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Coax length

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Coax length

Post by DaFuzz » Friday, 20 February 2015, 12:10 PM

in a mobile setup does coax length really matter and what will it affect? Does the coax have to be 18'? Do multiples of 3' matter? The guy that sold me my gear to set up my cb from scratch said it doesn't matter as long as it's quality coax but it's best to have a multiple of 3' so he cut it at 15'. I see on Firestiks tech page that they seem pretty set on 18'.

I'm still chasing my tail as far as having 2-2.25 SWR no matter what I try so I'm wondering if my coax has something to do with it. 15' is the perfect length with no loops or extra cable but I could do 18' and just put the extra behind my extended cab back seat without making any loops. Thanks in advance for any insight you can provide.


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Re: Coax length

Post by Bombero » Friday, 20 February 2015, 12:27 PM

Hey..

You only need enough coax to get from the antenna to the radio.

The 18' or 3 ' increments is not necessary...as far as I have read.

If your coax is 15' and you don't need it all .. coil it and put a plastic tie in the middle of it...sort of a figure 8.

Good quality coax is a must IMO.

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Re: Coax length

Post by 231 » Friday, 20 February 2015, 12:46 PM

DaFuzz wrote:in a mobile setup does coax length really matter and what will it affect? Does the coax have to be 18'? Do multiples of 3' matter? The guy that sold me my gear to set up my cb from scratch said it doesn't matter as long as it's quality coax but it's best to have a multiple of 3' so he cut it at 15'. I see on Firestiks tech page that they seem pretty set on 18'.

I'm still chasing my tail as far as having 2-2.25 SWR no matter what I try so I'm wondering if my coax has something to do with it. 15' is the perfect length with no loops or extra cable but I could do 18' and just put the extra behind my extended cab back seat without making any loops. Thanks in advance for any insight you can provide.
As Bombero said, the correct length for a mobile is what it takes to get from the gear to your antenna system. And the guy is right...good quality coax is helpful.

Base stations are slightly different depending on the layout. Either way the goal is to end up with a 50 ohm impedance.

So if the guy installed your gear, why didn't he adjust the standing wave for you? What antenna and mounting system did you do on what rig? (Please be specific)

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Re: Coax length

Post by MDYoungblood » Friday, 20 February 2015, 12:57 PM

Since you said extended cab it's a pickup but we need to know is the antenna and mount? How did you run the coax into the cab? In checking the SWR are you using an external meter the one in the radio, they are not always that accurate. Where are you checking the SWR, in the clear is best, doors shut.

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Re: Coax length

Post by TheCBDoctor » Friday, 20 February 2015, 13:00 PM

DaFuzz wrote:in a mobile setup does coax length really matter and what will it affect? Does the coax have to be 18'? Do multiples of 3' matter? The guy that sold me my gear to set up my cb from scratch said it doesn't matter as long as it's quality coax but it's best to have a multiple of 3' so he cut it at 15'. I see on Firestiks tech page that they seem pretty set on 18'.

I'm still chasing my tail as far as having 2-2.25 SWR no matter what I try so I'm wondering if my coax has something to do with it. 15' is the perfect length with no loops or extra cable but I could do 18' and just put the extra behind my extended cab back seat without making any loops. Thanks in advance for any insight you can provide.
Hi DaFuzz,

This a subject with more posts than any other. If you look at any mobile antenna packaging from Wilson to Francis antennas it requires 18 feet of coax. Yes, in theory the length of coax shouldn't matter, but if the manufacture states to use a specific length of coax then use that specific length. Almost all mobile antenna manufacturers use 18 feet.

It is my believe it is because the antenna coax shield becomes part of the antenna system. Unlike Ham and commercial radios the case of the CB radio is not at chassis ground, but is in fact part of the antenna ground. Ohm out the chassis ground to the case ground and you will see a difference in potential. Once the antenna is hooked up the case and the chassis grounds are connected together at the antenna connector (SO-239). That is why the length matters in my humble opinion.

