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Old 06-27-2010, 10:58 AM
   
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Default what length coax for cb

hello i am driving a dump truck and i have a 29ltd chrome and am curious what lenght coax cable would be good to run, i will be using a 60" whip anntena from radio shack,it has a mirror mount bracket, my radio has been peaked and tuned, i have herd shorter the cable is better, any thoughts, thanks dan
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Old 06-27-2010, 12:06 PM
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Changing the length of your coax wont tune your antenna.


You should use the least amount of coax possible, and follow the procedures from Radio Shack on tuning that antenna.
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Old 06-27-2010, 3:59 PM
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I hate to give you a flippant answer, but the length of coax you should use is just enough to go between the antenna and the radio, with enough extra to allow for smooth turns and since you're antenna is mounted on your door, add enough slack to allow the door to move without pulling (or causing to pinch) the coax. It's also a good idea to use a bit extra to route the cable neatly and hidden.

If you get a length of coax, complete with connectors that's a bit longer than you need, sure tuck the rest away neatly where it doesn't block any air movement or interfere with driving or passenger comfort. If what you get is way long, trim it to the correct length and properly install a new connector.

If your antenna is properly designed, the length of coax will not affect its tuning. If it does, you have issues with your antenna design or installation.
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Old 06-27-2010, 5:01 PM
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I drive a big truck and have found that an 18 foot piece of coax works best for me I have tried shorter coax but could get my swr down where I wanted it. My answer to you would be to buy the length of coax you can make work for your set up. Check you swr and if its high add 3 more feet and try again. Don't add 3 feet to the coax, go and buy another piece 3 foot longer and start all over. Better off if you start with either 18 foot or 21 foot of good coax and do the work outside of your favorite CB shop and away from power lines and metal buildings. This way you get a good swr reading. And since your going to mount your antenna on the mirror bracket make sure you check your swr with the doors closed. Hope this helps!
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Old 06-27-2010, 5:29 PM
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Yeah... I have always heard that 18' is the proper length for radio to antenna. And 6' for radio to linear amp, then 18' from amp to antenna.
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Old 06-27-2010, 7:07 PM
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You heard wrong.

If the antenna will not tune except for a specific length of cable then something is seriously wrong, the antenna is broken or the antenna does not have enough counterpoise (ground plane) to work against.

In the case of a mirror mount antenna on a fiberglass cab, the shield of the coax is trying to become the other half of the antenna because there is no counterpoise to speak of at 27MHz and there will be lots of RF current flowing on the outside of the cable. Its much better to provide an additional ground plane by running wire or conductive tape from the mirror mount to the other side of the cab (under the headliner) and several wires branching out in different directions is best.

Most CB amplifiers (I don't call them linear's because there usually not) have no specific input tuning and provide a poor match to the radio. In that case you will find a cable length between the radio and amp that makes everything seem to work better. Buy a crappy amp and you end up with band aids like custom length cables to smooth out problems.
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Originally Posted by OBXJeepGuy View Post
Yeah... I have always heard that 18' is the proper length for radio to antenna. And 6' for radio to linear amp, then 18' from amp to antenna.
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Old 06-27-2010, 7:54 PM
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Terms like Resonance, Wavelength, Counterpoise, outer shield radiatation are alien in the world of CB; but trust the Dude at your local truckstop CB shop, you know the guy with the recorded announcement running 24/7 on channel 19.
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Old 06-28-2010, 7:39 AM
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"I drive a big truck and have found that an 18 foot piece of coax works best for me I have tried shorter coax but could get my swr down where I wanted it."
Generally 16' is closer to the mark but let's not split hairs.
"My answer to you would be to buy the length of coax you can make work for your set up."
Yeah, generally 16 to 18' should do it for a mobile.

"And 6' for radio to linear amp, then 18' from amp to antenna."
And what are you doing with an illegal amplifier you bad little boy?

"You heard wrong."
No, here we depart from theory and get down to brass tacks.

"If the antenna will not tune except for a specific length of cable then something is seriously wrong, the antenna is broken or the antenna does not have enough counterpoise (ground plane) to work against."
There is nothing wrong with your antenna. For the next hour we will control....
OOPS, wrong TV show, not the Outer Limits, we want the Dukes!
Here's the theory, when you measure VSWR you're taking both current and voltage into account and for reasons beyond the scope of this discussion phase angles mean nothing in the virtual (V) world.

For all practical purposes SWR meters (notice no V) only measure current so the phase angle which determines impedance varies along the length of the coax. Think of it as a sine wave and if you know a bit of vector math you get the picture. The long and the short of it (;->) is depending on velocity factor a half wave appears every 16-18' along the cable so at those points you get out just what you put in impedance wise and the meter will read exactly what it does at the antenna.

