Great explanation Mike, probably the best I've seen in the time taken. Another thing to note too is that if you change your coax length and the SWR changes - then you have a problem with the antenna - either common mode current or it's not tuned correctly. As you point out - if you have a 50 ohm load (antenna) connected to a 50 ohm feedline into a 50 ohm transmitter, it doesn't matter what the length is - besides loss of course. Cheers mate
👍Thanks for video. Optimum co-ax length is as little as possible. Maybe even consider balanced feeder which will generally have lower loss than co-ax - expecially if you have any mis-matches. I watched some of Stan W1GV's videos a while back - I did not know he was SK.
Using a smith chart is a very good way to illustrate the concept. As far as tuning a coax length can be beneficial sometimes. I used a trap dipole on a band it was not intended for and the auto Atu would not match it. Looking on the smith chart, I was able to see that I may find an impedance value that might better suite the atu. I added some coax length and that rotated the impedance to a value the auto atu was able to cope with. Another instance I had was after repairing a faulty commercial radio antenna and getting a good match was horrified to watch the transmitter trip on high reflected power. The problem was revealed that the transmitter was producing a significant spurious product that was only a few db down on the fundamental and that was outside the band the antenna was designed for. On consulting with the owners transmitter engineer he told me that the transmitter pa was susceptible to this problem and some experimental trials of different coax lengths between the transmitter output and the main feeder would likely cure the problem. I tried and it did work, the spurious dropped many db and was in spec. Not a brilliant design for a broadcast installation.
Thanks Mike. It is so frustrating to see this topic being continually misrepresented on the internet. People make videos of themselves "cutting their coax to tune" and it's a shame as it wastes their time (and their viewers') and lots of coax too! Just tune the antenna, knowing that real-life antennas have a complex impedance depending on all sorts of factors (but definitely NOT the coax length!)
what can cause confusion and perpetuate the myth are certain antennas that recommend you have x length of coax for tuning or counterpoise purposes before passing through a 1:1 such as certain designs of windom and iirc the g5rv
@@battlestarone no i agree with you im just saying "some" antenna manufacturers suggest using certain minimum lengths of coax with some of their designs which may cause some confusion. clearly a properly tuned monoband shouldn't care how long the feeder is and a multiband is even less relevant!
I've had many an argument about this too! People measuring PHYSICAL lengths and getting annoyed when I asked about velocity factor! That being said I then mentioned about multiband usage with examples, as yourself. The myth started when someone noticed their swr reading changed with length, probably due to common mode affecting measurements. Imagine having a top band antenna on a car and having to store virtually a full reel of coax in the boot!!! Thereupon lay another preconception, is it better to coil spare coax in a feedline or leave it untidy! I feel your pain!!!
This reminds me of G4NSJ 's video on doublet antennas, look at 8 mins, when he gets on about his mates design where his feeder stops ten feet above the ground, lol
very good mike. things change if the antenna needs to use the shield as counter poise . in that case like a 39 in moonraker CB antenna the instructions have a length of cable. I have seen portable have trouble with length of cable. turns out the end fed half wave had no counter poise or some choke . he did that problem solved. and in some case a matching stub in needed to tune antenna then the length of stub matters and wear placed. and if phasing antennas length does matter. delay line . change pattern of far felid. that for more than 1 antenna on same transmitter. but one radio0 one antenna . length does not matter unless bad coax or lossy coax. 73's
I managed to take a perfect 50ohm antenna, a perfect half wave length of coax, and a tuned radio. I managed to keep my tune the same easily across all bands at the same time., I simply left the volume knob turned all the way to left on my radio. This had the added benefit of not over stressing the circuitry or LED's in the radio, and the fuse has never even seen current, so it's as good as the day it was made. Budget friendly RF usage with the extra added bonus of lack of RF interference on any device in any situation. I think I'm onto something here........however, contacts were exceptionally quiet, one might even go as far as to say, inaudible, so there is a slight chance of a minor downside to my idea.
Thank you Mike. I would point out that if you only had 75 ohm coax your schematic still applies. If the TX and ANT are 50 ohms a multiple of half wave lengths of coax will work as you have diagramed. Not ideal, but it works.
