The myth of coax length.

[M0GVZ]

I posted this explanation in response to a post in another thread but thought it was worthy of one of its own seeing as it gets brought up so much....

I'll tell you how this whole nonsense came about. (This is simplistic).

Lets take a simple dipole antenna. First of all read this to get a basic understanding of how it works and keep in mind the following graphic. As we can see, current flows in both halves equally which is a good thing. Each leg is an adequate source and return path for the flow of electrons to and from the other leg. A dipole is effectively a perfect self contained antenna from an electrical point of view.

Your vertical is for all intents and purposes one half of a dipole and you need to make up the other half with a RF ground in order for electrons to flow. That is usually done by using radials.

Now here is how the myth gained traction. With antennas with effectively no or poor RF ground the antenna tries to use anything it can both as a source of electrons and giving them somewhere to go, in short to try to balance the antenna. The one thing it has it can use is your coax. On a dipole as you change the length of the legs it alters the tuning of the antenna. Because your coax has become part of the antenna and is effectively being used as the "missing leg", altering its length alters the tuning of the antenna.

CBers (and sadly too many amateurs) not having a requirement for much technical knowledge to put up an antenna didn't realise they had a crap installation with very little or no RF grounding. Some however did notice as they altered the length of the coax the SWR altered and that if they cut it to multiples of a quarter wave long that they got the lowest SWR and bingo, the myth of having to have coax that is 9ft, 18ft, 27ft, 36ft etc for CB was born. It then got regurgitated endlessly. And it was all bollocks but because of the sheer number of poorly installed CB antennas and the fact that this appeared to work it became regarded as fact.

You do not want the coax to be part of the antenna. It means more noise on received signals, it can mean noise on your transmitted audio, you can get RF burns and in a vehicle it can interfere with the ECU, you'll set off neighbours burglar alarms, you'll cause TV interference to you and your neighbours, you'll pick up your transmitted audio on your hifi speakers because we have stray RF going where we don't want it to go. Simple truth is that if altering the length of your coax alters the SWR then you've an inadequate RF ground and you need to be addressing that at the antenna.

So does the myth work? Yes it does but only because there is a fault in your antenna system and this allows it to "work" the same as any botch you can do on a car or an appliance in your home to keep it running for example. The SWR will be low, you'll be able to make contacts but you'll experience the issues I highlighted above and it doesn't fix the problem.

If anyone ever comes out with the "you want to be cutting your coax to X, Y or Z length", it is literally the best indication that they actually don't know what they're talking about and it is advisable to pretty much ignore anything that comes out of their mouths when it comes to antennas and to treat other information with caution. As has been said there are times when you do need coax to be a specific length such as phasing or when using it as filtering in a repeater but for a typical home base or mobile antenna install, no you don't.

 

[wavrider]

Well Said

 

[redbeast]

I posted this explanation in response to a post in another thread but thought it was worthy of one of its own seeing as it gets brought up so much....

I'll tell you how this whole nonsense came about. (This is simplistic).

Lets take a simple dipole antenna. First of all read this to get a basic understanding of how it works and keep in mind the following graphic. As we can see, current flows in both halves equally which is a good thing. Each leg is an adequate source and return path for the flow of electrons to and from the other leg. A dipole is effectively a perfect self contained antenna from an electrical point of view.

Your vertical is for all intents and purposes one half of a dipole and you need to make up the other half with a RF ground in order for electrons to flow. That is usually done by using radials.

Now here is how the myth gained traction. With antennas with effectively no or poor RF ground the antenna tries to use anything it can both as a source of electrons and giving them somewhere to go, in short to try to balance the antenna. The one thing it has it can use is your coax. On a dipole as you change the length of the legs it alters the tuning of the antenna. Because your coax has become part of the antenna and is effectively being used as the "missing leg", altering its length alters the tuning of the antenna.

CBers (and sadly too many amateurs) not having a requirement for much technical knowledge to put up an antenna didn't realise they had a crap installation with very little or no RF grounding. Some however did notice as they altered the length of the coax the SWR altered and that if they cut it to multiples of a quarter wave long that they got the lowest SWR and bingo, the myth of having to have coax that is 9ft, 18ft, 27ft, 36ft etc for CB was born. It then got regurgitated endlessly. And it was all bollocks but because of the sheer number of poorly installed CB antennas and the fact that this appeared to work it became regarded as fact.

