How To cut your coax to nulls or coax tuning
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- Опубликовано: 31 июл 2024
- Not a lengthy discussion here. You found this video because you want to cut your coax so the jumpers are invisible in your system.
The same rules apply to longer lengths as well for instance if you wanted to cut a huge length to go to your antenna, just do the math to calculate it in a full wave,
Simple, to the point, enjoy your how to... 
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OK, OK, OHH KAY !!!! THERE IS NO REASON to "cut coax to nulls" or to a specific length when using coax as a feedline!!! The CORRECT LENGTH for a coax feedline is THE LENGTH THAT IT TAKES TO GET FROM THE TRANSMITTER TO THE ANTENNA!!!!
Yes, you can cut a feed line to resonance. But the SWR remains unchanged. Map it out on the Smith Chart to verify.
The author in this video is suggesting that achieving resonance via the feed line is equivalent to a unity SWR.
In an experiment there might be some SWR movement due to a flawed test procedure or parasitics, but in theory, the length of a feedline is irrelevant for changing the amount of reflected wave.
I have heard this myth for years and am still amazed people believe this!
I have never heard of making your coax invisible in your system. Someone care to explain. In all my radio setups, I have always setup with the length to go from radio to antenna, and tuned the antenna for best resonance, lowest vswr. Put a 1:1 choke on as needed for CMC depending on antenna design.
Cutting coax to a specific length to accommodate the antenna besides connecting it, that tells me your antenna is the problem, NOT the coaxial cable. Some exceptions do exist, but are very antenna specific. it is not a blanket rule.
There are books and educated folks to help with this. Some of us are also RF engineers!
I always utilize ferrite beads, typically around 15 pieces, to achieve an impedance of about 150-200 ohms at the operating frequency, equivalent to approximately 2-3 kohms. This is crucial because RF currents distort antenna matching, etc., and I prefer to avoid RF interference in my car. Inside my vehicle, I've inserted the ferrite bead choke balun into a brass tube and fastened it to the roof. While it might sound excessive, it's also to address vibrations and mechanically relieve strain on the coaxial cable. After all, it's nearly 500g in mass, and the brass aids in cooling the ferrite cores.
Thanks for the simple explanation. Your method works great. I usually talk on some lower frequencies below 11m channel 1 so I implemented your advice when I made my next jumper but made it a little over 6' (6'-5") and it came out perfectly, in fact I was able to cover the entire 11m band up to 27.425 down below 26.865 and my SWR never went above 1:1.2. During my years in the radio game some have told me it doesn't matter and others have said differently but now I know it does. Thanks again, I'm off to make a few more jumpers.
wait are you actually using 1/4 waves stubs to interconnect components so the coax is "invisible" to the connected devices LOL Oh you must be a CB tech LOL
As a retired electronics engineer, this is nothing but a bunch of male bovine excrement. Don't get me wrong Coax stubs are very useful in making things like phasing lines or precision phase corrected lab jumpers, But it's useless for interconnecting things like antennas and amplifiers to your radio.
This is a prefect example of an old CB wives tail, some CB fool read in a ham radio book that a 1/2 wave stub will present the same impendences at the load as it does at the source and that it meant that the coax should always be cut to the nulls LOL Damn our school systems are failing us.
Typical. Look at the "great video" and "thanks, just what I needed" responses.
Grit my teeth.
I know, wives tale. I'd need a 44 foot trimmed jumper lead for our 60m band, it is by chance about 50 feet but I didn't precisely trim it like this shows, and 550 feet for our 2200m band! Trim the aerial not the coax.
Why do they think things change up to the SWR meter? Why don't they tune the whole system, jumper, SWR meter and coax by adjusting the aerial if it bothers them?
G4GHB.
Actually it's not 550 feet, that's metres, it's more like 1700 feet!
@@bill-2018 Answer, because they don't understand electronics theory. Actually, hardly any hams do. They take a tiny (probably, that a half wavelength of coax repeats the Z) fact and distort it out of proportion! The sad part is whenever you see one of these and you check the responses then you see how ill informed and gullible the viewers are. One of my favorites, Fine Tune CB - he gets people to pay him good money to "Tune" a brand new radio, as if the factory didn't know how to do it.
@@feeatlastfeeatlast5283 Can't be bad charging people money for something already working. I think I've seen a couple of his videos. They are obsessed with trimming jumper cables as if that cable before the SWR meter is a problem and no problem beyond it.
73, G4GHB.
exactly, coax length does not tune the antenna system It only changes the z seen by the transmitter.
