CB Coax Length.

[Schlepporello]

OK, I've always been told that the proper length for CB coax should be done in 3' increments, with the optimal length being 18'. I've heard that if you cut your coax in lengths shorter than 18', setting your SWR's would be at best difficult if not impossible. So my question is, if I wish to put a CB on my bike, do I need to find a place to stuff 18' of coax?

[Snaggletooth]

One of my oldest friends has run a a CB shop for most of his adult life and we have discussed this more than once. Seems to be no real way around the 18' number for maximum performance.

 

The only thing that becomes critical in hiding the coax is making sure that you wind it in a loose figure 8. Zip tie it in the middle then zip around the loops to hold it togther so it don't wander on you.

 

Under the pan of the seat is about the only place it is possible to tuck it in and that is tight.

[Schlepporello]

Thanks Snag. That's what I thought.

[flb_78]

With the horrible ground plane a motorcycle offers, I don't think that the length of coax will be a factor. Make it 1 foot longer then needed to reach from the radio to the antenna.

[MiCarl]

With the horrible ground plane a motorcycle offers, I don't think that the length of coax will be a factor. Make it 1 foot longer then needed to reach from the radio to the antenna.

 

Agreed. How much aggravation is it worth to transmit another 6 feet?

 

When riding with a group I always set the CB to LO power anyhow so I don't over drive their receivers or interfere with someone using the frequency farther away.

 

By the way, the stock cable has about 1 foot of slack.

[Bob Myers]

With the horrible ground plane a motorcycle offers, I don't think that the length of coax will be a factor. Make it 1 foot longer then needed to reach from the radio to the antenna.

DING, DING, DING! We have a winner. If cutting the coax changes your swr then your antenna is not resonant any way. If the ant is not resonant then it will not matter if your coax is 1meter or a full 11meters long. A much better way to check antenna is with an analyzer, swr is just relative to the amount of power you feed it, and analyzer will tell you if you have a true 50 ohm load and what the swr is at the frequency you dial in.

[Schlepporello]

Boy the trolls are out tonight. I stand by my answer.

Well, my apologies if my original post seemed trollish. The only experience I have with CB's that I can fall back on is in how it relates to installations in semi-trucks and my personal pick-up. I have no experience with motorcycle installation applications. I knew that if I were to use my rational for installing a CB on my bike, I'd have to find a way to stash a whole lot of coax. Were I to try to put dual antennas, then there'd be 36' of coax I'd be trying to hide. I figured that in order to make the application fit, there had to be a better formula. And in this discussion, one thing was brought to my attention that I hadn't even considered. flb_78 mentioned the ground plane. Compared to the applications I am familiar with, a motorcycle has none. All the information mentioned in this thread has been very helpful to me.

Thanks guys.

[Condor]

I ran acrossed this antenna manufacture that builds 'no ground' pretuned antennas.

http://www.everhardtantennas.com/index.html

[Schlepporello]

I've heard of their antennas before.

I can't remember what I heard about them though.

[GeorgeS]

The correct length for an antenna transmission line, made of Coaxial Cable ( ie: RG58 or RG 58U ) is, An " Odd " number of 1/4 wavelengths. Wavelength is calculated for the Frequency band being used. If useing coaxial to feed any antenna from 2 to 50 MHZ, thats the general rule of thumb for a " Tuned, Resonant Antanna "

Thats what your CB antenna is, a Resonant antenna.

 

17.5 to 18 feet is the correct length.

 

You can nit pik it till the cows come home, but remember you are dealing with a 5 Watt transmitter, and ground wave out 5 to 15 miles.

 

Wether your antenna radiates 2.5 Watts or 4 Watts, ain't going to make that much difference.

 

And the SWR, if you have a commercially made 1/4 wave antenna, set the adjustment at about Mid Range of the adjuster, and your SWR is well in the ball park.

 

Probably perfect, on channel 19 !! I have been building Ham Radio Antenna's since about 1959, CB antanna, Ain't no different.

 

73, from N7UY !!

[Bob Myers]

Boy the trolls are out tonight. I stand by my answer.