You do not want the coax to be part of the antenna. It means more noise on received signals, it can mean noise on your transmitted audio, you can get RF burns and in a vehicle it can interfere with the ECU, you'll set off neighbours burglar alarms, you'll cause TV interference to you and your neighbours, you'll pick up your transmitted audio on your hifi speakers because we have stray RF going where we don't want it to go. Simple truth is that if altering the length of your coax alters the SWR then you've an inadequate RF ground and you need to be addressing that at the antenna.

So does the myth work? Yes it does but only because there is a fault in your antenna system and this allows it to "work" the same as any botch you can do on a car or an appliance in your home to keep it running for example. The SWR will be low, you'll be able to make contacts but you'll experience the issues I highlighted above and it doesn't fix the problem.

If anyone ever comes out with the "you want to be cutting your coax to X, Y or Z length", it is literally the best indication that they actually don't know what they're talking about and it is advisable to pretty much ignore anything that comes out of their mouths when it comes to antennas and to treat other information with caution. As has been said there are times when you do need coax to be a specific length such as phasing or when using it as filtering in a repeater but for a typical home base or mobile antenna install, no you don't.

I posted this explanation in response to a post in another thread but thought it was worthy of one of its own seeing as it gets brought up so much....

I'll tell you how this whole nonsense came about. (This is simplistic).

Lets take a simple dipole antenna. First of all read this to get a basic understanding of how it works and keep in mind the following graphic. As we can see, current flows in both halves equally which is a good thing. Each leg is an adequate source and return path for the flow of electrons to and from the other leg. A dipole is effectively a perfect self contained antenna from an electrical point of view.

Your vertical is for all intents and purposes one half of a dipole and you need to make up the other half with a RF ground in order for electrons to flow. That is usually done by using radials.

Now here is how the myth gained traction. With antennas with effectively no or poor RF ground the antenna tries to use anything it can both as a source of electrons and giving them somewhere to go, in short to try to balance the antenna. The one thing it has it can use is your coax. On a dipole as you change the length of the legs it alters the tuning of the antenna. Because your coax has become part of the antenna and is effectively being used as the "missing leg", altering its length alters the tuning of the antenna.

CBers (and sadly too many amateurs) not having a requirement for much technical knowledge to put up an antenna didn't realise they had a crap installation with very little or no RF grounding. Some however did notice as they altered the length of the coax the SWR altered and that if they cut it to multiples of a quarter wave long that they got the lowest SWR and bingo, the myth of having to have coax that is 9ft, 18ft, 27ft, 36ft etc for CB was born. It then got regurgitated endlessly. And it was all bollocks but because of the sheer number of poorly installed CB antennas and the fact that this appeared to work it became regarded as fact.

You do not want the coax to be part of the antenna. It means more noise on received signals, it can mean noise on your transmitted audio, you can get RF burns and in a vehicle it can interfere with the ECU, you'll set off neighbours burglar alarms, you'll cause TV interference to you and your neighbours, you'll pick up your transmitted audio on your hifi speakers because we have stray RF going where we don't want it to go. Simple truth is that if altering the length of your coax alters the SWR then you've an inadequate RF ground and you need to be addressing that at the antenna.

So does the myth work? Yes it does but only because there is a fault in your antenna system and this allows it to "work" the same as any botch you can do on a car or an appliance in your home to keep it running for example. The SWR will be low, you'll be able to make contacts but you'll experience the issues I highlighted above and it doesn't fix the problem.

If anyone ever comes out with the "you want to be cutting your coax to X, Y or Z length", it is literally the best indication that they actually don't know what they're talking about and it is advisable to pretty much ignore anything that comes out of their mouths when it comes to antennas and to treat other information with caution. As has been said there are times when you do need coax to be a specific length such as phasing or when using it as filtering in a repeater but for a typical home base or mobile antenna install, no you don't.