Whatever length is suggested by the manufacturer that is what you should use, then trim the antenna to get the SWR down if needed. I have been doing this for 30 years and I follow the manufacturer's recommendation and it always works. 90% of them state use 18 feet of coax.

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Re: Coax length

Post by DaFuzz » Friday, 20 February 2015, 13:36 PM

My setup is as follows:

Truck is a 2003 GMC Sierra HD ext cab short box with a sled deck on it. Sled deck is a 1"X2"X1/8" steel structure with an 8'X8' plywood deck.
Radios a Uniden 880 all stock no mods powered directly from my backup isolated battery.

Coax is 15' of something labeled Belden Pro Trucker and some unreadable numbers with soldered on PL259 ends run from under cup holder along passenger side under door step plastic, out passenger side of cab and up to antenna mount

Antenna mount is a Firestik SS64a mounted on top rail of sled deck in center so about 8-10" above top of bed rail. 10 gauge wire is run from a mounting bolt on the SS64a directly to a wire wheeled clean, shiny new hole in the drivers side frame rail where it's bolted on with stainless hardware. ANT stud is a Firestik K4a

Current antenna is a Firestik II 5', also have in the garage a Firefly 3', Wilson Flex 4' and an Everhardt STT 4'.

I have been testing SWR on a local frozen lake to limit all possible issues with adjacent structures, trees etc. All testing has been done with doors and windows shut and truck idling, nothing and no one around the truck. Testing has been done using an Astatic PDC2 and the radios internal SWR, both of which consistently show very similar results. I have tried installing a Firestik heavy stainless spring and a Firestik QD during testing but all antennas show too long with either accessory, both of which would be useful to me if I can ever get them to be on with a decent SWR.

I have checked the coax for continuity and the center tests good, shield tests good with no continuity between the shield and center conductor. I have checked for ground with the coax disconnected from the radio by checking continuity from the antenna mount to the sled deck, also to the trucks bed and to the exhaust, all show good continuity so ground shouldn't be an issue. Antenna shows no continuity to any of those grounding surfaces so antenna stud should be fine.

My testing show the 5' Firestik II to have the best SWR, ch 1-1.93, ch 39-2.14, ch 19-1.6. This still seems to be higher than what I should be able to get with such a sufficient ground plane and decent antenna. Most antennas are showing 3+ no matter how I tune them, all of them are showing electrically long too even without the spring or QD. In a perfect scenario I'd like to have the 5' mounted on the QD for everyday use and would be able to take it off and replace it with the 3' on a spring and QD for when going on the forest roads or trails around here where contact with branches is probable but not much range is needed.

Thanks for your help, let me know what I missed with needed info about my setup and I can break it down more

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Re: Coax length

Post by 231 » Friday, 20 February 2015, 16:23 PM

More questions:

- First of all I'm not sure what a 'Sled Deck' is? Does it sit on top of the OEM bed or does it replace the bed entirely?

- How close to the cab is the antenna?

- Is the sled deck ground to the rest of the chassis or truck?

Is this the mount you have?

Image

If so, first thing I would check is to make sure the center lead hasn't pulled through. I do not like those stud assemblies because they fail internally...and with a heavy antenna will literally pull the center lead through. Prior to that though often they'll short out. You say you've checked that but with an volt/ohm meter it may not be enough power to show the failure. But with the transmission of the radio is. With your volt/ohm meter move (wiggle / flop around) the antenna and while it's settling down, check center lead/shield at the coupling (PL-259) to see if you see it short. If not, then it's likely okay and indicates to me a counterpoise connection problem.

Until I know how the sled deck attaches to the vehicle I'm not sure what to tell you about improving your counterpoise. Just because something is wirewheeled and and bolted doesn't mean it's sufficient for the counterpoise connection. If your STT-4' is showing over 2.5:1 and won't adjust any better, I'd say that's the likely culprit and/or it's too close to the cab. My money would be on the counterpoise connection because it's almost always the culprit. Antennas are not forgiving to that either.

