Ham and CB Radio Myth - Coax Transmission Lines Must Be A Specific Length.

[@oobihdahboobeeboppah]

At last someone with a no nonsense, straightforward explanation to end all arguments on this topic. Thanks Mark!

[@BustRadioMyths]

You got it!

[@mostlypostie1]

A great video. What Walter Maxwell W2DU wrote in his "Reflections III" book will change how people think about SWR, antennas and feedline forever. It is an amazing read. Thanks for mentioning this.

[@myroncornett414]

Finally someone who makes sense of it all. Excellent speaking. Thank you for making this video.

[@Team-fabulous]

Yip... I was that soldier.. Back in my 11 meter day's I was meticulous when it came to measuring the coax length.... Very interesting video..

[@dandypoint]

Excellent! You hit the nail on the head again! There is advantage if the coax is 1/2 wave long (taking the velocity factor into account. In that case the impedance at the far or antenna end will be exactly what the input impedance to the coax is. Important if you want to measure antenna impedance without actually being at the feed point! Also if the coax is 1/4 wave long it can transform a high impedance to low and vice versa. These are cases where we are using the fact that the line operating with an SWR becomes an impedance transformer as you mentioned.

[@BustRadioMyths]

Good points!!

[@dandypoint]

@@BustRadioMyths I am glad someone out there is putting out good info! I am trying also!

[@dus777]

yes, exactly! I use "tuned" lengths so my analyzer is effectively at the feed point.

[@davep6977]

Always remember your antenna is 90% of your system

[@BustRadioMyths]

This is the truth!

[@radioguy19510]

I would love to see you do a video on transmission line transformers such as coxial baluns etc...

[@kumasu]

Thank you for educating us with facts. There's no much incorrect information out there . . .

[@BustRadioMyths]

Yes the number of myths is staggering!!!

[@Rocketman88002]

Thank you for mentioning the velocity factor rating. A coax data sheet has that and other useful information.

[@FjHenderson]

I have never calculated a certain length of coax. I always run extra so I can make a coil of at least 6 to 7 wraps before going into the shack for a choke to help with the undesirable's

[@BustRadioMyths]

Good!

[@lonewulf5150]

Lol

[@davemason2290]

I have heard about this in the past but didn't think it was still something that people did. I just use the shortest run of coax that I can, plus a couple of meters extra for an air choke at the antenna end.

[@kensmith5694]

Yes and also add a "drip loop" at the shack end. You want rain that lands on the cable to go away from the wall of the house. The same thing can save your gear in a thunderstorm. Use a heavy grounded iron wire to hold the bottom of the loop in place by winding a couple of turns around the cable.. A nearby strike will send an RF pulse down the cable and the pulse will see a handy way to go to ground that isn't though something expensive.

[@ajbrown2013]

Start by doing a 20’ check everything then go down same everything just keep using the same end as you remove it every foot. When you get the coax tuned to your frequency you want then check it again. During the entire time check for how the radio sounds check swr check power resistances resonate impedance. Try it

[@BustRadioMyths]

Did you listen to the video at all?

[@Aimsport-video]

If there is larger mismatch at antenna feedpoint, the reflected power standing wave has a greater effect, as you said, changing the impedance along the length. Coax trimming doesn’t change the feed point SWR, but it can fool people into thinking there antenna has low SWR by bringing impedance closer to 50Ohms at the radio.

[@BustRadioMyths]

True!

[@paulm0hpd319]

Matching impedance at the radio would allow it to produce its maximum power into the system, so a good thing ?

[@Aimsport-video]

@@paulm0hpd319 yes, but sub-optimal to matching at the feed. A portion of that standing wave will keep being reflected and as it goes back and forth both the cable length traversed and losses multiply.

[@paulm0hpd319]

@@Aimsport-video not if feedline is upgraded to a less lossy one

[@Aimsport-video]

@@paulm0hpd319 lmr400 too. How many times does a standing wave traverse a given feedline every second at your chosen frequency as it smashes against feedpoint? How far is that… approximately? Fortunately a percentage of the reflected standing wave does force its way past the impedance mismatched feedpoint with each go… so not a total writeoff. Feed point impedance matching is crucial.

[@dpetervan]

Excellent! Entertaining as much as edifying! Thank you! 73 Pete KD2OMV

[@BustRadioMyths]

More to come!

[@bill-2018]

I'm glad you did this, Mark. I seen a few videos and one where a C.B.'er said he cut off one inch increments and re-soldering the PL259 back on each time. I said it didn't matter because the SWR meter and jump lead made it all one coax and no longer ½λ. His reply was he wanted the best out of his system. The C.B. shops tell them it's about impedance matching. Do they think the SWR meter is an isolating device? Why not ½λ from the tx to the meter? Imagine ½λ for 472kHz. Why would they listen to us after all we are only amateurs! That was said to me in a pub years ago. G4GHB

[@BustRadioMyths]

Wow!!

[@paulm0hpd319]

The cb shops are not far wrong saying its about impedance matching just that the concept gets misunderstood

[@jerryKB2GCG]

what about using at least 50ft with an efhw ?

