The myth of coax length.

[The DB]

Cbphreaker, there is a big difference in what you and Captain Kilowatt are saying. You are saying that an SWR reading on a 1/4 wavelength of coax will be different than it is taken at the antenna unless the antenna is at a perfect match. Captain Kilowatt is saying that in that situation, as well as all others, the SWR will be the same irregardless of the match and length of coax. You are saying SWR changes with the coax's transformation of impedance, he is saying that even though the coax transforms the impedance, SWR stays the same.

To say you guys said the same things is simply not true.

For the record, the test you mention I have actually done, although it was more an extension of another test. SWR did not change with coax length when using a VNA to measure various lengths of coax into any number of custom dummy loads. These dummy loads ranged from a so called "perfect match" to 5:1. The only exception to this is if you have enough coax and said coax is lossy enough, the match will seem to get closer to a good match with coax length, i.e. SWR will gradually seem to get better the longer the coax is. This is not caused by a change in impedance, but by losses, and is not anything like what you are saying.

This happens weather or not there is a perfect match at the far end, or weather or not you have a specific length of coax. It is so reliable in fact that when tuning an antenna I will put a 3 foot jumper on to make sure SWR stayes the same, and if it doesn't I know there is a problem that must be dealt with. Every antenna, when I am done with it, has a consistent SWR no matter the length of coax, unless there is very long length as I mentioned above.


The DB

 

[cbphreaker]

Your not kidding about common mode current!

A quote from one of my "bibles", The ARRL Antenna Handbook

From the sub title "SWR Change with Common Mode Current"
"In theory at least, we have a situation where a change in the length of the unbalanced coaxial cable feeding a balanced dipole will cause the SWR on the line to change also. This is due to the chaining common-mode impedance to ground at the feed point. The SWR may even change is the operator touches the swr meter, since the path to ground is subtly altered when this happens. Even changing the length of an antenna to prune if for resonance may also yield unexpected and confusing results on the SWR meter because of common-mode impedance." (pp26-16)

According to the ARRL Antenna Handbook SWR can change. ( I must have forgot cuz it feels like new info)


Another "bible" of mine "Understand and Repairing CB Radios" claims

"Coax length changes apparent SWR when standing waves are present but not that the fact the antenna is causing them."

Further reading garners:

"The cure for suspicious SWR readings is to measure at the antenna itself where practical. Base antennas use standard coax sockets ... Use a 1/2 wavelength or it's multiple (2/2, 3/2, 4/2,5/2, etc) instead. Such a length repeats whatever impedance actually exists at the antenna while a 1/4 wave length feed line inverts it."

A final read using the same book reads:

"To prove this fact to yourself or to a stubborn customer, make a test cable shown in figure 8-9 (pp 315). " A graphic shown is a 12 foot 50 ohm coax with a T connector and two 50 ohm restive loads on either end. " By paralleling two 50ohm dummy loads you've made a 25 ohm non-reactive impedance. The SWR meter when placed as shown" Picture shows the SWR meter at the far end of a 12 foot coax." 'should read 2:1. You'll find that it does read this with 12 foot (1/2 wave length) but reads something quite different with a 6 foot (1/4) cable! This should end the argument..." The diagram is now the SWR meter at the end of a 6 foot piece of coax with the same 2:1 load.


Would one of you please point me in the direction where published information directly discredits these accepted publications?

Cheers -

CBPhreaker

 

[LeapFrog]

So coax length matters in certain situations, for most CB base station setups using 50 ohm coax with a proper ground plane it shouldn't matter as much?

I better go read up on common mode currents, because the confusion is setting in!!

 

[vkrules]

No !.If it matters you have an issue with your installation.

 

[cbphreaker]

This is exactly how misconceptions are started. It's ALWAYS best and prudent to verify any and all information from respected sources before committing them as fact in your head.

No, the length doesn't matter. But for perfection sake I should note that good quality coax should be used for line loss minimization.

Line Length matters when there are standing waves on your line or again feeding a balanced antenna with unbalanced line. When it comes to making accurate measurements a diode based SWR meter will be driven batty if the antenna isn't tuned right and the length of coax is random. But really, thats about it.

I mean when your looking at a meter showing 3:1 or 4:1 SWR who cares if it common mode, tuning or something else the fact is somethin' aint workin' right. In that case the absolute value is almost irrelevant.

Most CB ops don't have a VNA.