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Re: Coax length

Post by jessejamesdallas » Friday, 20 February 2015, 16:30 PM

Coax lengths do not mater...The Antenna builders like Firestick,Wilson, and Francis are wrong...Period.

ONLY TIME coax lengths make a difference is when your trying to install a multiple antenna array and your wanting to create a particular signal pattern, then jumpers between antenna's need to be certain lengths to get the correct out of phase your looking for... like 90° or 135° out of phase or whatever.

Here's one of the best write-up's on the subject I have came across that explains coax lengths in plane English.

http://www.stu-offroad.com/misc/myth-1.htm" onclick="window.open(this.href);return false;
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Re: Coax length

Post by DaFuzz » Friday, 20 February 2015, 16:51 PM

The sled deck consists of a frame that's sitting in the pickups bed and is pretty much a platform that sits about 2-3" above the bed rails level. The trucks bed is still there, the sled deck is removeable. My sled deck sits on a plastic bedliner so isn't grounded from contact but is grounded to the trucks frame via a braided ground strap.

The antenna is within 6" of the cab where the roof meets the rear window so it's not far away but I can't get any further back than that since the sled decks edges are outside any other antenna mounting surface options like bed rails or rear bumper.

Yes, that's the mount I'm using. I've checked it over thoroughly and it's in good shape, nothing broken and center lead is solid, no changes on multimeter no matter how it's moved/jiggled.

When installing the cable grounding from antenna mount to truck frame I wire wheeled to nice clean steel on both sides of frame and drilled a brand new hole so steel was a nice shiny holes it's plenty of nice clean shiny steel on both sides for washers to make contact.

What other way is there to mount the ground cable that would be better than wire wheeling to clean steel and drilling a brand new hole for the bolt? I'm definitely open to modifying that if it's the likely culprit of high SWR. Next question I have is that I was hoping since the FireStik 5' is about 80% above the roof and is a top loaded antenna,couldn't that make the proximity to the rear of the cab less of an issue?

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Re: Coax length

Post by 231 » Friday, 20 February 2015, 19:36 PM

DaFuzz wrote:The sled deck consists of a frame that's sitting in the pickups bed and is pretty much a platform that sits about 2-3" above the bed rails level. The trucks bed is still there, the sled deck is removeable. My sled deck sits on a plastic bedliner so isn't grounded from contact but is grounded to the trucks frame via a braided ground strap.

The antenna is within 6" of the cab where the roof meets the rear window so it's not far away but I can't get any further back than that since the sled decks edges are outside any other antenna mounting surface options like bed rails or rear bumper.
And why is the bedrail out of the question? 6" is too close...antennas need to be above the image plane as much as possible. And being 'next to' metal can throw them off (even top loaded antennas). Firestik antennas are fairly narrow banded and not very forgiving to some installations. At this point I wouldn't take anything for granted.
DaFuzz wrote:Yes, that's the mount I'm using. I've checked it over thoroughly and it's in good shape, nothing broken and center lead is solid, no changes on multimeter no matter how it's moved/jiggled.

When installing the cable grounding from antenna mount to truck frame I wire wheeled to nice clean steel on both sides of frame and drilled a brand new hole so steel was a nice shiny holes it's plenty of nice clean shiny steel on both sides for washers to make contact.
I'd still use a self tapping screw. Just throw one through the braided ground strap with a stainless washer. Do it to both the sled and frame point you did. I've seen plenty of what appeared to be good counterpoise connection points and come to find out they weren't.
DaFuzz wrote:What other way is there to mount the ground cable that would be better than wire wheeling to clean steel and drilling a brand new hole for the bolt? I'm definitely open to modifying that if it's the likely culprit of high SWR. Next question I have is that I was hoping since the FireStik 5' is about 80% above the roof and is a top loaded antenna,couldn't that make the proximity to the rear of the cab less of an issue?
Is there a reason it has to be mounted in the bed? From what I'm picturing the antenna is almost in the tub of the bed? Is this an appearance thing or something else?