If you're antenna is resonant then coax length isn't going to change that.
this is just a perfect example of illiterate people reading something in a book and distorting it.
This was a game changer on my station . Completely eliminate my reflect on my accurate meters (Bird) . Very good information .
The UK is still feet and inches. The kids are being into meters etc. most adult radio guys are still talking feet and inches. GL 73.
Wow your in NY. ME TOO I do similar I made a small dummy load on a T. and plug the coax into one side to show it has no reaction. Great job. Simplicity is everything for people. Thank yo
I'm in Watkins Glen NY 🤘 #12...
Great video, really appreciate the info!!
Thanks for the video.
Exactly the information I was looking for.
73
OMG, the schools (or lack of!) are failing!
Ya cause he made it real simple and u still don't understand simple science
Do you think an SWR meter is somehow an isolator? One side of the SWR meter is not connected to the other?
It's like a coax connector joining two cables together. The inner two are connected together and the braids are connected together as in one cable.
G4GHB.
Making a half wave is totally different than a quarter wave. You need a tee and a dummy load on the MFJ.
That's how I do it. T with small direct dummy
Thank you sir
Ótimo trabalho parabéns bazuka Foz do Iguaçu PR Brazil 🇧🇷🇧🇷
Thank you
Just curious .. Years ago (mid 1980's) I talked a local guy who said he took like a 12 foot piece of rg-213 coax and kept chopping it down a few inches at a time until he got a good match between the his radio and amp. I asked him about the mismatch (very high swr) and he said - "It's the coax, it needs to be the correct length between the amp & radio, or they'll always be a extremely high swr." This was a cobra base cb radio, base boomerang 250 amp to a vertical 5/8th wave antenna. I don't know what length he ended up with, but said he got it. Is there some (sweet spot length) magic length between a radio and amp that needs be got right?
Thank you ....
Did the math with RG8 comes out to 5.6 ft
wow! Thank you for the instructions. Do you plan to provide some messurements with an antenna connected? I mean different length to the exact length with SWR comparision and the resulting RST by same QSO partners? That would be great.
Greetings: Yeah, everyone should have their own Mighty Fine Junk. Remember there is no load and Ur hand effects also
Thx 4 the share.
Man unless you have a twin that sounds like you.,,back in the day did you build a suburban 2 tube (2) 3cx3000 ..the suburban was a old school and you had 2 sticks on top..Al Capone BYU got down...if not even tho I known this info ,Love the videos big bro keep up the good work...
So... a different length of coax is needed for each band??? Example: someone working the 10m band will have to use a different length of coax to work the 40m band???
Good question. This is fundamental error in "coax tuning"
Or on different parts of band. Many ham bands are wide 7-9% of central frequency, which gives HUGE difference on phase.
When making phasing cables or stubs the correct constant is the free space in a vacuum which is 492 not 468!
I remember someone telling me many years ago, in multiples of 14 feet, I was just getting into cb radio back then, one guy said length didn't matter, but I cut my coax to what your showing and apparently it does matter. Thank you so much for this video, I had forgotten alot since I was a kid.
Whoever told you that had no idea what he was talking about. But instead of investigating and learning you ate it up. Hopefully you no longer do that. Info is available, books still are available.
Put a 50 Ω dummy load on your coax and see what your power is. Try another length of coax. Is there a difference? 6 inches or 20 feet should make no difference unless it's very lossy coax. Why do you believe in a magical ¼λ coax?
G4GHB.
You say it matters. How did it change things?
I've never cut coax to a specific length. I'd need 44 feet on my 5.262 MHz inverted vee. I haven't.
G4GHB.
Using 468/freq. To find half wave in free space will get you close. But this is formula for a half wave dipole, which is about 5 % shorter than free space length, due to end effect of the wire. Best to use 492/freq. to get half wave length in free space then apply the velocity factor of coax you are using for a more accurate number.
The 468/F in MHz allows for a 95% velocity factor.
If you can use a nano vna what do you set it on
SWR, impedance, usually all I use. U can actually see the sine waves and nulls in a vna. Put your curser at your frequency then start trimming till the null slides over to your frequency. Done
@@electromechanicalstuff2602 ok thanks
Solid & simple way of helping the average RTO make more reliable contacts.
im in ny also. where in ny are you?
Why would you want the coax resonant? It then tends to radiate making the coax part of the antenna Coax resonance is a bad thing
Correct me if i'm wrong, but you cut length in odd multiples.
Sir, do you the same procedure for the coax from the antenna down to your shack? Thank you.