[@BustRadioMyths]

Think you are referring to when the coax shield is used as a counterpoise for an EFHW. If that's what you are doing, yes. I prefer to use a separate wire as a counterpoise because the coax shield runs into the shack and can cause rf in the shack problems since it is part of the antenna.

[@jerryKB2GCG]

@@BustRadioMyths yup, requires a common mode choke just to be sure

[@mudduckfarmer9122]

Thanks for ending this Myth

[@BustRadioMyths]

It was a pleasure.

[@M0RMY]

Mark this is a brilliant summary of a myth that needs to be debunked. Thank you. YT is full of people cutting their coax to "null" it for some reason. This in a simple systems not using matching stubs or such like. My instinct is to ask how come the coax in my car is the same length for all bands from 80m to 6m with a great match from the same antenna? Well, I have a Tarheel screwdriver antenna that adjusts its length. I also don't have multiple different feedlines running to my Hexbeam or DX Commander or loop - get the antenna "right" and the rest is straightforward.

[@BustRadioMyths]

Someone commented on this video talking about cutting coax to a "null" and I didn't know what the heck he was talking about!!

[@dylanschulz2404]

CB/freebanders, I believe, make a big deal about this as they run tremendous amounts of power on one single frequency. I think what they are doing is getting it perfectly in phase so that it nulls out the common mode current. I have researched broadcast station feedline length and all I could find was that they try to make the couplings in their hardline at odd wave lengths as that is where arcing can occur because voltages are higher at the peaks. In TV, it is so broad spectrum that they just run oversized feed line to prevent the possibility of arcing at the couplers. I have never found any engineering writings of calculating optimal overall length of feedline from broadcast stations. I have spoken over the air to fellow hams (who were retired broadcast engineers) about the subject and I have never got a definitive answer if over all feedline length was part of their engineering calculations. Obviously for loss figuring, but not for matching. In our ham radio world we just want to work every frequency we can so the shorter the feedline the better! 73, KF0BBU

[@BustRadioMyths]

At AM radio stations for example there is a tuning circuit at the base of each antenna so there is a perfect or near perfect match along each line which means no standing waves. This works fine for the radio station because it only transmits on one frequency. It's a different story for ham radio operators who transmit over a broad range of frequencies.

[@dylanschulz2404]

@@BustRadioMyths yep they know to put the tuner at the antenna to reduce feedline losses.

[@Larry-AK0Z]

Good stuff! Thanks.

[@ricdonato4328]

After Helene and Milton, I hope you are safe.

[@BustRadioMyths]

Yeah thanks no problem here on the east coast. Main problem here was tornadoes.

[@Dark_Knight_USA]

Greetings: Vwry true he is correct. I I defy the CB nuts 2 prove different. We know some will try. I knew a guy that swore by it and condemned me 4 my builds. Yet my signals prove pisitive results he fould not explain. Fortunately he is dead now. Thx 4 the share.

[@TheFlyingSquerill]

mine is 15' 6" cut to null and it works better than just enough length to reach my antenna....why is that if the length does not matter ???

[@smorrisby]

What do you mean "cut to null"?

[@TheFlyingSquerill]

@@smorrisbycoax cut to 1/2 wavelength x=0 (null) r=50 reading on antenna analyzer...no need for tuner or balun...in my situation this works best for me...not downing you in any way just saying it does work, and works well for me...tks...

[@smorrisby]

@@TheFlyingSquerill I think I've worked out what you are doing. You have not cut your antenna to the exact dimension for the frequency, so you are cutting the coax to eliminate reflections. You are using the coax as an ATU at best and a 50 ohm dummy load at worst. Your radiation pattern will be appalling. You have to get the antenna right 1st.

[@TheFlyingSquerill]

@@smorrisby well all I can say is my antenna is tuned at 1.01 flat across with r=50.4 x=0.64 and I have worked Australia, West Indies, Hawaii, United Kingdom, Canada and almost all the states except Alaska.....so my pattern can not be to shabby, I talk in all directions vertical....Hawaii and Alabama at the same time ... NY and Washington State at the same time.....for example ... did not mean to stir anything up really what your saying works I understand and agree but it takes a tuner and Balun, that's awesome, gets you on the air and making contacts many situations are different though....just in my situation it works well for me...I guess I may be the exception that proves the rule ....but it works for me....less the tuner and Balun.....thanks for the reply....

[@smorrisby]

@@TheFlyingSquerill Add 5m to the coax and it will make no difference. Just out of interest, what RF Analyser are you using?

[@Calico5string1962]

Thanks Mark! I have long known the truth about SWR and the feedline length myth. I'm not a scientist nor an engineer - just an old guy... but I can (and do) read what the scientists & engineers wrote, and I try to share my knowledge when I can. Nonetheless, there are STILL - to this day - hams who refuse to believe the facts. My favorite myth: Reflected rf going back into the transmitter and "blowing up" the finals! 😂). Maxwell had it right, waay back in his day... Anyway, keep putting out the truth! It shall set us free... 73 from west Texas!