Antennas need to be above the image plane to properly work. That's not to say they won't in less than optimal circumstances...some will some won't. If the coax, mount, and meters all check out, then that leaves only one thing...the mount and antenna. Being you have a few antennas to choose from and all of them aren't tuning properly, my energy would be on the mount location and/or counterpoise.

I would be considering another location myself...but without an image of the rig sort of premature for me to suggest. I could be picturing it entirely wrong.

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Re: Coax length

Post by DaFuzz » Friday, 20 February 2015, 20:03 PM

I sure wish I could figure out how to attach a pic to this thread. If you google search sled deck and look at the first few images that pop up that's very similar to what I'm working with except I have no headache rack, just a front bar going across behind the back window.

If you imagine laying a sheet of plywood across the top of a pickups bed rails at the front of the bed, that's pretty similar to what a sled deck is but the deck sits higher about 4-6" due to the framework underneath. There's only about 8-10" of antenna that's below the top of the trucks roof, that's why I thought it'd be almost as good as a roof top mount but a lot sturdier for inadvertent tree limb contacts. I have 4'+ of the 5' antenna above the roof of the truck.

After reading the comments I'm really leaning towards working hard at better ground strapping this weekend. It's gotta be something with the ground plane or counter poise or whatever it is. When I re do some ground straps should I ground everything or is there a possibility that it's too much ground plane? I could isolate the sled deck and just use that steel framework isolated from the truck frame and bed and everything. Or I could just check over all ground straps and redo the main 10 gauge cable that I've got going directly from the antenna mount to the frame rail and use stainless self tapping screws instead of bolting it through the frame rail. Or I could just use a regular braided ground strap and self tapping screws into the pickups bed since that way it'd be a lot shorter than the 4.5-5' of 10 gauge cable.

Thanks again for all the replies and help! I'll keep answering any questions about my setup since its kind of a hard thing to visualize, not exactly a standard pickup installation

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Re: Coax length

Post by 543FtWorth » Saturday, 21 February 2015, 1:32 AM

If the antenna is installed properly the coax length doesn't matter. For mobile installs I like to run an electrical 1/2 wavelength of coax to the antenna (12 ft of big rg8 or 213) so the swr reading is fairly accurate. As for my base I have no idea how long the run is, for most people the length doesn't matter. With certain antenna systems and connecting drivers to larger amps you can run into phasing issues but those at that level have already or will have to figure it out.

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Re: Coax length

Post by 231 » Saturday, 21 February 2015, 1:35 AM

You'll be able to post limited sized images at 10 posts. We have security functions built into the forum software to protect from hecklers and spammers posting undesirable images (which has happened before). So as you post you'll have more functions automatically available to you.

Okay, I googled the sled and see what you are working with. Indeed, while there is allot of metal there if not bonded completely may not be enough to act as the counterpoise.

Antenna systems 101: There are two parts to the antenna. The antenna itself (main radiator) which is isolated from ground (center lead of coax), and the counterpoise (aka 'ground') (shield of coax). The counterpoise connection needs to be an RF ground, not just electrical ground. It's a 3rd component related to the skin effect of RF. There is no room for error...either it's good enough or it's not, there is no middle ground. Every antenna has a counterpoise, whether through elements, conductive materials (like the vehicle), etc. In mobile applications the vehicle is the counterpoise. With magnet mount antennas it's through capacitive coupling but still uses the vehicle. Base stations, through matching networks and/or elements most often. But all have it.

