Mobil install..Coax length??

[Marconi]

That's a load of crap too.

Basically the length people are saying you have to use are multiples of a quarter wave. I run a Hexbeam. It works on 6m, 10m, 12m, 15m, 17m and 20m bands. How long should I make my coax? I actually don't have a clue how long my coax is. Its merely the length needed to get from the radio to the antenna when the mast is up at its highest plus a length for the rotator loop. SWR is 1:1 on all of the bands.

The only time that you need to use odd multiples of a quarter wave of coax is when the antenna design is using the coax as the other half of the antenna or you've done such as piss poor job of installing the antenna that it has to use the coax as a counterpoise. And this is where this old wives tale of needing to use 9ft of coax has come from - loads of people who have done piss poor antenna installations and found by using 9/18/27/36ft of coax that they can get a low SWR. The myth is therefore in their minds confirmed and they assume it must be true because they don't have the knowledge to understand why it isn't and why they're seeing the results they are.

M0GVZ, your Hex may work perfectly on all the bands just as you note above, but a lot of guys operating CB type antennas experience problems associated with CMC, and they are just looking for a little help with what they have around 27 mhz if there are any simple solutions.

Experts often suggest to these guys to fix their antennas or maybe tune them better...and then they too can make similar claims as you do above. IMO this is not always of much help to guys without some good prior understanding, experience, or knowledge to fix their antennas...while just wanting to talk on their radios and not interfere with neighbors.

Below is a reference that is often presented to help with such problems. This guy appears to be a countryman of yours. I'm not sure about his abilities and background, but I think he tells us he has tested all the fact and details he presents. I have not heard anyone being critical regarding his work on chokes noted below. Do you have an opinion you can share about his work?

Common-mode chokes

 

[Captain Kilowatt]

My favorite thing is when people say to use a 1/4 wavelength of coax and suggest 9 feet for 27 MHz. These people have no idea whatsoever what they are talking about because they obviously have never heard of velocity factor or if they have, they have no idea what it is or what effect it has on lengths.

 

[jazzsinger]

As CK says 9FT is nonsense as a 1/4 wave in most cb coax that has solid dielectric is 6ft, but if you must use any multiple and trust me it won't change a thing at the antenna end, it would be 12 ft half wave multiples, or impedance repeaters,

which would give you most accuracy when measuring swr in remote locations as the antenna feed impedance is repeated at 12 ft intervals along cable, but you still have to realize that it only works accurately on 1 frequency, the centre frequency the system is tuned for and that accuracy degrades not only with reactance as you move away from that frequency but also with cable losses depending on cable run,

if its a very lossy cable even a halfwave repeater will show significantly lower swr at radio end than real swr at antenna end, but its usefull on quality cables when tuning and reaching antenna is too difficult to check swr at feed point where it should be checked,

you must always figure in cables velocity factor when doing these type cable runs and always remember nothing you do between antenna feedpoint and radio will change swr at feedpoint, that can only be changed by tuning antenna. And not all cables have .66 velocity factor.another point worth noting is unless you only operate on 1 channel you will always have a degree of reactance present as even if your gear isn't accurate enough to measure it, its still present, however small.

Only times you would use a 9ft or odd multiple of, piece of coax it would be 75 ohms used either for cophasing antennas or too match a quad antenna or any other antenna that has a 100 ohm feed point nominal impedance.

 

[hammer0630]

Gold turds, Enriching their lives, The ignorant??? Wow this stuff seems to have hit the heart and sole for you!? Hey what do you think about base antenna coax it seems that ONLY 10 foot sections should be used !!!!

I was just kidding LiL Yeshua about the coax length on a base station but their is still a lot of mysteries concerning this topic.

 

[The DB]

Just a note or two on jazzsinger's post.

12 feet of coax with a velocity factor of .66 is CB channel 8. Channel 1 is 12' 0 and 13/32". Channel 40 is 11' 10 and 3/32". The middle frequency of the CB band should be 11' 11 and 5/16", not that any of this really matters for simple SWR only measurements. Such a length for tuning SWR is irrelevant as SWR doesn't change with feedline length except due to losses in the feedline. Should you choose to try and get one of these lengths accurate I wish you luck, 32's of an inch is not an easy length to cut to accuracy with coax cutters...

Link to the calculator I used above, punch in the frequency, choose the velocity factor, and the results match my calculations done by hand...