Hi Mike, i was always asked this question by foundation and intermediate candidates what "electrical" length should my coax be. I think this topic will remain shrouded in mystery for some just like the rf earth. Simple enough explanation here Mike, i'll direct future learners to this video.
Fully agree - except when I buy a pre made up length that may be longer than needed which then becomes the perfect length in my book. I may even wind a (usually un-needed) choke if it has good excess length . If not a pre made one is used and if neccessary a matching unit.
The only time ive thought about the length of coax is when making stubs. There you want a set impedance so you want to calculate the length from the shorted end. I always think of coax as a infinite number of sections of matching networks where their input impedance matches their output impedance. So x ohms in is x ohms out no matter how long it is. The longer it is the more attenuation thats all
Tnx Mike! I’ve used the velocity factor (.66) of the coax to determine the lenght of coax. A simplified example: on 10 meters a good lenght of coax would be 6,6 meters ( or 13,2 etc.) A second myth I suppose ;-) ?
Hi @kariolavi There is no requirement to do this - the correct length of coax is what is needed to connect the radio to the antenna. Forget working out coax length according to frequency. If this were true, how do multiband antennas work? VF is used to determine matching stubs and using the coax as a tuning device. This is not what we are discussing.
Additional comment: Back then, we used to have L1/2 x vf or multiple for the feedpoint impedance measurement for each band. Today, hopefully every OM has a Nano VNA or better. Every main feed line is calibrated with 50 ohms and the calibration data is documented and saved. And of course, every line is addressed and labeled. This is the only way to get the actual conditions at the feedpoint. Without glossing over SWR through coaxial losses... A serious OM makes the effort once and knows forever what is really going on.
The only counter i would have, which is, im sure, well known: a user using the wrong coax for the band they intend to operate on. Dont run long lengths of cable for EHF operation because you'll lose more signal due to bad choice in coax. ❤
If it does do something then would it not make sense to tune your coax to the natural resonant frequency of your antenne? When you tune your radio all your doing is exploring what your antenna is seeing.
@@mike-M0MSN It should not fall off like a cliff, just like the antenna does not, ideally the resonance to frequency fall off curve would match the antenna giving you the best possible bandwidth window.
Hi Mike - thanks for this - I have a question for you please: if I have an EFHW antenna with the 9:1 balun, and two VHF antennas and connect their shields electrically by way of the grounding of a mounting plate where all three antenna wires terminate outside my shack ( in theory, they are going to be connected electrically via the grounding of the radios they are connected to as well) - do you think that the EFHW antenna could either interfere, or pick up interference due to the fact that the balun actually electrically connects the EFHW shield and core?
you forgot velocity factor and odd multiples and the phase inversion of even multiples .i suggest you read maxwells book reflections to be better informed . yes correct on single frequency usage
I've had this with C.B.'ers, carefully trimming and testing their coax to the aerial in one inch increments to get it right. What about to the radio from the SWR meter? Why does that not matter to them? I tell them it is all connected together as one length of coax through the SWR meter that their carefully trimmed coax has been a waste of time and they still say they want the best out of their system and will carry on doing what works! G4GHB.
This is why an atu at the antenna feed point is the correct vway to do it,,,not having the atu in the shack,,also a Lc match at antenna feed point would do,,,ye just have to go out in the rain to change band.
Mmm,,,of tuner is at shack end you still have high swr in at antenna feed point and in coax,,,reflected power in coax is dissipated as heat and can get so bad it will flash over inside the coax in sone cases,,get the tuner out at antenna feed point and see the difference in performance.
The only time it has seemed remotely important about length is using cheap coax for hf on long runs. Like rg6 250 feet long. With reasonable good coax, does not matter.
Incorrect. Your statement @ 4:18 is true only if the antenna has no reactive component (antenna's 50ohm impedance is purely resistive). At that point, it isn't an antenna, it is a dummy load. Anything that can be called an "antenna" has a reactive component to it's complex impedance.
It's so easy. Until there is nothing to transform, you get 50 Ohm in and 50 Ohm out. The mess began if an antenna isn't 50 Ohm.... beside this - length plays NO ROLE!
