 30 minutes or less and the customer is happy because he is talking.

A municipality has the funds and will pay $300 to have an antenna installed. You do not want any future headaches from a city so you do it by the book, why not you have the car all day?

Respectfully,
Best Regards,

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Re: Why is it bad to coil up your coax?

Post by jwalker48 » Tuesday, 20 October 2009, 8:22 AM

OK> :lol: I've been playing with this subject since I saw it somewhere else. What IS missing is (in many cases), a lot of people "tune" their antenna with "coax length" and never prune or adjust the antenna itself: they believe the coax is the way to do it............period. But what they don't KNOW is, they STILL got an out-of-resonance antenna at the other end. SURE! It will work, but with reduced performance and higher ACTUAL SWR (GASP) that is not apparent to the meter!!!!!! :shock: You can "tune" an antenna with a 50 ohm, 10 watt resistor across the antenna terminal! The radio will be very happy. Oh.......but you won't have much of a signal! It is called a "dummy load", btw! :lol: But you already know that.
I just find it amusing that people, mostly those in the CB world, that firmly believe this is the way to "tune" an antenna. It isn't, but if it makes the customer happy, and he believes his antenna is "tuned", that's fine business. He will likely never know the difference.

It's time to go to lunch now!


73

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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 9:41 AM

jwalker48 thats what i was saying also.You dont add or subtract coax to tune your feedline impedance.You must tune the antenna by the total length or by the matching network at the base of the antenna.Where your matching low impedance coax with a high impedance point which is the antenna.Coax like rg8x or rg58u is super lossy by adding more lengh your just fooling the meter.You cant just subtract it either you still need to tune the antenna.

with a little effort you should be able to get any antenna or wire antenna down to a flat swr.

The higher the swr the more power you have backing back up into your amp or radio if you dont have an amp which can cause a false power reading also causes your coax to turn into a radiating element that means more tvi.Also will make your mic more likely to capture stray rf and squeel like a piggy.

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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 9:43 AM

for those who tune by cutting and adding coax get your hands on a mfj analyzer that measures impedance.
Im sure you will be suprised.

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Re: Why is it bad to coil up your coax?

Post by jwalker48 » Tuesday, 20 October 2009, 12:55 PM

scottm wrote:for those who tune by cutting and adding coax get your hands on a mfj analyzer that measures impedance.
Im sure you will be suprised.

I declare! I was going to mention that, and I flat forgot because I was so wrapped up in "playing" around with this subject. The 'screwdriver' antenna seems to bumfuzzle people, and I get a big kick out of seeing this discussed on the boards. Anytime coax length comes up (You gotta have 18 feet!!!!) somebody will mention multi-band HF antennas that cover from DC to Daylight, and folks won't TOUCH it with a 10 foot pole!!!! :lol: :lol: I don't believe there IS a way to answer it!!!!
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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Tuesday, 20 October 2009, 14:34 PM

Hi jwalker48

I would say 90% of the drivers out there are using their antennas with 18' of coax. They are doing fine. If it works don't fix it. They follow the manufacturer's instructions and they are getting out.

Most drivers do not have the understanding or have the time to make the antenna right so that any length of coax may work. Sometimes the antennas are properly tuned and maybe some are not, but if putting on the manufacturer's recommended coax length works, then who will tell the manufacturers any different.

If a driver can't get out 500 feet, and an 18 foot coax will work 90% of the time allowing the driver to get out 5 miles, then what is crime? You and I have a line checker and I am sure most Ham clubs have one. That is no way to run a business when a driver has 30 minutes before he has to hit the road. Just put on the antenna with 18 feet of coax and trim the whip if necessary until he reads a good SWR.

He talks and is on his way and I move on to the next customer. I wish I had the time make it perfect. I doubt if he has money to pay me for my time. That is why I have a Day Job where having the proper line termination and feed point is critical, but then I am making good money doing it the company's way. If that is what the company wants to spend on my services, then who am I to argue. :icon_e_smile:

For the drivers I suggest putting on the antenna of your choice with the manufacturer's recommended coax, trim the whip if needed and be on your way spending less time and money. Marconi will get over it. Time is money and this thread is beating a dead horse.

The Hams won't accept anything less than perfect and can spend the time and money to get it right; they have their entire life to do it. The Drivers using CBs for the most part just want to talk to their friends and keep the cost down. If their antennas are working, they see a low SWR, and they make contact, then so be it. As long as Hams and CBers are happy doing what they want, then there is no harm? I have never seen a CB fail because the radio "Sees" a good SWR . It may not be the proper way to tune an antenna, but that is the way it is.

After All This is America,

Respectfully,
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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 16:03 PM

When running big power its a must to have a low swr.Plenty of guys run big power in their cars or trucks.