If you have something more advanced than an SWR meter I suggest learning a bit more about antenna theory to be able to use the device to its fullest. How X and R change over the length of a feedline is well documented. For example, lets assume R = 50 and X = 35. If the antenna is on the other end of a 1/4 electrical wavelength of feedline the antenna's R and X will be R = 36.25 and X = -26.25. This can be done with any length of feedline, all you need to know is the feedline's electrical length for the given frequency, R, and X. This doesn't factor in losses, but over short feedline lengths at cb frequencies that won't make to much of a difference.

Also the 9 foot odd multiple (well near that anyway, use the 1/4 wavelength figures from the calculator above...) for the impedance transformer, that is before the velocity factor as well, so make sure you factor that in here to.


The DB

 

[M0GVZ]

Below is a reference that is often presented to help with such problems. This guy appears to be a countryman of yours. I'm not sure about his abilities and background, but I think he tells us he has tested all the fact and details he presents. I have not heard anyone being critical regarding his work on chokes noted below. Do you have an opinion you can share about his work?

Common-mode chokes

He has done quite a lot of work and experimentation with antennas etc. The broadband hexbeam is actually an antenna he designed. He is quite well known and respected in the amateur radio community in regards to his work with antenna systems.

 

[hammer0630]

He has done quite a lot of work and experimentation with antennas etc. The broadband hexbeam is actually an antenna he designed. He is quite well known and respected in the amateur radio community in regards to his work with antenna systems.

I have in the past totaly believe in "Marconi" and have followed his advice and it has turned out wonderful!

 

[M0GVZ]

I have in the past totaly believe in "Marconi" and have followed his advice and it has turned out wonderful!

You will probably find that the advice from Marconi and Steve G3TXQ is pretty much the same. Nothing in Marconi's posts I've read contradicts what Steve has posted on similar subjects on amateur forums and vice versa.

You won't go wrong listening to Marconi or Jazzsinger for that matter.

If you're really interested in antennas the ARRL does a very good book. Although aimed at amateurs everything in it can be applied to CB. The only difference between amateur stuff and CB stuff is the measurements involved - you just need to scale them to 11m. There's no such thing as amateur only antennas or CB only antennas. RF doesn't know or care what the hell its being used for.

 

[Lil'Yeshua]

I was just kidding LiL Yeshua about the coax length on a base station but their is still a lot of mysteries concerning this topic.

That's cool.
Finding the right teachings on the subject will cure a lot of that.

 

[t77chevy]

If an antenna is not a proper 50x Jo then the X and j will swing back and forth traveling through the coax.the 18 feet is 180 degrees "1/2 wavelength " so the radio will See the true X and j from antenna

you can see using the smith chart for an antenna close to resonance the difference the coax length has on the X and J at the radio end... Note an antenna with x50 and J0 "for 50 ohm coax" will read x50 j0 no matter the length.

90 degrees is 9 foot ruffly.
180 degrees is 18 foot ruffly

 

[W5LZ]

That 9 and 18 feet thingy is a VERY rough estimate, not close really. You have to factor in the velocity factor for coax if you want an electrical 1/4 or 1/2 wave length. That velocity factor changes for the different 'types'/'sizes' of coax, they are not all the same. If you use an SWR meter at the radio end of a feed line then you are reading a combination of the antenna's and feed line's impedance, NOT one or the other. It can't tell you enough for you to really do anything with. Nasty thought? Yep, but that's the way it is...
- 'Doc

 

[The DB]

If an antenna is not a proper 50x Jo then the X and j will swing back and forth traveling through the coax.the 18 feet is 180 degrees "1/2 wavelength " so the radio will See the true X and j from antenna

you can see using the smith chart for an antenna close to resonance the difference the coax length has on the X and J at the radio end... Note an antenna with x50 and J0 "for 50 ohm coax" will read x50 j0 no matter the length.

90 degrees is 9 foot ruffly.
180 degrees is 18 foot ruffly

So far so good, you have gotten fairly accurate lengths in freespace. My question is why did you stop there and not finish the calculating for the subject at hand, namely feedlines?

Lol, an android smith chart app, neat. I always did them by hand... I hope you can zoom in like you can in web browsers, otherwise I would think it would get rather difficult to see... What is the name of that app?


The DB

 

[jazzsinger]

Just a note or two on jazzsinger's post.