I run a front fender mount on my pick up(s). There are a variety of locations a person can mount the antenna but generally speaking antennas like to be above the image plane / mass of reflective counterpoise. They are forgiving some to that, but not forgiving to the connection. Your mounting the antenna on the sled is probably just fine, it's the bonding between the the sled and truck bed, and bed to chassis that may be missing. I realize you wire wheeled the connection points and sometimes that's good enough. But truly all that's needed to finish the RF counterpoise connection is something to really tightly dig into the metal.

Below (2nd image down) is an image of my ball mount connections on my Jeep. It's just an illustration to you of how simple the counterpoise connection can be, nothing more. I think if I were you I would add a secondary ground wire from the bottom of the mount (1st image) where the stud connection meets the angle mount additionally. I've seen doing that help. Here is sort of an image of what I'm talking about adding a secondary wire:

Image

My Jeep's connections for illustration:

Image

My front fender mount with Everhardt SOTT-5'

Image

I think your idea of working on the bonding between the sled, bed of the truck, and chassis is a good one. I'm telling ya', it doesn't have to be a large self tapping screw to make the connection. Some folks have luck with star washers too...but I've found the best using a small stainless self tapper. Even my fender mount is held on by the strong body fender bolts, but I have 2 self tappers additionally which sole purpose is to insure the counterpoise connection.

Just a few thoughts additionally that might help you visualize where to focus.

Let us know how it goes.

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Re: Coax length

Post by DaFuzz » Saturday, 21 February 2015, 18:46 PM

Well this forum ROCKS! After some serious ground strapping my 5' FireStik II SWR is now ch1-1.2, ch39-1.28, ch19-1.17 and I'm even able to use the QD now with those results. 3' Firefly with QD and spring is ch1-1.57, ch39-1.65, ch19-1.4. 4' Wilson Flex and 4' STT are in the low to mid 1s too so all antennas now SWR good. Connected the sled deck to the bed, bed to frame rail using 2 gauge stainless flat braided ground straps and stainless self tapping screws, three screws per connection point in case one corrodes or vibrates loose.

Thanks to everybody who provided advice and assistance in this thread, it was a fun little problem solving project with a positive end result thanks to your help.

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Re: Coax length

Post by 231 » Saturday, 21 February 2015, 23:58 PM

That's great news...congrats'!!! Always a nice feeling to get that going correctly. Now you can enjoy the performance they were intended for.

By the way, now that you have the counterpoise connection(s) decent drop some silicone on them to keep moisture and corrosion out. Any of them will work...doesn't have to be anything special.

Thanks for letting us know you are in business.

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Re: Coax length

Post by DaFuzz » Sunday, 22 February 2015, 8:59 AM

Good idea on the silicone, I'll do that.

231, it looks like you shrink wrap your antenna spring and most of you QD? Does that affect performance/tuning at all? Does the spring still flex fine or is it pretty rigid? When messing with my 5' FireStik II I found that even the heavy spring is way too soft at highway speeds, that antennas all over the place and laying over pretty good. The 3' Firefly works good with the spring but I think the 5' is too much for it

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Re: Coax length

Post by drdx » Sunday, 22 February 2015, 10:38 AM

I am like 543, I like to run a half wavelength based on the velocity factor of my coax, but in reality on my stuff that happens to be almost the exact length (within a foot) from my setup to the antenna and I have even ran a choke at the antenna feed on some. Maybe I'm just superstitious but it works for me. The only thing I haven't tried is a choke at both ends but don't rule that out of my future. It really shouldn't matter but after decades of reading that's what I do. On a low power setup I wouldn't bother but once the wattage rises all kinds of gremlins tend to appear.


And remember, even under the mount, once that coax center leaves the shield, it is all part of the antenna length.

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Re: Coax length

Post by 231 » Sunday, 22 February 2015, 16:36 PM

DaFuzz wrote:Good idea on the silicone, I'll do that.