Rule of thumb: When tuning an antenna always measure the SWR >AT< the antenna and you can't go wrong, anywhere else and you're fooling the meter (and yourself). On the other hand you can just tune for the lowest reading whatever it is and screw it. (;->)
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Old 08-28-2010, 10:54 PM
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Default lenght of coax

if your runnig a 102" steel whip, you have to tune with the lenght of coax, and always be sure you have a good ground at the antenna, the ground plane can be a killer
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Old 10-04-2010, 3:13 AM
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Default Let's simplify this, shall we?

Hi,

kb2vxa has it right, but bombarded some of us with technical stuff.

He is right though.

For measuring your SWR you should do it at the antenna. This is because an SWR meter is a "dumb" device. It measures current. It only measures current that is present on the coax where it is attached. The currents vary depending on the location on a given length of coax, even if the antenna is perfectly tuned. The meter doesn't know it is not at the antenna or instead at a certain point on a piece of cable, it just indicates the current where it is.

At 8 feet away from the antenna on 27mHz it may show a very high reading, even if the antenna is tuned. At 16 feet it may show a low reading for the same antenna. (notice I said "may".) The currents can be different at different points on the cable. The meter will show only these currents. It doesn't know where it is on the cable, ergo the term "dumb device".

Now ... do not confuse this with the performance of the system. If the antenna is tuned, the length of coax has no effect on the systems performance (radio, coax & antenna) in radiating the energy at the antenna. It also has no effect on the tuning of the antenna itself, contrary to many opinions. It only effects the meter reading (the measurement), depending on where the meter is on the length of the coax.

So tune the antenna with the SWR meter as close to the antenna as practically possible. This eliminates any chance of a false reading due to varying currents on a cable. A tiny piece of coax cable, like 6", is good. Even better, if you have the space, one of those male-to-male UHF adapters are great, if you can get the meter in near the feedpoint like that.

Oh and try to step away from the antenna when making measurements, it can throw off the tuning by you being in close proximity.

Tuning the antenna this way you know it is tuned, and you are not reading currents on the cable. This way you know you can use any length of coax and the system will radiate the energy properly.

BTW, this effect of varying currents at different points on a coax cable is why people think the length of coax effects tuning. It doesn't. They just assume it is, because they are being just as dumb as the meter. They do not understand the measurement (the false reading) and are making bad assumptions based on incomplete knowledge. Tuning is not usually affected by coax length. Measurements of current always are. The two are not the same thing.

Was that simple enough ... I don't know. :-)

John LeVasseur, W2WDX
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Old 02-11-2011, 8:27 AM
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By the end of this topic I think he's gonna throw out the cb and use a nextel..
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Old 04-20-2011, 11:18 AM
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Quote:
Originally Posted by OBXJeepGuy View Post
Yeah... I have always heard that 18' is the proper length for radio to antenna. And 6' for radio to linear amp, then 18' from amp to antenna.
Though the coax is shielded, try to keep coax from running along power wires like the ones going to a linear...any length of coax will work just make sure the excess is tucked away and free of possible movement and crimping from doors or seats moving parts.
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Old 01-01-2013, 12:36 PM
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Quote:
Originally Posted by W2WDX View Post
For measuring your SWR you should do it at the antenna.
How are you supposed to do this with an NMO antenna?
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Old 01-01-2013, 1:23 PM
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The problem with using SWR to 'tune' an antenna system is that it doesn't actually 'do' any 'tuning'. There are two parts to tuning an antenna, making it resonant and matching impedance to the rest of the antenna system. All that SWR can tell you is if the -apparent- impedance on the front side of the meter is close to the impedance on the back side of the meter. That means that -where- you do that measuring make a humongus difference. If it's done at the antenna feed point it can tell you how well the impedance of the antenna matches the impedance of the feed line. (It doesn't say anything about how well the antenna is working/radiating.) You can make that impedance matching 'better' by either making the antenna's impedance match the rest of the system, or by making the rest of the system match the antenna's input impedance. Make the antenna 'fit' the system or make the system 'fit' the antenna.
If that SWR measuring takes place at the transmitter end of the antenna system then it means that everything between that meter and the tip of the antenna is figured into the 'mix', feed line and anything elese between the meter and antenna. That introduces a huge number of variables into the equation, and since nothing is perfect, it means that the chances of it all being 'right' are about like winning the lottery, good luck.
Are you sick of all this yet? It gets more complicated.
The quick'n'dirty answer is that if the length of the coax makes any difference then the antenna ain't right. Each and EVERY installation is unique/different. There is no 'one size fit's all' antenna. A 'pre-tuned' antenna may be you to the ball-park, but the chances of it getting you to your seat is almost a miracle (never count on it). And then it's a matter of just how important is being in the 'right' seat to you...
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Old 01-01-2013, 4:51 PM
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The SWR does not change along a feedline, except for the small change due to line loss, with the SWR somewhat less at the radio than at the antenna.