 

This is an excerpt taken from the book called "Ultimate Guide to 11 meter(CB) antennas. Not that i wish to call you wrong by standing by your answer, but, to only illustrate what I was taught 39 years ago in electronics school. I'm reasonably sure the theory has not changed.

 

Why does my coax length affect the SWR of my antenna?

How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistant SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistant SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistant SWR readings when the SWR meters position on the coax or length of the coax is varied. In practice, generally you will find varying the coax length seemingly effects the SWR reading. Most SWR meters (built into radio and external type meters) and impedance "humps" in coax lines and connectors will cause minor variations in SWR as jumpers and coax length are varied. In reality, the mismatch at the antenna's feedpoint / coax junction is unchanged. Therefore - the actual SWR is unchanged.

Another reason SWR could vary is from the situation where the coax is acting as part of the antenna. Not a favorable or normal situation. The signal is traveling back down the outside of the of the coax braid (note power should only be traveling on the inside on the coax braid). Therefore, the coax is part of antenna system and changing the coax length will change the SWR. This situation is more likely to occur in mobile installations. You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidily between two values, you might have a intermitant (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change.

[GeorgeS]

I'm trying to keep it simple. ( for those who are not electrical inclined )

 

The commercially built antenna is allready Resonant on the CB Band.

The antenna length adjustment set about mid range, is good enough.

 

Useing and odd number of 1/4 wavelengths of coax is not going to " Tune " the antenna. Or change the SWR, It simply puts more power into the antenna, or you can keep it simple, and say it just works better that way.

 

Actually any length of coax will work, ( Note Yamaha, used the exact length between the antenna and transmitter ( Intersting ?? ) It works just fine.

 

However an odd number of 1/4 wavelengths will work a little better, ( but nobody will ever know )

 

This is NOT a Ham Radio Station, transmitting with 1000 Watts of power.

 

Yes: IF I was building an antenna from scratch, I would use all the test equipment, and bla bla bla, you know the drill,

 

But this is just a CB system, 5 Watts, All this technical talk is simply not necessary.

[Bob Myers]

I'm trying to keep it simple. ( for those who are not electrical inclined )

But this is just a CB system, 5 Watts, All this technical talk is simply not necessary.

You are absolutely right George, it is not necessary! And neither was the crack made earlier by Stoutman. So I'll sit down and be quiet again.

[Phaedrus]

You guys stopping now? This was just starting to get fun! You were only about two posts away from having the moderators jump in here.

 

Stoutman, you were right about one thing, it was probablly more than they wanted to know.

 

Wayne, I hope you can make sense of this all, cause I was starting to get confused. I guess it is all about who comes across as the most knowledgeable. From what I've read so far you should:

 

Leave the coax at 18 feet, just don't coil it to tightly.

 

Cut is as long as it has to be to reach the antenna.

 

If you have to cut it, go through stoutman's mind numbing explanation. ( By the way stoutman, the CB you helped me put in my truck is working great. Glad you didnt' explain it to me, just made it work.)

 

Make it whatever length you want because it won't affect your SWR. (don't know who said it would affect SWR, but OK).

 

Just put it in, cause it's only a 4 watt CB and it doesn't matter that much anyway. (George sounds like the winner here. 73s)

Edited June 23, 2009 by Phaedrus
clarity

[flb_78]

Boy the trolls are out tonight. I stand by my answer.

 

I take offense to being called a troll, sir, assuming you know the proper definition of "troll".

 

73's - N9OHM ex.KB9CQW

 

In Internet slang, a troll is someone who posts controversial, inflammatory, irrelevant, or off-topic messages in an online community, such as an online discussion forum or chat room, with the primary intent of provoking other users into an emotional response [/url]or to generally disrupt normal on-topic discussion.

[LilBeaver]

I am curious here - With the stock setup on the venture, what is the approximate range of the CB for a clear transmission (assuming optimal conditions, no powerlines or other sources of interference, etc.)?

[Stoutman]

I take offense to being called a troll, sir, assuming you know the proper definition of "troll".

 

73's - N9OHM ex.KB9CQW

 

[/u]

fb_78, You are not a troll. I am the troll. I hereby restrict myself to read only access.

[rbjohnsn]

I am curious here - With the stock setup on the venture, what is the approximate range of the CB for a clear transmission (assuming optimal conditions, no powerlines or other sources of interference, etc.)?