Exactly. It's nonsense!
I've never in 40+ years trimmed coax to any specific length. Tune the aerial not the coax.
G4GHB.
6 inches of coax or 20 feet of coax unless very lossy still produces 4 Watts. Try it on a 50 Ω dummy load in place of the aerial with different lengths of coax.
Why do you believe in a magical ¼λ length of coax? The reason you might see a difference is R.F. on the outer of your coax.
G4GHB.
Hey G4GHB!! Make a video and show us how it's nonsense. Share your knowledge G4GHB. KCU3589.
@@fido3561 I'm thinking I will do a video showing it doesn't matter.
I set things up yesterday for 24.94 MHz as my oscilloscope isn't good at 28 MHz, it's not made to go that high.
First read 50 Ω at the tx: 50 Volts, then the same 50 Ω with the magical ½λ: 50 Volts, adding 39" of coax should throw it off, right? No, still 50 Volts. Add another 18½" and still no difference.
Output test at the 50 Ω carbon resistor adjusted to read just on the red line of an r.f. pick up meter (meter, diode and capacitor), meter from a hi-fi or something. Meter and coax trapped on the desk so nothing moves. On the ½λ it's just at the red line, add 39" at the tx end so nothing is disturbed: no difference, add 18½" more and still no difference. The same power output sensed by the meter.
C.B. theory says I should have a mismatch due to different coax lengths and less power at the end.
I did use a ferrite ring so no r.f. gets on the outer of the coax as indeed it shouldn't anyway being a resistive load but to make sure I used one.
If coax length makes a difference it means there is r.f. on the outer braid of the coax.
The SWR meter can go anywhere too as I proved.
Coiling coax had no effect. No figure 8 needed. Again if it does then ferrite rings are required.
Just all things I have known for years.
73, G4GHB.
How long do you want this cable? Only about a week. LOL.
There is no reason to cut a coax cable to a quarter wave (or other specific length) for jumper cables. you can use any practical length to connect your radio to whatever you need to connect it to. I f for example your Watt meter is only 2' away from your radio, you can't need a 6' piece of cable (it isn't going to affect the performance of your radio in any way). Making a coax cable any longer than you need is just a waste of coax!
we can argue that the excess cable is a loss of signal, but certainly not a gain in invisibility to the system! Even if only the difference of 4 feet. imagine if we are working at V/UHF freqs, and each foot adds up with loss.
I worry more about the connectors chosen and lets get rid of every extra foot of cable in the system that we dont need. the only extra footage MIGHT be the drip loops and room to move gear but past that its all BS. You are absolutely correct!
@@ChrisRobinsonKF6NFWYou speak about different thing
@@jacuswoczega9180How so? Explain. I speak of the exact same. I may have explained differently and put into different context. However, extra coax is extra loss. Do not cut a cable any longer than you need it. There just is no need. It's pure waste of money and rf power. I am willing to explain and show the math and work though to anyone. I'm also willing to admit where I am mistaken if you show me that also
@@ChrisRobinsonKF6NFW I agree. Only small clarification, most of thread is about magic length. Loses are generally known, and almost no controversial (one exception: microvawe low loss cables fof CB)
@@jacuswoczega9180 but the point being, the magic length is whatever it takes to get from radio (point A) to antenna (point B) any length longer is a waste. If you wish to cut for some wavelengths to solve an antenna issue, this is clearly not the way to do it.
I allways asked cb tec that buid my coax cables for how long was best 6 ft was unless used steal whips antenna then little longer
nice and simple.. these self proclaimed engineers below.. are not likely real in the true sense.. this tuning.. is important in the uhf and above as when an antenna is made to 46ohm and the transmitter is 46 ohm and the coax is 51 ohm...or other variations and this is a true senario every thing being a true 50ohm dosnt always happen.. this 1/4 trick is nessassary.. also with long lengths of cable the velocity rating of the coax cant be used.. as this changes with frequency...this video and other methods should be used
This is the correct way to do a 1/4 wave length of coax. But when you do a half wavelength of coax you need to short the center conductor to the shield while you analyze the coax. Good video.
Great video, you just got a new subscriber. 73👋🏻
That makes a lotof sense but how about from amp to antena on Mobil
On the frequency I use every day my quarter wave jumper would be over 41 feet.
For me more important is not HOW cit, but WHY. 1. Impossible to "tune coax" to wider frq range. 2. Normal rezonant antena don't require coax tuning, only very rare or bad tuned antennas
Why did you apologise to the UK for using feet as a measurement?