If the SWR 'measurement' changes in any significant way based on feedline length it means there is current flowing on the outside of the coax shield, the SWR meter is defective, or both.
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Old 01-01-2013, 6:21 PM
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Quote:
Originally Posted by rubicon05 View Post
if your runnig a 102" steel whip, you have to tune with the lenght of coax, and always be sure you have a good ground at the antenna, the ground plane can be a killer
Or you can loosen the set screws on the rod holder and trim the rod. If you have a molded on rod holder you can use bolt cutter to trim the tip then put on one of the replacement balls if you have to have one.
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Old 01-01-2013, 7:17 PM
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There is a lot of good and bad info in this thread and its interesting to hear all the silly comments like you must tune your coax when using a 102" whip or VSWR doesn't change along the coax and so on.

Its been mentioned before in some version but if the antenna match at the end of the coax changes with the length of the coax, then the coax has common mode RF current flowing on the outside of the shield and its part of the antenna and radiating, which you don't want. In this case measuring the SWR (or VSWR, same thing) will vary at different points along the cable and its supposed to.

In many cases like mirror mount CB antennas on a big rig, there is not enough ground plane or counterpoise and the coax becomes part of the antenna and it will radiate. Coil up the cable or route it differently in the vehicle and it will change the SWR. Make it longer or shorter and the SWR measured at the radio end will change. Its supposed to.

The fix is to provide an adequate counterpoise at the antenna end, which for a mirror mount can be a similar whip inverted (pointing down) and connected to the grounded side of the mirror mount. Both whips need to be tuned to resonance and when done correctly, there will be minimal common mode RF on the outside of the coax and cable length will be a non issue. BTW, when you have a lot of hot RF on the outside of the coax, its power that is radiated and wasted inside the vehicle and not outside where you want it.

You can also experiment with counterpoise wires attached to the ground side of the mirror mount and run away from the antenna. Try to use two or more of equal length since only one counterpoise wire will tend to make the whip and counterpoise wire into an inverted V dipole and you can radiate some of your signal in an unwanted polarization which is not so good. A single counterpoise wire will radiate but two or more of equal length running opposite directions will cancel each other and not radiate.

You can also choke off the common mode RF near the antenna by various means (several turns of coax around a #43 mix ferrite toroid core) but it may be impossible to obtain a decent match on the antenna without the counterpoise. In the above cases where the coax has common mode current flowing it would be a bad idea to measure the SWR at the antenna end of the coax because the coax is part of the antenna system and the radio is seeing a totally different picture. For other types of antennas where the coax has little or no interaction with the antenna then it may be appropriate to tune the antenna at the antenna end of the coax, but not on a mirror mount CB antenna.

Also, most SWR meters are simply a pair of directional couplers with detector diodes and the basic circuitry is the same as most RF power meters including a Bird 43. If the coax does not have common mode RF current flowing then the SWR or power meter will read essentially the same at any point along the coax not counting coax loss. In the presence of common mode current they will read different at various points along the feedline.

Yadda yadda, ymmv.
prcguy
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Old 01-02-2013, 11:00 AM
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Basic facts
#1. If the antenna system isn't a perfectly matched 50 ohms impedance from radio to antenna then SWR will change along that feed line. That's by definition.
#2. The chances of there being a perfect impedance match is almost non-existent. There may not be much, but there's always some mismatch.
#3. The absence of CMC is NOT determined by SWR, other factors enter into it. There doesn't HAVE to be any CMC if the SWR is really bad, other factors enter into it.
#4. SWR only does one thing, measures impedance matches. Can't tell you anything about how well an antenna 'works'.
#5. An SWR meter is a dumb 'animal'. It can't discriminate between the 'parts' of impedance, 'R' and 'X' (or 'J'). Both of those 'parts' have to be right before what they make up (impedance) is right. So, an SWR meter only half 'axe' measures things.
Think about it...
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Old 01-02-2013, 12:47 PM
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There are plenty of great engineering textbooks on antennas, feedlines and SWR. One that I particularly like because of its readability is 'Reflections: Transmission Lines and Antennas' by Walt Maxwell, W2DU. It was originally printed as a series of articles in QST Magazine. Walt was the director of engineering for RCA Laboratories and designed antennas for RCA satellites as a member of their Astro-Electronics division. I highly recommend 'Reflections' for anyone who wants to understand antennas and feedlines from HF through UHF.

Walt Maxwell and all of those engineering texts agree with this well established fact:
SWR does not change along a feedline, except for small changes due to feedline loss.

The reason SWR appears to change is because most SWR meters cannot handle the common mode currents flowing on the outside of the coax shield. In that case the coax has become 3 conductors - the RF from the transmitter flowing on the center conductor and the inside of the shield, and the common mode current flowing on the outside of the shield.

Walt passed away last year but the 3rd edition of 'Reflections' is still available from the CQ Magazine bookstore.
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Last edited by k3td; 01-02-2013 at 12:53 PM..
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Old 01-02-2013, 6:53 PM
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... and ol'Walt was never mistaken about anything, was he.
- 'Doc
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