231, it looks like you shrink wrap your antenna spring and most of you QD? Does that affect performance/tuning at all? Does the spring still flex fine or is it pretty rigid? When messing with my 5' FireStik II I found that even the heavy spring is way too soft at highway speeds, that antennas all over the place and laying over pretty good. The 3' Firefly works good with the spring but I think the 5' is too much for it
The Firestik antennas are just too heavy...there isn't hardly a HD spring that'll keep it reasonably vertical at highway speeds that I know of. They flex worse than the 102" whip...so I'm not sure what to tell you.

Yes, I shrunk wrapped them and there is no performance difference whatsoever. I don't think it stiffens up the spring that much (if at all) but it sure wouldn't hurt to try it. Maybe even a couple of wraps? I run my heavier antennas on it and they don't flex too much with or without it. The spring is really to help share impact energy with the antenna and help it not have to absorb the entire brunt of it. Give it a try...it might help a little.

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Re: Coax length

Post by TheCBDoctor » Monday, 23 February 2015, 9:41 AM

Good morning DaFuzz,

It is good to hear you solved your SWR problem. There are a couple of things I would like to add. The first is that a ground plane is not a few ground straps connected to a mount. A true ground plane on an Isotopic antenna is a sheet of Iron (Or Steel that a magnet can attach to). It should be 6 feet in diameter. This is not always possible or necessary.

If you want an antenna that will not lay over on a mounting spring I recommend using a Francis antenna. Francis used to make antennas from 3 feet to 8 feet in length. Today they only make 5 1/2 foot antennas. That is a shame, but what can we do?

An SWR of 1.5 to 1 will provide a 4% loss. At 2 to 1 the loss is 11% at 3 to 1 the lose is 25%. Anything under 2 to 1 is acceptable. No one will notice an 11% loss since you must cut your power in half or double it to make any significant changes to a receiving station when transmitting line-of- sight.

I have found that when using an HR-2510, which has a common chassis ground at both the main board and the case, the coax length will not matter. It does matter on a CB because the case ground and chassis ground are not the same. You are working with a coax length under the full wave length of the CB band (36 feet).

All antenna systems are mounted differently when it comes to base stations. In this situation I have found that coax length does not matter even for CB radios.

18 wheelers used to made of steel and iron; mounting an antenna was not a problem back then. Today, they are made of plastic, Kevlar, and Aluminum. This presents a problem. Most new Cabs use an impedance matching unit to fool the antenna into working even with a poor ground plane.

It has been noted that Francis and Firestix are wrong for recommending 18 feet of coax. If so, then they have been wrong for the past 30 years and they must employ monkeys instead of engineers to design their antenna systems. I doubt they employ monkeys and have been manufacturing antennas long enough to know what works and why.

Theory is one thing; practicality in the real world is another. 102 inches of a stainless steel quarter wave whip may take any length of coax without effecting the SWR. Most antennas are 5/8 wave and anything in between. The ground planes on modern vehicles, especially 18 wheeler are poor; The case ground and chassis ground on the CB radio are not the same. The antenna manufacturer's suggests specific coax lengths for a reason. One company I bought antennas from suggested 9 feet of coax. In theory if one doubles the length of coax or cuts a length of coax in half it should not change an SWR.

I have been doing this for 30 years and I am retiring. I am not going to disagree with others as to coax lengths. I can only speak for myself and what I have found works in the real world. As I mentioned in an earlier post there are more posts on this subject on this website than any other subject. No one seems to be able to agree. For me? I don't care what a theory book may say. I worked in freezing cold, rain, snow, heat that would fry an egg; I have been doing this for a while. No theory book has ever installed an antenna system under the conditions I have for as long as I have.

You do have a good SWR, but the next time you need to swap out coax try 18 feet of coax and I will bet dollars to donuts your SWR reading will be flat across channels 1-40. If you ever get an 18 foot of coax just run it from the antenna to the CB and see what your SWR from 1-40 is. You won't need to run it through the cab, just go direct as an experiment and see what happens and what readings you get.