MOGVZ-Could you go farther on mobile base loaded antennas? Its my understanding that thebase load is to make up for lack of length and thereby match for frequency. Seems the size, style, mounting of antennas on different vehicles will affect output. Am i correct in assuming that mag mounts are part of that "ground" circuit? I tend to look at antennas (maybe too simplistically) as an electrical circuit..........poor ground, high resistance ground = low current flow. As i am no expert in radio or electronics, certain basics still hold true, so my understanding of what youre saying is that poor ground plane = poor tx......i get that, but is ground plane "size" related to antenna base load or length? Im coming from the cb\mobile side, and am just trying to learn. It seems to me that like auto ine way radio, coax shield acts like a drain, running induced rx to ground....poor ground, lotsa static. It is the same with cb yes? I do have it right that the whole antenna transmits, ive heard in the past that only the tip transmits.......which kinda makes sense from a tuning aspect.....getting the length correct for frequency peaks. Watching your illustration it appears highest output at tip of dipole relates to lowest output at "ground plane", shield, what. One thread i was reading talked about "moving the .....node.... for peak swr-think ive got that right; that sez to me that that......"node"".....should be whats basically your antenna(tip?) Hey, just asking, trying to get a handle on things. Also, thread ohms vs swr wasnt the idea to be more concerned with getting x right as opposed to swr at 1:1? Just seems, and i admit pretty simplistically since i donthave radio lernin', that were still basicallytalking about an electrical circuit, and hi R=lo I.

 

[M0GVZ]

Th

MOGVZ-Could you go farther on mobile base loaded antennas? Its my understanding that thebase load is to make up for lack of length and thereby match for frequency.

No, its purely to provide a 50 Ohm match, it does not make up for lack of length. The wire in the coil on an antenna doesn't count as additional length so if there is 5ft of wire in a coil that isn't adding 5ft to the length.

Seems the size, style, mounting of antennas on different vehicles will affect output. Am i correct in assuming that mag mounts are part of that "ground" circuit?

Magmounts are a connection to the ground circuit using capacitive coupling to achieve it.

I tend to look at antennas (maybe too simplistically) as an electrical circuit..........poor ground, high resistance ground = low current flow.

Pretty much spot on there.

As i am no expert in radio or electronics, certain basics still hold true, so my understanding of what youre saying is that poor ground plane = poor tx......i get that, but is ground plane "size" related to antenna base load or length?
[/quote]
Its related to the wavelength of the frequency. When you read articles about making it the size of the antenna that is usually when talking about a compromise installation where you don't have sufficient room to run longer radials.

Im coming from the cb\mobile side, and am just trying to learn.

www.k0bg.com has a wealth of information on. Its an amateur radio site but RF is RF and it is all applicable to CB too.

It seems to me that like auto ine way radio, coax shield acts like a drain, running induced rx to ground....poor ground, lotsa static. It is the same with cb yes?

The coax shield is there to "contain" the signal within the coax and prevent outside interference. RF flows on the surface of a conductor, not through it. So it flows on the outer of the centre conductor and the inner of the braid. The problems come when we have a poor RF ground and then you end up with RF being passed on the outside of the braid. And that is when you start causing interference, experiencing "mike bite" when you transmit etc.

I do have it right that the whole antenna transmits, ive heard in the past that only the tip transmits.......which kinda makes sense from a tuning aspect.....getting the length correct for frequency peaks.

The whole antenna transmits. How much of the signal is radiated from a specific point depends on the current flow in that part of an antenna. Its length in wavelengths, loading coils, capacity hats all affect where that current maximum is. In general the higher up the antenna it is, the lower the take off angle so the better for DX it is.

Also, thread ohms vs swr wasnt the idea to be more concerned with getting x right as opposed to swr at 1:1? Just seems, and i admit pretty simplistically since i donthave radio lernin', that were still basicallytalking about an electrical circuit, and hi R=lo I.

X when its at its lowest value is effectively the value that shows where an antenna is resonant. It indicates reactance which is an opposition to the passage of AC current (RF is AC) so a value of X that is anything other than zero at its lowest point indicates we've not got as much current flowing as we could have. There are many reasons why this could be which are to do with the antenna design and also the environment the antenna is installed in with objects nearby having an effect on it (such as the car stereo antenna or a luggage rack or the vehicle body if you're using a bumper mount) and not just RF grounding so I won't go into this. We're also interested in the value of R at resonance too because this can be used as an indication of how efficient our RF ground is. Why we're more interested in what R and X are rather than the SWR is because SWR doesn't tell you anything other than the radio is seeing 50 Ohms. An impedance of 50 Ohms can be made up from many values of resistive and reactive impedance and the SWR isn't going to tell you what that mix is. Its a complex subject that really needs time reading electronic and antenna theory books if you want to get into the meat and bones of it all because it involves a fair bit of AC theory.