High swrs is one of the main killers for transistor boxes tubes can take more but you still get all that rf coming back down the coax which causes all sorts of problems like more tvi.

Plus with a high swr you see inacurate power reading on your power meter and the radio or amplifier doesnt swing or work as it should.

Its really important to have a nice low swr if your only on a 5 watt radio so you can get out as far as you can and also hear as best you can.

Its actually the eact oposite hams could care less about a low swr they think a 2.1 is low.
They just hit the auto tune button on the radio but could care less if their tube amps sees a 2.1 swr.

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Re: Why is it bad to coil up your coax?

Post by 'Doc » Tuesday, 20 October 2009, 17:42 PM

:)
I'd sort of be careful with those generalizations. To the best of my knowledge, most hams do try to get that SWR down to an acceptable level (meaning matching impedances). With some tube type amplifiers, a 2:1 SWR IS acceptable. With others it's not.
And just for grins. All that reflected power isn't getting back into that transistor amplifier, that isn't what's causing a problem. The problem is when it can't get that power out of the amplifier because a reactive load won't take it. It sort of backs up inside there until it has to find a place to go. What's commonly known in the 'trade' as 'RF constipation', cause by final PA 'sphincter' cramps due to excess reactance. Reducing that reactance reduces the cramping, relaxes that 'sphincter', lets all that power go where it should, out the antenna.
[This is secret stuff the pro's seldom even acknowledge, much less confirm. So don't expect them to.]
- 'Doc

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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 18:39 PM

Your right it goes both ways with swrs when it comes to hams.I hold a general class ticket most of the guys i have talked to think 2.1 is fine.I just made that comment because the cb doctor said hams go out of their way to get a 1.1.
your right if you dont have a good match all the power cant go into and then radiate off of your antenna it gets blocked and goes back into your radio or amp if you have one hooked up.Some people think by hooking up an antenna tuner beween your amp and antenna or radio and antenna will take care of the problem it wont.Yes it will make the device in front of it see a good swr but you still have the mismatch between the antenna and the tuner.

I personaly wont stop until i have a good swr which is a 1.2 or less on the frequency i plan to talk on.And i dont acheive this by changing coax lengths.

Point im trying to make is you will notice some sort of increase in performane with a low swr vrs a 1.5 or higher.

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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 18:44 PM

This is what was said
The Hams won't accept anything less than perfect and can spend the time and money to get it right; they have their entire life to do it. The Drivers using CBs for the most part just want to talk to their friends and keep the cost down. If their antennas are working, they see a low SWR, and they make contact, then so be it. As long as Hams and CBers are happy doing what they want, then there is no harm? I have never seen a CB fail because the radio "Sees" a good SWR . It may not be the proper way to tune an antenna, but that is the way it is.

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Re: Why is it bad to coil up your coax?

Post by 'Doc » Tuesday, 20 October 2009, 20:41 PM

The only contention in that last part was a 'marked increase in performance' with SWR below 1.5 or higher. Sorry, there isn't enough difference between a 1.5:1 and lower that you will ever notice it other than on a very accurate measuring device under controlled conditions, which is not any commercially and readily available watt meter. As far as a difference in someone else's receiver? Never happen. Sorry, been there done that (under those lab grade controlled conditions). Doesn't hurt to try, but don't expect miracles.
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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 21:16 PM

i noticed a difference on my tv and the way my power output and how it would swing on rms with my bird 43.
guess it all depends :bounce:

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Re: Why is it bad to coil up your coax?

Post by 'Doc » Tuesday, 20 October 2009, 21:35 PM

I guess you're right, it all just depends. I've never yet used a TV to define how well my antenna system is tuned. They are very handy to how 'un-well' antenna systems are tuned though. And that just depends on TV too.
Oh well. The original question was answered a long time ago.
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Re: Why is it bad to coil up your coax?

Post by scottm » Tuesday, 20 October 2009, 22:18 PM

I was talking about interfernce.But i have a feeling you already knew that.
Higher swr equals more interference boy some people get moody out here.
I come out here to help I nor anybody else out here really can say they know it all.

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 21 October 2009, 4:13 AM

Hi Doc,

Your sentence: "What's commonly known in the 'trade' as 'RF constipation', cause by final PA 'sphincter' cramps due to excess reactance. Reducing that reactance reduces the cramping, relaxes that 'sphincter', lets all that power go where it should, out the antenna."

I guess that is one way to put it. :icon_e_smile: The only thing missing from the sentence is the A AAHHHhhhhhhh and the courtesy flush. I will get back to you, I have to relieve some reactance, but I will be back.

Respectfully,
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Re: Why is it bad to coil up your coax?

