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 Post subject: Re: CB radios
PostPosted: Thu Feb 11, 2010 7:37 pm 
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Guys with a short coax aways say that. :roll: :lol:

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 Post subject: Re: CB radios
PostPosted: Thu Feb 11, 2010 8:12 pm 
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Location: Green Valley/Vegas
andy02 wrote:
Goat wrote:
Brad's right! The thing you have to remember is to NOT cut any more cable than you have to. The antenna cables have to be 18.(something) feet long to perform well.


I could go into a lot of numbers, velocity factors of RG-58 coax, wavelengths and such but the 18 foot thing is just a myth. You only need enough to reach from your radio to your antenna.



WHAT?...I CAN'T HEAR YOU!!!

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 Post subject: Re: CB radios
PostPosted: Fri Feb 12, 2010 12:05 pm 
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Location: Henderson, NV
I agree with Andy ! My rig works really well, I've talked and listened at ranges of 7 to 8 miles. My coax runs from the hard-mounted base loaded whip on the roof, down the front window post on the left , under the dash and to the radio.. About 7 feet !

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 Post subject: Re: CB radios
PostPosted: Sat Feb 13, 2010 7:41 am 
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Interesting discussion about coax length. I install lots of radios, antenna's etc. in cars, trucks, jeeps, or whatever. Usually 9' or 18' works really well. If you measure the coax on a magnet mount system it frequently is 17'. I think that has to do with the way a mag mount is coupled or grounded to the vehicle. Seems like the bottom line is if you get a good "match" aka swr reading of 1.5 to 1 or lower, your transciever will work good and last a long time. While you can make a non standard length work, you usually have to trim a lot of the whip to get the antenna to a resonant length. Just my opinion.

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 Post subject: Re: CB radios
PostPosted: Wed Feb 17, 2010 8:22 am 
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Location: Durango & 95
Okay, here we go.

A CB works on a frequency of 27.4 MHz and has a wavelength of 36 feet. A little math tells us that a half wavelength is 18' and a quarter wavelength is 9'. All sounds good so far. The reason that people claim that this is good because supposedly the RF sine wave would be at zero at the end of the cable and if our coax were perfect conductors, I would agree with them. The problem is that RG-58 coax has a very well known resistance. Known in radio circles as velocity factor. The VF of RG-58 is .66 So, if we multiply the 18' by the velocity factor we get the length we should be using if the length were an actual issue. 18x.66=11.9

In other words, in order for the sine wave to drop back to perfect zero the cable would actually have to be 11.9 feet long. With that, you either have to admit that you are actually adding to the problem of high SWR by using a cable that is to long or you have to admit that cable length has no bearing on SWR. So why do antenna companies keep saying that you need 18'? Because they are using an old wives tale to pad their bottom line. Go to a truckstop and find out the price difference on a 10' section of coax and an 18' section of coax.

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 Post subject: Re: CB radios
PostPosted: Wed Feb 17, 2010 7:19 pm 
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Again just my opinion. I don't use rg58 I use rg8x. It is Belden brand and I buy it in 500' rolls. I go through 1-2 rolls per year. It is 97 % percent shielded but I use this brand for price, not because of the heavy shielding. I measure the swr with an MFJ analyzer but don't remember the model #. 259 or259b, not the one that goes to 440 mhz. Anyway, when I check the match, I am happy with a match of 1.5 or lower and an impedance of 40-60 ohms. A friend of mine is a factory trained cb technician servicing Ranger and other brands. He likes multiples of 12' for mobile installations. He doesn't use the same coax I use. It probably has to do with the velocity factors of the differing cables but I don't like the match I get with my coax at 12' multiples. My cables match well with good impedance at 9 and 18'. For my use those lengths work well so if it aint broke don't fix it. Using the 12' lengths, I can get a respectable match if I trim a lot of the whip, or wire coil at the top of a fiberglass antenna like a firestik etc. I don't see the point in making an antenna shorter for no reason. 73 KZ7L

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 Post subject: Re: CB radios
PostPosted: Thu Feb 18, 2010 8:46 am 
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Joined: Sat Aug 08, 2009 9:03 pm
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Location: Winnemucca, NV
I use what came with the antenna. Because, like I originally stated, I use my CB very infrequently, and I'm not trying to talk to the left coast. As long as I can be heard by the one or two vehicles ahead and behind me, I'm happy ... :wink:

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 Post subject: Re: CB radios
PostPosted: Thu Feb 18, 2010 1:23 pm 
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Joined: Sat Dec 03, 2005 9:24 pm
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Location: Henderson, NV
andy02 wrote:
Okay, here we go.

A CB works on a frequency of 27.4 MHz and has a wavelength of 36 feet. A little math tells us that a half wavelength is 18' and a quarter wavelength is 9'. All sounds good so far. The reason that people claim that this is good because supposedly the RF sine wave would be at zero at the end of the cable and if our coax were perfect conductors, I would agree with them. The problem is that RG-58 coax has a very well known resistance. Known in radio circles as velocity factor. The VF of RG-58 is .66 So, if we multiply the 18' by the velocity factor we get the length we should be using if the length were an actual issue. 18x.66=11.9

In other words, in order for the sine wave to drop back to perfect zero the cable would actually have to be 11.9 feet long. With that, you either have to admit that you are actually adding to the problem of high SWR by using a cable that is to long or you have to admit that cable length has no bearing on SWR. So why do antenna companies keep saying that you need 18'? Because they are using an old wives tale to pad their bottom line. Go to a truckstop and find out the price difference on a 10' section of coax and an 18' section of coax.


Holy Calculus! Batman!