[@BustRadioMyths]

So good to meet a fellow truth seeker. These videos only express in my own flawed way what I have learned from engineers like Maxwell. And yes, reflected power destroying your transmitter is one of the best myths LOL.

[@paulm0hpd319]

Totally agree about Maxwell and the myth of reflected power killing the transmitter, the main reason why it doesn't kill the transmitter is that the impedance at the transmitter has changed something that's hard to fathan by some 👍

[@davep6977]

Qualifier. You can get out better with a system with low SWR. That because the "new" radio's roll back the power when the swr get's to certain point. I found that true by accident when I saw my 100 watt radio putting out 10 watts, the I realized I had the coax switch to the wrong antenna. Good video

[@BustRadioMyths]

Get an antenna tuner and you will get out just fine even with a higher SWR.

[@paulm0hpd319]

@@BustRadioMyths maybe a topic for another video, tuners

[@jimmyroger3026]

100% true , ive cut a coax for 2M with Nano vna . Transmission and reception was the same before and after Lol

[@jtc1947]

What I have NEVER been able to understand is about the ANTENNA itself? WHY do we HAVE to have Specific measurements for the radio antenna? Anybody remember the OLD VHF TV SETS? You put up an antenna and the TV TUNER did the WORK? Why does THIS not work for ham radios? One antenna does it for ALL of the various bands? Can anybody dumb this down for me? Just really makes NO sense to me. Sorry!

[@BustRadioMyths]

Pretty much anything works as a receive antenna. Even your TV rabbit ears were of a length appropriate for those frequencies - around a quarter wave. Transmit is a lot more critical electronically.

[@jtc1947]

@@BustRadioMythsOur antenna was mounted to the chimney of our house and I don't remember the antenna arms being of DIFFERENT LENGTHS for the various channels.

[@BustRadioMyths]

Well no not for each channel. One length would be fine for all the TV channels. Again,. with transmitting the antenna will not present an acceptable load to the transmitter if it is not the right size, generally a half or quarter wave, depending on the design.

[@patriot9455]

You mean that you need to be able to make an electric connection over long distance, if you want to be able to send or receive any signal. HOODA THUNK IT

[@JxH]

I got as far as 0:22 and was forced to comment. I hope that you realize the exception to the rule... In some antenna systems a *specific length* of coaxial cable (i.e. 0.25 wavelength, yes considering the velocity factor of course) is used to act as an impedance transformer. For example, if you had a folded driven element and its Z was around 112 ohms (due to nearby reflectors and directors), then a quarter wavelength of 75-ohm coax could be used to flip that 112 ohms around to about 50 ohms. This is not some unknown theoretical exception, even good old Radio Shack used to sell "Twin" CB whips that used that same impedance matching concept (except for paralleling two whips). There are plenty of other traditional Ham radio antenna designs that require the specified feed system (the ARRL handbooks, especially the older editions, are full of examples). It's essential that when explaining the rule, that you also acknowledge the exceptions. But yes, you're generally correct. A dog's legs need to be long enough to reach the ground. Same thing for coaxial cables in most antenna systems, i.e. ones that are not designed and intended to employ a specific length of feedline as part of the matching concept. To preempt the obvious rebuttal: When using a *coaxial* quarter-wavelength matching, I would consider that to be part of the antenna 'system', but it's not intended to be part of the antenna itself (the antenna elements that radiate). Pedantry aside, noobs would easily mistake it for a feedline, and taking your incomplete advice they might trim it. Their only clue might be that it's perhaps not a 50-ohm coaxial cable.

[@BustRadioMyths]

Yes I understand that. The video was not about quarter wave transformers.

[@JxH]

@@BustRadioMyths Yep, that's why I mentioned it in the comments. "...Myth - Coax Transmission Lines Must Be A Specific Length" '...Fact - Coax Quarter-wave Transformers (may be made from coax) Must Be A Specific Length' The challenge for noobs is to distinguish these exceptions. Thus worth mentioning.

[@dennistowerofpower5808]

This is the second time on your videos you are very vague, which leads new operators astray… there are many articles throughout the years that stress, the importance of velocity factor, whether it’s solid dielectric, or foam dielectric and the importance of velocity factor when feeding an antenna array, some require odd multiples of a half wavelength in order to reach the feed point some require even multiples of a quarter wave length to reach the feed point…it does play a role in the performance of the antenna array… and another reminder that operator should understand tight circles of foam, dielectric coax will cause center conductor to cross talk to shield if you are running the legal limit or beyond the legal limit….

[@KiloWatt304]

this is true but also not true .... there are too many other factors involved mounting hight, ground conduction, reflection from nearby objects and such ,,,, remember we are dealing with RF AC power ... i have used coax baluns and had poor field radiation readings ... changed coax lenght and removed baluns and presto good readings ... i could go on like a book but this is not the place .... great video

[@ramonbennett8416]

Again always sharing how the the real life actually works!!! KN6JHC