Post by RatsoW9 » Wednesday, 21 October 2009, 7:18 AM

scottm wrote:it is possible to acheive a perfect swr flat 1.1 in a car with the right antenna.Its a little more difficult with magnet mount types I think it has to do with grounding and havng to go through paint.
No, coiling coax will have no effect unless, as Alhefner said, your winding it for the purpose of acting as a rf choke. Using coax length to tune an antenna system in basically fooling a bad match into thinking it's a good one. Then end result is more power lost to ground losses and into the body of the vehicle rather than radiating from the antenna. Also, achieving a 1:1 swr is not always a good indicator of a good match between tx and antenna. Mobile antennas (installations) are in general very poor radiators, most radiating only a small percentage (under 10%) of the power fed to them. Why do you think most people need a 100 amp to talk 5 miles down the road? Google "L. B. Cebik, W4RNL". He wrote a great article on optimizing mobile antenna installations. You really have to have or have access to a good antenna analyzer to really optimize a mobile antenna.

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 21 October 2009, 8:13 AM

Hi RatsoW9,

I would say that 12 watts peak is fine to get out 5 miles. As long as the antenna is doing its job, the terrain is flat, and near the seashore. Those that use 100 watts to get out can usually reach a station 20 to 35 miles away. If the antenna is not doing its job then even 100 watts will not get out a mile, if that.

It is all in the antenna system. I just don't lose sleep over worrying about the difference between a 1 to 1 SWR or a 2 to 1 SWR. I don't care about coil loss and cable length. If I can make a positive contact on the first try then as far as I am concerned my radio system is working. My only concern is causing excessive TVI. Since I use SSB that is far less likely to happen.

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Re: Why is it bad to coil up your coax?

Post by scottm » Wednesday, 21 October 2009, 8:26 AM

yeah when your on ssb your neighbors just cant understand what your saying.lol
fm is the cleanest mode

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 21 October 2009, 10:14 AM

Hi Doc,

I use an old B+W TV to see what a radio is doing. Not that it longer matters, but channel 2 which is double the CB band in frequency is an indicator that something may be wrong. You are right though it never tells you something is right. I keep a TV on my work bench as well as a spectrum analyzer. For those that do not know it the analog TV was a compromise between bandwidth and performance.

The video is an AM signal with one reduced sideband. Both sidebands are detected on an analog TV. The carrier is used to lock on the fundamental frequency.
(54 MHz on channel 2) The audio is FM with a deviation of +/- 49 MHz. The total bandwidth of the signal was 4.5 MHz. Well it no longer matters, the machine that lead to the discovery of radar is now a thing of the past.

Remember when you knew that an airplane was flying overhead because a ghost image would appear to one side of the picture and move to the other side as the plane passed by. You have to be a "COB" like you Doc to remember those days. :icon_e_wink:

Kids today do not have a clue.

Respectfully,

If it isn't sideband it is not a real radio. :icon_e_surprised:
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Re: Why is it bad to coil up your coax?

Post by scottm » Wednesday, 21 October 2009, 13:34 PM

Its alot harder to sound good on am and to run am then ssb.
Listen on 3.885 or 3.880 to the guys talking at night.Some of them sound better than am broadcast stations.
This is where the smart guys talk the kinda gus that work for nasa,radio stations,henry,ameritron etc etc.

I do also talk on ssb my yaesu has also been modified to sound great on ssb with external rack gear hooked up to the back of the radio.Coupling caps have been changed along with other things which i dont wish to disclose but i can acheive essb audio smooth and lots of low end.

I prefer am though I like talking to the guys on 75 meter am i lear somthing new everythime i get on and talk to them.

No carrier with ssb.Its easier on your radio your amp and your neighbors.
If your on a ham radio unless you know the right tech its not going to sound like a cb on am you can get good audio but not loud and punchy.When you crank the audio up on a stock ham radio on am or talk loud into the mike your audio get crunchy.

Rci/ galaxy etc export radios dont count they sound just like a cb.

Im talking about kenwoods yaesus icoms etc Even the older yaesu need some attention to sound good on am. The yaesu 757 and 767 sounds really good righ out of the box.

Am uses more bandwidth then ssb. ssb is what like 3khz or so am is what like 6khz some guys run up to 10khz wide and sound really good but splatter like hell

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Re: Why is it bad to coil up your coax?

Post by TheCBDoctor » Wednesday, 21 October 2009, 21:56 PM

Hi Doc,

I am sorry I meant plus or minus 49 KHz for the FM deviation on the old analog TV's audio.

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Re: Why is it bad to coil up your coax?

Post by jwalker48 » Saturday, 24 October 2009, 10:46 AM

Question for CB Doctor,

This is just my curiosity, but I've noticed that CBers call "SWR" "SWR'sssssssssss" plural. How did that get started? There's only ONE "SWR" in ONE feedline. :lol: Now it doesn't matter if you call it "reflected power", or lost power or SWR's, of course, but this is also a thing that seems to be unique to CB. :icon_e_smile:

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