Just my humble opinion and Respectfully as always,
Best Regards,

Rick, The CB Doctor


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Re: Coax length

Post by DaFuzz » Monday, 23 February 2015, 10:17 AM

Ok so it sounds like another test is in order. I'm game to test an 18' of coax since I won't have to run it thru the grommets and under interior panels etc, if it makes a difference in performance and having a well set up truck its a small price to pay. What brand of pre made PL259 ended 18' coax would you more experienced CB guys recommend? I'm not really interested in putting on my own ends at all at this point, maybe somewhere in the future I'll jump that kinda hurdle but not yet.

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Re: Coax length

Post by TheCBDoctor » Monday, 23 February 2015, 12:57 PM

DaFuzz wrote:Ok so it sounds like another test is in order. I'm game to test an 18' of coax since I won't have to run it thru the grommets and under interior panels etc, if it makes a difference in performance and having a well set up truck its a small price to pay. What brand of pre made PL259 ended 18' coax would you more experienced CB guys recommend? I'm not really interested in putting on my own ends at all at this point, maybe somewhere in the future I'll jump that kinda hurdle but not yet.
Good afternoon DaFuzz,

It won't be necessary to change coax since a 1.7 to 1 SWR is an insignificant loss. I would be interested in seeing if you do get a flat SWR. It may be 1.7 on all 40 channels, but it would flat. Any changes to get it down to 1 to 1 would have to be made at the antenna. If the SWR remains the same, then my opinion would be wrong.

If others are correct, then any change in your SWR, Bad or Good, means the coax length makes a difference for the CB radio. What does the manufacture suggest for your particular antenna? It would be on the packaging.

Good luck
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Rick, The CB Doctor


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Re: Coax length

Post by DaFuzz » Monday, 23 February 2015, 13:37 PM

The package suggests the standard 18'. I'm aware that it's not necessary to change coax. When I posted that I'm interested in getting some quality coax in an 18' length with PL259 ends I wasn't interested in lowering my SWR which is now acceptable, just getting my set up as good as I can get it. I'm interested in what particular coax to buy or where to buy quality coax from, does anybody have any good leads from their personal experiences?

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Re: Coax length

Post by The DB » Monday, 23 February 2015, 15:21 PM

There are two different points of view expressed in this thread, namely what TheCBDoctor is saying, and what everyone else is saying. They are both correct, at least to a point. They are looking at the situation from different points of view.

I don't recall ever reading on this or any other forum what is happening in this case , why it is suggested, what it does, and why it is right or wrong. I think part of the reason for that is it gets complicated. I'll try and keep this as simple as possible.

Now, there are varying reasons why you may need or want to use various lengths of coax, and those reasons, while both calling for 'a half wavelength of coax' apply the measurement of a half wavelength differently.

For example, if you have a device that can read more than just SWR, such as an antenna analyzer, or some of us may mention a VNA, which is to most people a real fancy antenna analyzer, then some parts of what you are measuring change with coax length. In this case, the half wavelength measurement has to take into account the material between the center conductor and the inside of the shielding. Because the material here is not air, the signal will travel through it slower than if it were air. This is where the velocity factor comes into play that drdx mentioned above... Note that in this case we are referring to specifically the signal that is traveling inside the coax, namely the signal heading towards the antenna and any reflections due to SWR not being perfect.

Another signal in play is a signal that is generally referred to as "common mode currents". This signal travels on the outside of the coax shield, and is caused by something else entirely, an imperfect groundplane is but one possible and the most likely cause. Because it is on the outside of the coax shield, the velocity factor on the coax itself does not apply to it, and instead it has its own velocity factor based on the outer covering of the coax and the air around the coax. The velocity factor of this signal will be much higher as most of the signal is actually in the air around the coax. It is so high in fact that you are close enough to a half wavelength without factoring it in.