You are wrong if I need 30 meters of coax between my radio and antenna and I use 200 meters of coax I will have more loss based on the coax loss if I shorted it to the lenith needed between the radio and antenna that is the correct coax lenth
Great explanation Mike, probably the best I've seen in the time taken.
Another thing to note too is that if you change your coax length and the SWR changes - then you have a problem with the antenna - either common mode current or it's not tuned correctly. As you point out - if you have a 50 ohm load (antenna) connected to a 50 ohm feedline into a 50 ohm transmitter, it doesn't matter what the length is - besides loss of course. Cheers mate
Yep all the way!
If there were an amateur radio faculty , you sir would be one of the head professors. (Defence Against Misinformations 👍👍👍👍)... 73 TA1XTA
love that.. 👍👍
👍Thanks for video. Optimum co-ax length is as little as possible. Maybe even consider balanced feeder which will generally have lower loss than co-ax - expecially if you have any mis-matches. I watched some of Stan W1GV's videos a while back - I did not know he was SK.
Open wire feeder is better BUT, you must match at the Xmtr side and the antenna side to the characteristic impedance of the line. Then you're good
Thanks Mike ... What Brilliantly 'Simply' explained .... Best to ALL from ChCh, NZ
Very welcome
Excellent explanation and I fully agree as my ATAS-120A works absolutely flawlessly
Using a smith chart is a very good way to illustrate the concept. As far as tuning a coax length can be beneficial sometimes. I used a trap dipole on a band it was not intended for and the auto Atu would not match it. Looking on the smith chart, I was able to see that I may find an impedance value that might better suite the atu. I added some coax length and that rotated the impedance to a value the auto atu was able to cope with. Another instance I had was after repairing a faulty commercial radio antenna and getting a good match was horrified to watch the transmitter trip on high reflected power. The problem was revealed that the transmitter was producing a significant spurious product that was only a few db down on the fundamental and that was outside the band the antenna was designed for. On consulting with the owners transmitter engineer he told me that the transmitter pa was susceptible to this problem and some experimental trials of different coax lengths between the transmitter output and the main feeder would likely cure the problem. I tried and it did work, the spurious dropped many db and was in spec. Not a brilliant design for a broadcast installation.
Agreed, still needs must.
Is there a perfect length coax, of course there is. The shortest piece you can possibly use.
Spot on Mike.
Thanks Mike. It is so frustrating to see this topic being continually misrepresented on the internet. People make videos of themselves "cutting their coax to tune" and it's a shame as it wastes their time (and their viewers') and lots of coax too! Just tune the antenna, knowing that real-life antennas have a complex impedance depending on all sorts of factors (but definitely NOT the coax length!)
Well said!
what can cause confusion and perpetuate the myth are certain antennas that recommend you have x length of coax for tuning or counterpoise purposes before passing through a 1:1 such as certain designs of windom and iirc the g5rv
Yes indeed.
Not myth it's basic rf engineering but no really ideal for radio hams that use an antenna for multi band use,,,listen to whats being said.
@@battlestarone no i agree with you im just saying "some" antenna manufacturers suggest using certain minimum lengths of coax with some of their designs which may cause some confusion. clearly a properly tuned monoband shouldn't care how long the feeder is and a multiband is even less relevant!
IMO you can be too short.
Like a 43 vertical with 4:1 unun with 15 feet coax. I found 75 feet it tunes much easier.
Excellent, thank you!
Just enjoy the radio with what ever you have that's the main thing. Great video Mike 👍🏽😁 always great info.
73
Thanks 👍
I've had many an argument about this too! People measuring PHYSICAL lengths and getting annoyed when I asked about velocity factor! That being said I then mentioned about multiband usage with examples, as yourself. The myth started when someone noticed their swr reading changed with length, probably due to common mode affecting measurements. Imagine having a top band antenna on a car and having to store virtually a full reel of coax in the boot!!! Thereupon lay another preconception, is it better to coil spare coax in a feedline or leave it untidy! I feel your pain!!!
Sooo true 👍👍
Imagine the coax needed for 472 kHz or 136 kHz.
G4GHB.
😄 from radio to the antenna. Perfect answer.
😁
Cheers Mike, another well presented video. Nothing better than common sense.