 

[redbeast]

Th


No, its purely to provide a 50 Ohm match, it does not make up for lack of length. The wire in the coil on an antenna doesn't count as additional length so if there is 5ft of wire in a coil that isn't adding 5ft to the length.



Magmounts are a connection to the ground circuit using capacitive coupling to achieve it.



Pretty much spot on there.

As i am no expert in radio or electronics, certain basics still hold true, so my understanding of what youre saying is that poor ground plane = poor tx......i get that, but is ground plane "size" related to antenna base load or length?

Its related to the wavelength of the frequency. When you read articles about making it the size of the antenna that is usually when talking about a compromise installation where you don't have sufficient room to run longer radials.



www.k0bg.com has a wealth of information on. Its an amateur radio site but RF is RF and it is all applicable to CB too.


The coax shield is there to "contain" the signal within the coax and prevent outside interference. RF flows on the surface of a conductor, not through it. So it flows on the outer of the centre conductor and the inner of the braid. The problems come when we have a poor RF ground and then you end up with RF being passed on the outside of the braid. And that is when you start causing interference, experiencing "mike bite" when you transmit etc.


The whole antenna transmits. How much of the signal is radiated from a specific point depends on the current flow in that part of an antenna. Its length in wavelengths, loading coils, capacity hats all affect where that current maximum is. In general the higher up the antenna it is, the lower the take off angle so the better for DX it is.


X when its at its lowest value is effectively the value that shows where an antenna is resonant. It indicates reactance which is an opposition to the passage of AC current (RF is AC) so a value of X that is anything other than zero at its lowest point indicates we've not got as much current flowing as we could have. There are many reasons why this could be which are to do with the antenna design and also the environment the antenna is installed in with objects nearby having an effect on it (such as the car stereo antenna or a luggage rack or the vehicle body if you're using a bumper mount) and not just RF grounding so I won't go into this. We're also interested in the value of R at resonance too because this can be used as an indication of how efficient our RF ground is. Why we're more interested in what R and X are rather than the SWR is because SWR doesn't tell you anything other than the radio is seeing 50 Ohms. An impedance of 50 Ohms can be made up from many values of resistive and reactive impedance and the SWR isn't going to tell you what that mix is. Its a complex subject that really needs time reading electronic and antenna theory books if you want to get into the meat and bones of it all because it involves a fair bit of AC theory.[/QUOTE]
Yup, copy all that. Since my background is automotive, esp driveability, electrical, and electronic controls im drawing on that base and using that to try and "keep up". No doubt theres a lot i need to learn about rf , ac trans, and so on.....but trying to geta step above your normal mudduck..... (not a cheap shot MDM!!!). I guess mymain question is does the size of a vehicle affect ground plane performance, or is it more about antenna placement and connections.

 

[redbeast]

Its related to the wavelength of the frequency. When you read articles about making it the size of the antenna that is usually when talking about a compromise installation where you don't have sufficient room to run longer radials.



www.k0bg.com has a wealth of information on. Its an amateur radio site but RF is RF and it is all applicable to CB too.


The coax shield is there to "contain" the signal within the coax and prevent outside interference. RF flows on the surface of a conductor, not through it. So it flows on the outer of the centre conductor and the inner of the braid. The problems come when we have a poor RF ground and then you end up with RF being passed on the outside of the braid. And that is when you start causing interference, experiencing "mike bite" when you transmit etc.


The whole antenna transmits. How much of the signal is radiated from a specific point depends on the current flow in that part of an antenna. Its length in wavelengths, loading coils, capacity hats all affect where that current maximum is. In general the higher up the antenna it is, the lower the take off angle so the better for DX it is.