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 Post subject: Re: CB radios
PostPosted: Fri Feb 19, 2010 12:34 pm 
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Location: Durango & 95
rainman wrote:
Again just my opinion. I don't use rg58 I use rg8x. It is Belden brand and I buy it in 500' rolls. I go through 1-2 rolls per year. It is 97 % percent shielded but I use this brand for price, not because of the heavy shielding. I measure the swr with an MFJ analyzer but don't remember the model #. 259 or259b, not the one that goes to 440 mhz. Anyway, when I check the match, I am happy with a match of 1.5 or lower and an impedance of 40-60 ohms. A friend of mine is a factory trained cb technician servicing Ranger and other brands. He likes multiples of 12' for mobile installations. He doesn't use the same coax I use. It probably has to do with the velocity factors of the differing cables but I don't like the match I get with my coax at 12' multiples. My cables match well with good impedance at 9 and 18'. For my use those lengths work well so if it aint broke don't fix it. Using the 12' lengths, I can get a respectable match if I trim a lot of the whip, or wire coil at the top of a fiberglass antenna like a firestik etc. I don't see the point in making an antenna shorter for no reason. 73 KZ7L


I talked with a radio technician that I work with about this because after reading your post I had a lot of questions. From the extreme dumbed down version that I could wrap my brain around, we are both right. In a perfect world, cable length doesn't matter. He went into detail about transmitted power traveling through the center conductor and getting reflected into the outer conductor which should carry no power (after typing that I realize he was talking about SWR!). He told me that when that happens the coax basically becomes part of the antenna and that by coiling the excess you are creating a RF choke point that helps to mitigate the problem. He also told me that the choke point should be as close to the antenna as possible because the area between the antenna and the choke point is still acting as part of the antenna and is in essence wasted power. He told me that cable length doesn't really matter so long as you have between 6 and 8 coils. He also told me that the problem doesn't always present itself which is why some installations (Phil's for instance) can get away without coils and still not have the high SWR.

With that, I want to say thank you. I understand antenna and coax theroy a little better now than I did before and never would have asked the questions if it hadn't been for this discussion.

Edit: Just found something that explains it better than I can. Have a look if you like: http://www.signalengineering.com/ultima ... asics.html
I also found that I still really don't understand the concepts and get lost in the technicals of it all. :mrgreen:

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 Post subject: Re: CB radios
PostPosted: Sat Feb 20, 2010 8:32 am 
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Thank you also Andy for an intersting discussion. I am not formally trained in electronics, rf energy or other aspects of cb radio systems etc. I became interested in radio because I am a truck driver and got ripped off by one of the local cb shop's about 15 years ago. I too have had to deal with the wives tales repeated by people and some manufacturors. A lot of the hard theory is way beyond my scope of comprehension. I have had the benefit of having friends who are fellow ham radio operators and their experience has helped me a lot. Anyway, my family goes on NBR runs occasionally so I hope to see you on the trail.

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 Post subject: Re: CB radios
PostPosted: Sun Feb 21, 2010 10:11 pm 
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Now I'm looking at were to mount my CB. I just picked up a travel Trailer so I need to also make room for the brake control.

Anybody have any luck with those stake-hole antenna mounts?


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 Post subject: Re: CB radios
PostPosted: Mon Feb 22, 2010 2:06 pm 
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Location: Henderson...of course
>>> but the 18 foot thing is just a myth. You only need enough to reach from your radio to your antenna

Abso-LUTE-ly!

FYI, comparing CB radios (devices much less complicated than the Display Driver in your "sail" phone) is like comparing salty snack foods or picking out a dog at the Shelter: there is no discernible difference between them, except for What Works Fer Yew, so get one you like the looks of and take it home.

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 Post subject: Re: CB radios
PostPosted: Mon Feb 22, 2010 8:38 pm 
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For me a cb radio needs to be a full featured one like a Cobra 29 or a Uniden 78. I want a mic gain and rf gain knob. I also like the extra noise filters (anl and nb) to take out static and hash noise. A built in swr meter is handy also. Some people get by just fine with a simple radio like a Uniden pro510xl or a Cobra 19dxIV. I want more features and options. Space limitations can be an issue, but I can always make the bigger case fit somewhere. The bigger radio's usually have improved modulation. A little extra range can help in some situations. I forgot to add nothing helps more than a properly tuned antenna.

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 Post subject: Re: CB radios
PostPosted: Tue Feb 23, 2010 9:16 pm 
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 Post subject: Re: CB radios
PostPosted: Thu Apr 01, 2010 9:10 pm 
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This is an old thread so maybe I am wasting my time but what the heck. Some people are adamant that the evil antenna and coax manufacturors are making billions of dollars by selling you a coax cable that is 9' or 18' in length. They say that you only need to take a cable from your radio to your antenna and not waste your money spending all those hard earned dollars buying miles and miles of unneeded cable. Where do you get a cable of a non-standard length? Does everyone know somebody with a roll of coax cable laying around? Is everyone qualified to properly install a pl259 coax connector? If the 9' and 18' lengths work and the 3' multiples which some say are just wives tales help your cb system overcome various matching and broadbanding issues, what is the point cutting a cable to go only from your radio to your antenna? Truck stops and cb shops sell 3' multiples of coax because they work. If it works, why do you need to fix it? I have seen postings from some socalled experts trying to help you overcome these problems. In a discussion with Andy02, a man he worked with described a sittuation where you can use any length of coax as long as you make an rf choke of 6-8 turns of cable near the feedpoint of the antenna. That means you have a heavy loop of cable outside your vehicle below the antenna that can catch on things or will flop around on bumpy roads etc. Why would you do that? Just to save you those countless dollars so you can prove a point? I try to help people solve their problems and not re-invent the wheel. A good basic cb system will give you years of service with minimal upkeep. Again, just my opinion.

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