Thanks 👍
I agree 100%. I just roll out the coax and cut it to fit. 👍
Perfect.
Hi Mike, thank you for this good video you have done!...Simple an great description thank you very match !!!....thing can be so simple !❤ vy 73 DL1AQU
Thank you :)
This reminds me of G4NSJ 's video on doublet antennas, look at 8 mins, when he gets on about his mates design where his feeder stops ten feet above the ground, lol
very good mike. things change if the antenna needs to use the shield as counter poise . in that case like a 39 in moonraker CB antenna the instructions have a length of cable. I have seen portable have trouble with length of cable. turns out the end fed half wave had no counter poise or some choke . he did that problem solved. and in some case a matching stub in needed to tune antenna then the length of stub matters and wear placed. and if phasing antennas length does matter. delay line . change pattern of far felid. that for more than 1 antenna on same transmitter. but one radio0 one antenna . length does not matter unless bad coax or lossy coax. 73's
Absolutely 👍, I tried to keep it simple but you are right, there are always exceptions to this.
I managed to take a perfect 50ohm antenna, a perfect half wave length of coax, and a tuned radio. I managed to keep my tune the same easily across all bands at the same time., I simply left the volume knob turned all the way to left on my radio. This had the added benefit of not over stressing the circuitry or LED's in the radio, and the fuse has never even seen current, so it's as good as the day it was made. Budget friendly RF usage with the extra added bonus of lack of RF interference on any device in any situation. I think I'm onto something here........however, contacts were exceptionally quiet, one might even go as far as to say, inaudible, so there is a slight chance of a minor downside to my idea.
Brilliant 🤩
Thank you Mike. I would point out that if you only had 75 ohm coax your schematic still applies. If the TX and ANT are 50 ohms a multiple of half wave lengths of coax will work as you have diagramed. Not ideal, but it works.
Great point
perfectly said, and put.
thank you mike.
Welcome
Hi Mike, i was always asked this question by foundation and intermediate candidates what "electrical" length should my coax be. I think this topic will remain shrouded in mystery for some just like the rf earth. Simple enough explanation here Mike, i'll direct future learners to this video.
Thanks 👍
Fully agree - except when I buy a pre made up length that may be longer than needed which then becomes the perfect length in my book. I may even wind a (usually un-needed) choke if it has good excess length . If not a pre made one is used and if neccessary a matching unit.
Perfect John
Mismatch also applies to other than CW (the sidebands aren't the same frequency as the carrier --- to help support your statement)
The only time ive thought about the length of coax is when making stubs. There you want a set impedance so you want to calculate the length from the shorted end. I always think of coax as a infinite number of sections of matching networks where their input impedance matches their output impedance. So x ohms in is x ohms out no matter how long it is. The longer it is the more attenuation thats all
Thanks Mike. Always thought this was a myth too. 👍
👍
Tnx Mike! I’ve used the velocity factor (.66) of the coax to determine the lenght of coax. A simplified example: on 10 meters a good lenght of coax would be 6,6 meters ( or 13,2 etc.) A second myth I suppose ;-) ?
Hi @kariolavi There is no requirement to do this - the correct length of coax is what is needed to connect the radio to the antenna. Forget working out coax length according to frequency. If this were true, how do multiband antennas work? VF is used to determine matching stubs and using the coax as a tuning device. This is not what we are discussing.
Additional comment: Back then, we used to have L1/2 x vf or multiple for the feedpoint impedance measurement for each band. Today, hopefully every OM has a Nano VNA or better. Every main feed line is calibrated with 50 ohms and the calibration data is documented and saved. And of course, every line is addressed and labeled. This is the only way to get the actual conditions at the feedpoint. Without glossing over SWR through coaxial losses... A serious OM makes the effort once and knows forever what is really going on.
Yes the Nano VNA, the tool that every HAM should have.
Thanks Mike, great info. :)
Yes Doctor another great video Mike.
Thanks
Thank Mike
Hi Mike yes great video snd how is it all affected when using an ATU?
A whole new can of worms......
The only counter i would have, which is, im sure, well known: a user using the wrong coax for the band they intend to operate on.