 

Optimal Conditions in the 11 meter band! LOL, maybe 5 miles.

 

Find a high point, right weather conditions (skip) and you could make 1000 mile plus or minus 995 mile. The 11 meter band is the worst for RF communications. That is why the FCC gave it to use.

 

The whole idea of the SWR (Standing Wave Ratio) is to match the impedence of the radio to the attenna, which is an ideal 50 ohms. The longer the coax cable more insertion losses. Each connector has an insertion loss associated with the style of the connector.

 

An SWR of 3-1 is great, 2-1 Ok, 1-1 bad. A 1-1 SWR means all the power being sent to the attenna is also being reflected back to the source. When this occurs there is nothing being trasmitted, it's all returning to the transmitter.

 

Hope this helps.

[Bob Myers]

An SWR of 3-1 is great, 2-1 Ok, 1-1 bad. A 1-1 SWR means all the power being sent to the attenna is also being reflected back to the source. When this occurs there is nothing being trasmitted, it's all returning to the transmitter.

 

Hope this helps.

Sir, I do believe you have your numbers transposed. Technically a 1.0:1 is perfect match, 2:1 is acceptable but not desirable, 3:1 is danger zone

[Guest tx2sturgis]

An SWR of 3-1 is great, 2-1 Ok, 1-1 bad. A 1-1 SWR means all the power being sent to the attenna is also being reflected back to the source. When this occurs there is nothing being trasmitted, it's all returning to the transmitter.

 

Hope this helps.

 

Ooops...thats exactly reverse of the facts. 1 to 1 is a perfect match. It means that for every 1 watt transmitted (by the transmitter), 1 watt is radiated by the antenna. And a reading of 1.5 to 1 means that for every 1.5 watt transmitted, ONE watt is radiated, and so forth. A reading of 3 to 1 means that three watts are transmitted, but only one is being radiated, and two watts are returning to the transmitter, meaning: Not a good match.

 

On coax length:

 

If the antenna is matched perfectly, or nearly perfect, (1 to 1) the coax can be ANY practical length, 1 foot, 5 feet, 12 feet, 15 and a half feet, whatever. The meter will always read a perfect match no matter where it is placed in the feedline for measuring.

 

What trips people up is that if the antenna feedpoint match is off ( meaning not resonant), then the reading on the METER will change, depending on how many feet of coax are between the meter and what it is measuring. A typical SWR meter is actually a VSWR meter, and it is measuring VOLTAGE, Standing Wave Ratio. It is seeing a high VOLTAGE or low VOLTAGE along the Standing Waves in the feedline. Moving the meter up and down the feedline, or changing the feedline length, changes the high and low voltage points along the feedline, and the uninformed user may assume the match is changing, when actually, its only the meter reacting to different voltages along the length of the coax.

 

Again, if you have a good well matched, single resonant antenna, the coax can be ANY length.

 

Now if were talking about co-phased dual 'trucker' type antennas, then YES, you have to retain the pre-set length of a half wavelength per side ( 18 feet) , because most cophase antennas are made of 75 ohm coax, NOT 50 ohm coax, and it get very technical explaining the theory that makes this work. It doesnt apply to motorcycle CB anyway, so I'll stop there.

 

 

 

 

 

[BOO]

Darn, how did I miss such a lively discussion and right up my alley too.

I see there were some deleted postings also, damn hate when that happens.

Bottom line to all this discussion in my opinion is "if it works use it". I'm kind of a communications freak but I got to tell you I hardly ever turn the CB on on my bike.

Nice discussion though,

BOO

[Bob Myers]

I'm kind of a communications freak but I got to tell you I hardly ever turn the CB on on my bike.

BOO

Yep, sure paid a lot of my bills in the late 60's and 70's, I would bet I have a dozen radios, here and there, prefer my 2m or 6m radio on the bike.

[rbjohnsn]

Sir, I do believe you have your numbers transposed. Technically a 1.0:1 is perfect match, 2:1 is acceptable but not desirable, 3:1 is danger zone

You are correct. Sorry, I guess I am just getting to old anymore. I stand corrected.