From Wikipedia:
The foot ( pl. feet), standard symbol: ft, is a unit of length in the British imperial and United States customary systems of measurement. The prime symbol, ′, is commonly used to represent the foot. In both customary and imperial units, one foot comprises 12 inches, and one yard comprises three feet.
I prefer to make mine in half wave lengths. Use a dummy load on a t.
Why? So the only problem occurs before the SWR meter and not beyond it?
If you put a 50 Ω dummy load on a 10 foot length of coax and then add a another 10 foot, 20 foot or 30 foot it is still a 50 Ω load and still a 1:1 SWR. You don't adjust anything. Nothing changes and still a minimal r.f. loss.
You put an aerial on and adjust the aerial for 50Ω, put a jumper lead of any length in and it should still be 50 Ω on a well matched aerial. Tune the aerial not the coax.
It sounds to me if you have to cut jumper leads to length you have r.f. on the outer of your coax affecting things or the aerial is not a true 50Ω. I've never cut any to a specific length after years of making aerials. I'd need a jumper lead of around 44 feet (¼ λ) or 88 feet (½λ) for our 60m band and a ridiculous whopping 550 feet (¼λ) for our 2200m band.
G4GHB.
I was thinking the same thing. I've been into radio for 50 years and never once tuned coax on any antenna and was always able to obtain a flat match. If SWR is changing from length, that suggest common mode on the shield of the coax.
@@richb.4374 It's as if they believe a 6 inch length or 20 foot length of coax won't produce 4 Watts unless it's the magical ¼λ. They just don't get it. A 50 Ω dummy load proves otherwise.
73, G4GHB.
Actually it's not 550 feet, that would be 550 metres. It's even worse, about 1800 feet!
G4GHB.
@@richb.4374Most antennas aren't 50 Ohm perfect matches. By using 1/2 wave multiples, the radio will see the same impedance as the feed point, since that's what 1/2 wave sections do, repeat the impedance they see at the other end. This way, the matchbox matches to the feed point impedance and not some complex reactance that includes the feed line, and consequently it's length. If you do things this way, your antenna won't suddenly be out of match because you swapped a different patch cable in your shack. This is a real issue with VHF, your swr changes dramatically when you change patch cords around. Whenever your antenna isn't a perfect match to your feed line, your feed line becomes part of your antenna due to the swr. By using half wave multiples you nullify the effect as much as possible. The swr you measure in your shack truly represents your antenna situation by itself since it's impedance is seen at the radio end, not the complex conjugate. 1/4 wave stubs have their own special uses.
@@richb.4374 49 years radio here and in the early days I did not have an SWR meter, I just tuned for maximum r.f. out. After building the resistive SWR meter it proved maximum r.f. out was minimum SWR. In fact I built it only about ten years ago and calibrated it four years later!
G4GHB.
Just use an antenna tuner
SWR should match with both the coax and antenna. Not just the coax
sir this is only for HF range .please suggest for UHF range too regards
I'm sure glad this wasn't a college course.
Invisible coax lol, must be a cb thing.
Yes, CB myth
good information thanks for making this video..73s 007
He has the cb brain oh boy this is so wrong and so many variables are not correct. He must have a cb backround
Its so crazy its like the same 4 guys on every single cb or coax video crying in the comments about how ive been doing this 50 years and i dont know how to use a VNA to tune outputs i use 50 watts and have never had a problem with my set up. This is a waste of time...Bla bla bla. Its such a waste of time that they spend ALL of their time searching for these videos just to complain about them......
This is a complete waste of time.
Funny how operators dont believe this. Ha... Jokes on them. You run poweryour SWR's go up. Because your coax isn't tuned. Why tune everything else and not the most important thing.
What are swr's.. Heat. Whats heat do to our equipment... Burns it up.
My Dead key is 1000 bird. My pep is 4700.
My swr's... X=0... my swr needle doesn't move. Why because I tuned my coax too. 50 ohms...
So believe what you want and leave this stepout. #12-
Watkins Glen NY waving
CENSORSHIP?
My previous comment (part 1 of 2) was censored / deleted within 10 minutes of my posting it, so I’m guessing it was RUclips
I was bestowing my thanks and gratitude for this video and sharing our method of terminating, etc…
Other than a reference to the mentally challenged pederast who is currently an inhabitant of the White House, I don’t know how else I offended the Big-Tech Over,odds….?
Why? Just why? This a waste of expensive coax and shows a real disregard for physics and RF engineering.