Dont run long lengths of cable for EHF operation because you'll lose more signal due to bad choice in coax. ❤
"Is it true or is it a myth?"
"Yes."
Yes you are correct
Good video
Well put Mike..👍😀M0DCX
Thanks Darren, good speaking with you today.👍
If it does do something then would it not make sense to tune your coax to the natural resonant frequency of your antenne? When you tune your radio all your doing is exploring what your antenna is seeing.
Like I said, its only good for a single frequency as soon as you move off you no longer have a half wave of coax..
@@mike-M0MSN It should not fall off like a cliff, just like the antenna does not, ideally the resonance to frequency fall off curve would match the antenna giving you the best possible bandwidth window.
Hi Mike - thanks for this - I have a question for you please: if I have an EFHW antenna with the 9:1 balun, and two VHF antennas and connect their shields electrically by way of the grounding of a mounting plate where all three antenna wires terminate outside my shack ( in theory, they are going to be connected electrically via the grounding of the radios they are connected to as well) - do you think that the EFHW antenna could either interfere, or pick up interference due to the fact that the balun actually electrically connects the EFHW shield and core?
No you should be ok, the isolation of the inner cores and impedance differential due to the spread of frequencies should reduce any major issues.
EFHW should he 49:1 or 56:1 or 64:1 or something like that. 9:1 should be for a "random" wire.
@@paulsengupta971 yes, my bad - I meant 49:1 - confused myself with my other hobby project to setup a random wire using an AT-130
you forgot velocity factor and odd multiples and the phase inversion of even multiples .i suggest you read maxwells book reflections to be better informed . yes correct on single frequency usage
Keep it simple…
Top banana Mike!
I've had this with C.B.'ers, carefully trimming and testing their coax to the aerial in one inch increments to get it right.
What about to the radio from the SWR meter? Why does that not matter to them?
I tell them it is all connected together as one length of coax through the SWR meter that their carefully trimmed coax has been a waste of time and they still say they want the best out of their system and will carry on doing what works!
G4GHB.
You didn’t cover resistive losses nor complex impedances at the antenna. Do those matter?
Check out the Stan's video in the description...
My kenwood couldnt tune my antenna so I ve added about 4 m of coax and now it's tuning.
Line loss
Maybe you should tune the antenna? Rather than the radio? By adding coax you have lowered the SWR due to line loss.
👍👍👍
This is why an atu at the antenna feed point is the correct vway to do it,,,not having the atu in the shack,,also a Lc match at antenna feed point would do,,,ye just have to go out in the rain to change band.
And now let's return to the advertised schedule...
And now for something completely different.
Multiband antenna's prove that the length of coax is not vital. End of story.
Mmm,,,of tuner is at shack end you still have high swr in at antenna feed point and in coax,,,reflected power in coax is dissipated as heat and can get so bad it will flash over inside the coax in sone cases,,get the tuner out at antenna feed point and see the difference in performance.
@@battlestarone repeat after me, "SWR is not loss, reflected power is not loss, SWR is not loss, reflected power is not loss..."
The only time it has seemed remotely important about length is using cheap coax for hf on long runs. Like rg6 250 feet long. With reasonable good coax, does not matter.
Incorrect. Your statement @ 4:18 is true only if the antenna has no reactive component (antenna's 50ohm impedance is purely resistive). At that point, it isn't an antenna, it is a dummy load. Anything that can be called an "antenna" has a reactive component to it's complex impedance.
Apology for dumbing down a very complex subject. I just tried to make it simple.
It's so easy. Until there is nothing to transform, you get 50 Ohm in and 50 Ohm out. The mess began if an antenna isn't 50 Ohm.... beside this - length plays NO ROLE!
The perfect length of coax is zero. This is self-evident.
Coax is coax length is to get from a to b you can cut it where you like. Now co phase is another story.... Matching stub another story...
yep!
You are wrong if I need 30 meters of coax between my radio and antenna and I use 200 meters of coax I will have more loss based on the coax loss if I shorted it to the lenith needed between the radio and antenna that is the correct coax lenth
Did you actually watch the video?
Crystal
This obviously doesn't apply when on the FM. Gambio!
Hello Russ, hope you are keeping well.. :)