X when its at its lowest value is effectively the value that shows where an antenna is resonant. It indicates reactance which is an opposition to the passage of AC current (RF is AC) so a value of X that is anything other than zero at its lowest point indicates we've not got as much current flowing as we could have. There are many reasons why this could be which are to do with the antenna design and also the environment the antenna is installed in with objects nearby having an effect on it (such as the car stereo antenna or a luggage rack or the vehicle body if you're using a bumper mount) and not just RF grounding so I won't go into this. We're also interested in the value of R at resonance too because this can be used as an indication of how efficient our RF ground is. Why we're more interested in what R and X are rather than the SWR is because SWR doesn't tell you anything other than the radio is seeing 50 Ohms. An impedance of 50 Ohms can be made up from many values of resistive and reactive impedance and the SWR isn't going to tell you what that mix is. Its a complex subject that really needs time reading electronic and antenna theory books if you want to get into the meat and bones of it all because it involves a fair bit of AC theory.

Yup, copy all that. Since my background is automotive, esp driveability, electrical, and electronic controls im drawing on that base and using that to try and "keep up". No doubt theres a lot i need to learn about rf , ac trans, and so on.....but trying to geta step above your normal mudduck..... (not a cheap shot MDM!!!). I guess mymain question is does the size of a vehicle affect ground plane performance, or is it more about antenna placement and connections.[/QUOTE]
Also sounds like i should invest in a quality antenna meter, since all i have is an old 3window micronta. Obviously , not to negate radio performance, antenna setup is huge.....im guessing more so on ssb and the ham side.

 

[M0GVZ]

I guess mymain question is does the size of a vehicle affect ground plane performance, or is it more about antenna placement and connections.

Yes it does affect performance as a vehicle is quite small as a ground plane for HF amateur bands but is "getting there" for something half decent for CB. In countries like the UK there's not a massive amount of difference though in performance between a small and large car because the differences are not that large due to the fact that it is mostly the horizontal sheet metal (roof, trunk lid, hood, truck bed floor) that is doing the work.

Also sounds like i should invest in a quality antenna meter, since all i have is an old 3window micronta. Obviously , not to negate radio performance, antenna setup is huge.....im guessing more so on ssb and the ham side.

An antenna analyser is worth spending money on even if its just a MFJ259. There are kits you can buy to build your own such as this one from Fox Delta for $54 which is a USB analyser you can use with a laptop or Windows tablet

http://www.foxdelta.com/products/aaz-0616.htm

 

[redbeast]

Yes it does affect performance as a vehicle is quite small as a ground plane for HF amateur bands but is "getting there" for something half decent for CB. In countries like the UK there's not a massive amount of difference though in performance between a small and large car because the differences are not that large due to the fact that it is mostly the horizontal sheet metal (roof, trunk lid, hood, truck bed floor) that is doing the work.



An antenna analyser is worth spending money on even if its just a MFJ259. There are kits you can buy to build your own such as this one from Fox Delta for $54 which is a USB analyser you can use with a laptop or Windows tablet

http://www.foxdelta.com/products/aaz-0616.htm

MOGVZ..... nuthin for nuthin, has there been any tests or research done on different types of conductors in coax? Thinking in terms of spark plug wire technology, carbon with stranding, double shielding, etc. Not real knowledgeable, but seems a lot of concerns could be solved in this area. Maybe im just ignorant of whats out there, but most coax ive seen uses copper conductor and what, a stainless shield. It would seem that theres more could be had here, guessing that it might also be a $/ft issue too. Just wonderingi

 

[The DB]

Most coax uses a copper center and a copper shield. The silver foil that is sometimes used as the shielding is more often than not aluminum or tin. If you have a coax with a silver colored center wire it is generally steel (not stainless). Anything that uses something other than copper for either of these is generally considered lower end coax and is cheaper than all copper varieties..

There are also some types of coax that will have a double shielding, say a copper braid with an aluminum outer foil shield. These tend to be higher end varieties of coax and cost as such.

The big decider when it comes to a coax performance, however, is not the metals used, it's the material between the metals, we call it the dielectric. This will determine how efficient a given piece of coax is over a given length, and is directly responsible for determining the coax's velocity factor, and has other effects on the coax as well.

The reason this is the case is most of the signal in the coax does not actually travel inside the metal. Being that your transmitted (and received) RF signals are AC, most if it is in the area immediately around the conductors, between the center conductor and inside of the shield. Sure, some small part of an RF signal will travel within the metal, but a significant majority of it is in the areas of the dielectric.


The DB

 

[fourstringburn]

Nice article MOGVZ and well written and explained. Maybe some people will finally get it.