To try and show how it works, lets look at a 5/8 wavelength ground plane base antenna. One of the things that the typical four ground plane radials of this antenna design do is give low impedance path alternatives to the coax and mast going to the antenna. This low impedance path essentially absorbs and controls the currents in said part of the antenna. If you use a 5/8 wavelength antenna that doesn't have such a ground plane design, such as the Imax 2000, they are known to have a significant amount of common mode currents which cause other issues, and a wise person who owns such an antenna will find other ways to control said currents.

What the goal of using 18 feet of coax is trying to achieve is just the opposite as the radials on the 5/8 wavelength base antenna above. Instead of presenting the antenna with a low impedance option for the errant currents, you are feeding the antenna with a length of coax that will have a high impedance, and hopefully resist those currents, and thus force them elsewhere in the antenna system.

Note the word "hopefully". That is the key word here. While controlling said currents and keeping them off the outside of the coax is the goal, this is an imperfect method at best of doing that. For example, if that coax is run along metal for any length, which most vehicles are made out of, this will change the velocity factor of the outside of the coax, and thus the electrical length of the outside of the coax, and suddenly your 18 foot physical length isn't as effective as it could be. Further, it is possible that if the coax run is near enough metal that the effect will actually backfire and make the problem worse.

There is also the possibility that, no matter how high the impedance of the outside of the feedline is seen by the signal, that the impedance as seen on the ground plane, or perhaps mirror mount, or whatever the antenna is mounted to, may be even higher. If that is the case, then the 18 feet of feedline won't make a hill of beans worth of a difference.

So long story short, what the 18 foot length coax length does is give you a greater chance of not seeing an issue, and at best acts like a workaround for said issue. If you have such problems then other methods of dealing with them are just as if not more effective, and will have more consistent results.

Now here is how to tell if you have such a problem. All you need is your SWR meter, a female to female coupler for the coax, and 3 or 4 feet of coax. You measure SWR as normal, then you add the coupler and short coax length between the meter and the antenna, then you check your SWR again. If you have a significant change, in this case lets say 0.3 SWR or more, then you have an situation where the 18 feet of coax recommendation that some give may resolve your situation, but remember, even if it seems to resolve an SWR problem it is an imperfect solution at best.

There is another potential problem that I feel should be mentioned. If such common mode currents exist on the outside of the feedline then you cannot be sure that your SWR measurements are correct to begin with. On your SWR meter there are forward and a reflect settings. An SWR meter measures everything that passes through the meter, and that includes both the signals on the inside of the coax and the common mode currents traveling on the outside of the coax. The only way to get a good reading when common mode currents exist it to be at the point where the signal crosses the 0 point. Sounds simple right? Unfortunately, how can you be sure where that point in the coax is?

I could calculate a length for any given frequency to ensure that this happens, however, that length will only be correct for a perfectly straight piece of coax that exists in, essentially, a vacuum. Anything near it will affect it, especially metal... Because of this, if you have this issue, unless you are one of the very lucky few, the SWR readings you are looking at, even with this supposed "magical" 18 foot length of coax, are in error to begin with. How much of an error depends on the situation.

I hope I didn't get to complex, unfortunately this is not something that is easy to explain...


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Re: Coax length

Post by 1206FL » Tuesday, 24 February 2015, 3:37 AM

I have tried several different methods including the 18ft length and by using the VF and specific Freq. I have also just used whatever reached from the antenna to the radio. The last run I made I don't recall the length, I rolled out a 35' piece, measured what I needed and cut it. Installed it and trimmed about 1/2-5/8" off my whip. Vswr reading is about 1.1 on 40 and about the same on 20 and 1. Haven't put an analyzer on it yet as it is being used by a friend. But I did test this setup in a large field away from any obstructions. The length didn't seem to make any difference. This is with a mag mount setup and using lmr240. The run is over 20ft, I know that for sure, but exactly how long I am not sure. Just my experience.


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