12 feet of coax with a velocity factor of .66 is CB channel 8. Channel 1 is 12' 0 and 13/32". Channel 40 is 11' 10 and 3/32". The middle frequency of the CB band should be 11' 11 and 5/16", not that any of this really matters for simple SWR only measurements. Such a length for tuning SWR is irrelevant as SWR doesn't change with feedline length except due to losses in the feedline. Should you choose to try and get one of these lengths accurate I wish you luck, 32's of an inch is not an easy length to cut to accuracy with coax cutters...

Link to the calculator I used above, punch in the frequency, choose the velocity factor, and the results match my calculations done by hand...

If you have something more advanced than an SWR meter I suggest learning a bit more about antenna theory to be able to use the device to its fullest. How X and R change over the length of a feedline is well documented. For example, lets assume R = 50 and X = 35. If the antenna is on the other end of a 1/4 electrical wavelength of feedline the antenna's R and X will be R = 36.25 and X = -26.25. This can be done with any length of feedline, all you need to know is the feedline's electrical length for the given frequency, R, and X. This doesn't factor in losses, but over short feedline lengths at cb frequencies that won't make to much of a difference.

Also the 9 foot odd multiple (well near that anyway, use the 1/4 wavelength figures from the calculator above...) for the impedance transformer, that is before the velocity factor as well, so make sure you factor that in here to.


The DB

Must admit I rounded it down a little for simplicity, and only calculator i used was the one on my Blackberry,

the formula I know off by heart so don't trust online calculators. 300/27/2 x .66 x 3.28 to convert to feet, but as calculators are decimal and not imperial you will still get a decimal fraction of a foot and need to convert to inches, which didn't seem worthwhile for .02667feet, i suppose if I'd substituted 300, for the exact speed of light I'd have been even closer, but again wasn't required to prove the claims being made.

but the point i was making is neither 8,9 or 18 feet is the right length, its in the 12ft range, give or take an inch or two when using cable with .66 velocity factor and which frequency you use as centre frequency, i used 27.000 Mhz dead on, and rounded it down slightly.The actual figure I got was 12.026667 feet. I didn't really see the need to convert the .026667 to fractions of an inch or thou's to make the point

as I see 11m as 26-28 Mhz unlike you guys over the pond where many would use 27.205 as I assume you did,

and some would use 27.500 especially here in the UK where the two legal bands run from 26.965 - 27.405 FM and 27.60125 - 27.99125 FM with most use around 27.555 by illegal SSB'ers.

I personally don't care about UKFM, so tune my system centred on 27.000 as i'm often found around 26.285 Mhz or 27.415 - 27. 590 SSB.

Sacrificing a better swr up above 27.600 where there is too much UK Qrm, in favour of low end of 26 Mhz having a slightly better swr for amp.

But as I do occasionally stray onto UKFM i try not to have too bad a swr so use 27 Mhz dead instead of dead centre between 26 and 27.590. As long as my vswr is around 2.5:1 or less I'm quite happy to fire an amp through it, it ain't no biggie.

I never fully drive amps and generally run them on level 4 max, i don't care if i lose 10 or 20w or more, if my signal is cleaner and my amp runs cooler and lasts longer. Never had an amp or output blow on me in 35 years so can't be that far out, lol

 

[K4PIH]

Technically it "could" make a difference due to the velocity factor, coax quality, etc. However it is so insiginificant that it could best be measured in a lab under controlled conditions, not in a mobile application especially in the HF range. Use the length that fits and tune the antenna for lowest SWR, good to go.

 

[t77chevy]

So far so good, you have gotten fairly accurate lengths in freespace. My question is why did you stop there and not finish the calculating for the subject at hand, namely feedlines?

Lol, an android smith chart app, neat. I always did them by hand... I hope you can zoom in like you can in web browsers, otherwise I would think it would get rather difficult to see... What is the name of that app?


name of the app ....Smith chart.

and it wasn't intended to solve any matching scheme but to simply show X and j different at different lengths and give an idea why some "antenna manufacturers" say a certain length of feedline for their antennas.

personally I would stay away from any company who states this.

If the gentlemen is in or near conroe Texas I'll be more then happy to set it for him.

personally for mobile install I like to shunt match my antennas.

and for some reasons portions of the mobile app "smith chart " doesn't work ...or work the same with my new cell "galaxy s4" but to be honest I only use this app for demonstration purpose .

any real smith charts uses are still done by hand when needed and never for antennas .

good old fashion antenna analyzer and diy L match works every time.

73