However, I'm going to quote SP5IT from another thread where coax lengths does indeed come into play...

Coax length is very important IMO. Can't be shorter than distance between radio and antenna
Mike

 

[cbphreaker]

Wow, lots of information on this post.

Perhaps this may answer your question with a more practical application than theory of X's.

If you have a 1/4 wave length of coax the it's exhibits a Hi to Low conversion. In other words it's an impedance inverter.

If you have 1/2 wave length piece of coax it exhibits an impedance reflection. Meaning that the end your measuring will be the impedance of the "other" end.

This means that if you attempt to measure the true SWR of your antenna with a random length of coax the measured value meaning will be useless unless the antenna is 50 ohms.

As an experiment take a random length of coax (makes sure it's not 1/2 wave length or a multiple of that) and use a 50 ohm load, you sould see 1:1. Now change that resistor to 25 ohms and measure. It won't be 2:1, it will be something else. If its possible, start measuring at different places along the length and watch the SWR value change. You'll also find a 1:1 somewhere on that line even with a 25 ohm load.

Using a 1/2 wave length and using a 25 or 100 ohm load will show 2:1 the same as a 150 ohm load will show 3:1.


There in lies the myth of "cutting the coax till it reads 1:1". That is only apparent SWR (what the radio "see's") and thats when condition of noise, hot mic etc appear.
If the antenna is 50ohms its any length of coax is all good but when it's not the craziness appears.

Also, the number one cause of common mode current is feeding a balanced antenna with unbalanced line.

Hope this helps

CBPhreaker

 

[Captain Kilowatt]

Wow, lots of information on this post.

Perhaps this may answer your question with a more practical application than theory of X's.

If you have a 1/4 wave length of coax the it's exhibits a Hi to Low conversion. In other words it's an impedance inverter.

If you have 1/2 wave length piece of coax it exhibits an impedance reflection. Meaning that the end your measuring will be the impedance of the "other" end.

This means that if you attempt to measure the true SWR of your antenna with a random length of coax the measured value meaning will be useless unless the antenna is 50 ohms.

As an experiment take a random length of coax (makes sure it's not 1/2 wave length or a multiple of that) and use a 50 ohm load, you sould see 1:1. Now change that resistor to 25 ohms and measure. It won't be 2:1, it will be something else. If its possible, start measuring at different places along the length and watch the SWR value change. You'll also find a 1:1 somewhere on that line even with a 25 ohm load.

Using a 1/2 wave length and using a 25 or 100 ohm load will show 2:1 the same as a 150 ohm load will show 3:1.


There in lies the myth of "cutting the coax till it reads 1:1". That is only apparent SWR (what the radio "see's") and thats when condition of noise, hot mic etc appear.
If the antenna is 50ohms its any length of coax is all good but when it's not the craziness appears.

Also, the number one cause of common mode current is feeding a balanced antenna with unbalanced line.

Hope this helps

CBPhreaker

No. The SWR will remain the same however it is the impedance that changes. There are an almost limitless number of impedance combinations comprised of X and R values that will result in the same SWR. A half wavelength is useful for repeating the actual complex impedance to determine the values needed to create a perfect match using a tuner. With this length there is no change in the resistive or reactive component of the impedance. A random length will change those values.

 

[LeapFrog]

The SWR reading would change if the antenna is not properly installed (rf grounding issues) and one started trimming the coax length.

I believe that is how the myth started in the first place, people got a different SWR reading by changing coax length.....

Even if it is only an impedance change that alters the meter reading (after trimming) the number on the SWR meter changed, and people started believing that coax length matters.


This is a good thread.

 

[cbphreaker]

This means that if you attempt to measure the true SWR of your antenna with a random length of coax the measured value meaning will be useless unless the antenna is 50 ohms.

Is exactly the same thing as

The SWR will remain the same however it is the impedance that changes. There are an almost limitless number of impedance combinations comprised of X and R values that will result in the same SWR

Your right the SWR doesn't really change just the apparent SWR and a simple diode type meter doesn't understand complex X but a more simple measurement of Z.

As in

That is only apparent SWR (what the radio "see's")

I should have defined apparent swr.

 

[Captain Kilowatt]

Throw some common mode current into tbe mix and really watch your meter change with varying length of cable.