If you properly tune your antenna, use baluns where applicable, and don't feed balanced antenna's with unbalanced line you should not have any issues with a random length of coax.

You library should consist of the minimum books
1. The ARRL Antenna Handbook
2. The ARRL Handbook for the Radio Amateur
3. The ARRL RFI Book
4. Understanding and Repairing CB Radios for the Professional Technician

The term used by CB ops Flat or Flat-lined means no matter where you measure the SWR it's the same - or the antenna is FLAT or 1:1 or only 1 Standing wave on the line at any give time. I wonder how or why that term came about?

Cheers

CBPhreaker

 

[Road Squawker]

Your not kidding about common mode current!

A quote from one of my "bibles", The ARRL Antenna Handbook

From the sub title "SWR Change with Common Mode Current"...CBPhreaker

Nice try, but,..................... apples and oranges

this thread is about VSWR changing DUE to coax length.

you are adding a different variable (common mode current) and trying to use that to justify your incorrect theory.

 

[Road Squawker]

This is exactly how misconceptions are started.

No, the length doesn't matter...

Line Length matters when...
CBPhreaker

Well, which one is it?

 

[cbphreaker]

VSWR changing DUE to coax length.

I think it rather obvious that the VSWR would be reported as less than it really is at the end of a long length of coax...it's kinda common knowledge..don't you think?

My theories aren't really theories unless you consider my references as nonfactual.

You didn't really add anything to this thread other than you own "theory" which has no fact backing it up. Pointless.

Well, which one is it?

It doesn't really matter on the line length unless your tying to measure the SWR of the antenna at the radio end. To do that is when one needs a multiple of 1/2 wave length.

The reported VSWR will be somewhat lower at the radio end due to the loss of cable but if the line is flat it makes no difference on the length as far as getting out (again take into the considerations the losses as part of the total signal loss of the feedline. )

Cheers

CBPhreaker

 

[Tallman]

Now let's really mix this up with "Power Factors".
You can have an apparent wattage of let's say 600 watts. The actual truth of the matter is that the "Apparent Power" is inductive and capacitive in nature and is "Wattless power and is not consumed in the circuit. If the power factor is .8 the circuit is only consuming 480 watts. Not the 600 indicated. Throw in some RF common mode noise and it can almost impossible to figure it out. It can be done.

A low SWR ratio still does not indicate resonance of the antenna. The highest peak on a field strength meter at a measured distance from the antenna is a better indication of increased radiation.

 

[cbphreaker]

The actual truth of the matter is that the "Apparent Power" is inductive and capacitive in nature and is "Wattless power and is not consumed in the circuit

100% Agree!!



Cheers

CBPhreaker

 

[Tallman]

While this is a very nice graphic it could not be more wrong. The center conductor of the coax and the shield do not change polarity for the cycle. If you look at a 120 volt ac power plug you have three colors of wires. Green, White, and Black. The only wire that supplies the power is the Black wire. It and only it oscillates between positive and negative voltage. The white is the Neutral wire and is at ZERO AC voltage potential and is the return line for the power.
The Green wire is EARTH GROUND and is at ZERO Volts. The safety chassis ground cable which is in no way a suitable Station ground.
If radio transmitter outputs truly changed in that fashion you would need Dual Gamma matches on every beam antenna.

 

[bob85]

cbphreaker,

you say "It doesn't really matter on the line length unless your tying to measure the SWR of the antenna at the radio end. To do that is when one needs a multiple of 1/2 wave length"

That comes from one of lou's cb book of fairytales,
that's not what the ARRL or any other respected source says,

this topic has been done to death & explained with links to actual articles from respected sources such as the ARRL W2DU bird electronics, tellewave, mfj,

lou franklin did not understand antennas and feedlines when he wrote his books,

you don't seem to have understood what the ARRL said i suggest "another look at reflections" by walt maxwell to erase those cb myths.



thanks

 

[The DB]

A quote from one of my "bibles", The ARRL Antenna Handbook

From the sub title "SWR Change with Common Mode Current"
"In theory at least, we have a situation where a change in the length of the unbalanced coaxial cable feeding a balanced dipole will cause the SWR on the line to change also. This is due to the chaining common-mode impedance to ground at the feed point. The SWR may even change is the operator touches the swr meter, since the path to ground is subtly altered when this happens. Even changing the length of an antenna to prune if for resonance may also yield unexpected and confusing results on the SWR meter because of common-mode impedance." (pp26-16)

According to the ARRL Antenna Handbook SWR can change. ( I must have forgot cuz it feels like new info)

So you are quoting somethign that in the 22'nd edition is the second paragraph under the title "SWR Change with Common Mode Current". Chapter 24, page 38. This section of the chapter is specifically talking about SWR changes due to the presence of common mode currents. The impedance that is being discussed here, you will not in this section, and your quote, has the words "common-mode" in front of it. To explain this further, the first paragraph that you did not quote includes the text:

As could be expected the common-mode impedance in parallel with the dipole's natural feed point impedance has lowered the net impedance seen at the feedpoint,

"Impedance" and "common mode impedance" are clearly being treated differently. Further, using text from a section that specifically tells you that it is common mode currents that are causing this SWR change, and saying that the book is saying that it is impedance that causes this change, I'm sorry, but either you don't understand what it is you are reading, or you are trying to manipulate us with a selective choice of text taken out of context by a reputable source. If anyone wants to read it for themselves, I an happy to post the entire section.

When it comes to the other book you quoted, I have honestly not read it as it was not on my radar of gooks to make sure I got and read. I will look for a .pdf version and take a look at it when I get some time.

That being said, lets look at your first quote from that book...

"Coax length changes apparent SWR when standing waves are present but not that the fact the antenna is causing them."

Take a look at the part I put in red. This apparent SWR change happens when standing waves are present. Well, a well made choke can remove said standing waves. Nothing is said about impedance here.

About your other quote...

"To prove this fact to yourself or to a stubborn customer, make a test cable shown in figure 8-9 (pp 315). " A graphic shown is a 12 foot 50 ohm coax with a T connector and two 50 ohm restive loads on either end. " By paralleling two 50ohm dummy loads you've made a 25 ohm non-reactive impedance. The SWR meter when placed as shown" Picture shows the SWR meter at the far end of a 12 foot coax." 'should read 2:1. You'll find that it does read this with 12 foot (1/2 wave length) but reads something quite different with a 6 foot (1/4) cable! This should end the argument..." The diagram is now the SWR meter at the end of a 6 foot piece of coax with the same 2:1 load.

I have done this test, not with a T adapter and two 50 ohm loads, but custom loads that are at 50, 75, 100, 150, 200 and 250 ohms, this did not happen. SWR was the same with every coax I tested with these loads. Hell, if you go back far enough (we are talking years) there is a thread on this forum where you can see the results of my doing essentially this test (actually a bit more of an expanded test) with my VNA. The results that were talked about simply didn't happen.

If you really want some good reading on this subject, you should locate one of the three Reflections books (Reflections, Reflections 2, and Reflections 3). If you can't find any of those, do a search for a freely available .pdf article by a M. Walter Maxwell called "Another look at Reflections".

And finally, I will leave you with another quote from the ARRL Antenna Book series, Chapter 24, Page 4.

A fourth myth is that changing the length of a feed line changes the SWR. Changing a feed line's length does not change the SWR (except for losses) inside the line.

It goes on to explain that any change in SWR was due to common mode currents.

Nowhere does this or any previous versions of this book (I own all but three of the ARRL Antenna Book Series) does it say that SWR will change with coax length unless there are common mode currents present. Anyone who reads these books and thinks that said books are telling them that it is the coax's impedance mismatch with the antenna that causes this needs to do some serious reading and thinking, because they are misleading themselves.

It is strange though, what you are saying in your post after the one I quoted from is not the same as what you said before that. I agree with everything antenna related from that post. Talk about a 180 on your part... I have three of the four books you recommend, and actually multiple versions of two of books that you recommend, and can recommend other books, and in fact I already have...


The DB

 

[The DB]

Now let's really mix this up with "Power Factors".
You can have an apparent wattage of let's say 600 watts. The actual truth of the matter is that the "Apparent Power" is inductive and capacitive in nature and is "Wattless power and is not consumed in the circuit. If the power factor is .8 the circuit is only consuming 480 watts. Not the 600 indicated. Throw in some RF common mode noise and it can almost impossible to figure it out. It can be done.

A low SWR ratio still does not indicate resonance of the antenna. The highest peak on a field strength meter at a measured distance from the antenna is a better indication of increased radiation.

Mmm, power factor, don't see people talking about that much when it comes to antennas, or coax for that matter...

I am curious as to what part of the antenna system you think we need to worry about the power factor?


The DB

 

[bob85]

what is apparent vswr ?, i only know actual vswr,