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Does different coax affect swr?

At that frequency and length the common mode is not present, It works a lot better than most chokes. Most of your so called chokes are a band-aids, a lot of operators add chokes and all they are really doing is adding more inductance or capacitance. Not adding the needed resistance to "choke" off the common mode.


You do not need resistance to choke off CMC. You need inductive reactance (inductance) to offer a high impedance to the CMC. That is the whole idea of adding a choke. You WANT inductance to choke off the RF currents on the OUTSIDE of the cable. It has zero effect on currents INSIDE the cable. If resistance was what you needed you could insert a megohm resistor and all would be good. Using a specific length of cable (1/4 wave) does not get rid of the CMC it only creates a null at the equipment end and masks the problem.
 
..., I tune the coax for a perfect 1/4 wave length to reduce common mode current....

a "perfect" 1/4 wave? is that a 1/4 wl or a 1/4 elect wl?, or...?

there is a "proper" length for a coax cable, it's about a foot (YMMV) longer than necessary to go from the antenna to the radio,
 
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Well there you have it JoeDirt from 2 of the more knowledgeable members on the forum.

Coax lengths don't rid CMC nor do they tune antennas. There are exceptions for tuning like stub matching and multiband antennas that deliberately use part of the coax as a radiating element.

What I learned to use for CMC on a mobile antenna is making 7 turns of rg8x about 4 inches in diameter and use a mix 31 snap on ferrite. CMC issue resolved! This creates inductive reactance as Captn KW stated.
 
Well there you have it JoeDirt from 2 of the more knowledgeable members on the forum.

Coax lengths don't rid CMC nor do they tune antennas. There are exceptions for tuning like stub matching and multiband antennas that deliberately use part of the coax as a radiating element.

What I learned to use for CMC on a mobile antenna is making 7 turns of rg8x about 4 inches in diameter and use a mix 31 snap on ferrite. CMC issue resolved! This creates inductive reactance as Captn KW stated.
Mix 31 with 7 turns of RG8X with a 4 diameter. makes a great balun/choke and creates inductive RESISTANCE at the giving target frequency. That effective frequency is below 10mhz and not as effective at 27mhz.
 
Mix 31 with 7 turns of RG8X with a 4 diameter. makes a great balun/choke and creates inductive RESISTANCE at the giving target frequency. That effective frequency is below 10mhz and not as effective at 27mhz.

"Inductive resistance" doesn't exist, perhaps you are thinking of "inductive reactance"? Resistance and reactance have the same electrical symbol, and at first glance appear to act the same, however, they are not the same, not even close.

If you are turning coax through a ferrite, the diameter of the turn in the coax is not the important thing, its the number of passes through the ferrite. Its the ferrite that is doing the heavy lifting, not the coil, and in most cases the coil is incidental. Ferrite based chokes are essentially adding resistors to the outside of the coax, and each pass through a ferrite (be it the same ferrite multiple times or different ferrite beads next to each other) actually acts like resistors in series. Each pass through an FB-31-24001 core at 25 MHz adds 63 ohms of resistance, so seven passes would add 441 ohms of resistance to said common mode currents at that frequency. Raise that up to CB frequencies and you will have a slightly higher resistance as a ferrites resistance goes up with frequency, not down.

Also, in a ferrite based choke it doesn't matter where the resistance peaks as long as you have enough on the frequency at hand.

Reference link.


The DB
 
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Mix 31 with 7 turns of RG8X with a 4 diameter. makes a great balun/choke and creates inductive RESISTANCE at the giving target frequency. That effective frequency is below 10mhz and not as effective at 27mhz.
It will be effective across the bands and not just a target frequency as I use this for multiband mobile operation from 3.5 to 29 MHz. Mix 31 beads are effective from 1 to 300 MHz. Mix 43 beads are from 20 to 250 MHz.

You should do a bit more research on CMC.
 
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Mix 31 with 7 turns of RG8X with a 4 diameter. makes a great balun/choke and creates inductive RESISTANCE at the giving target frequency. That effective frequency is below 10mhz and not as effective at 27mhz.

No such thing as inductive resistance. There is inductive reactance which is the opposition to current flow thru an inductor and it is measured in ohms however it is not a resistance per-sea.
 
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Here are three models that I think might shed some light on what we heard from Joe and the 1/4 wave minimum vs. 1/2 wave maximum currents on the feed line issue.

It is important to consider what W8JI writes in this link Does different coax affect swr? Also consider the quote below that is noted near the bottom of W8JI's article. I think this problem was also demonstrated quite well in the article with a 1/2 wave horizontal dipole...just in case you missed it.

"Common mode currents are also low with a 1/4 wave feeder. In many or most cases of dipole height between 1/8th and just over 1/4 wavelength, a balun is NOT necessary provided the feed line drops straight down through open air to ground, and the feed line is grounded when it reaches the earth."

This is what you see in the attached PDF file below where the feed line is connected directly below where it touches the ground as apposed to a greater distance away at your radio and the electrical service pole.

1. the antenna with no choke at 36' feet with an even number of 1/4 wavelengths for the feedline.
2. the same antenna with a choke added to help mitigate some of the CMC.
3. the same antenna raised 9' feet for an odd number of 1/4 wavelengths of feed line.

The red lines represent currents in the models and the further away the line is from the element...the greater the magnitude of the current.

Did W8JI get it wrong or do CBr's just miss the point of what he is telling us?
 

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  • Common mode current issues..pdf
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Did W8JI get it wrong or do CBr's just miss the point of what he is telling us?

W8ji is correct, however, that is not a complete explanation of what is happening in the stated situation, namely a mobile environment. A few questions for you.
  1. Does an outer coating on a wire change the velocity factor of said wire? Of any currents flowing on the outside of coax?
  2. If a radiating antenna is next to metal (as the feed line would be inside a metal vehicle for instance), does it change the electrical length of said antenna?
  3. Do common mode currents radiate like they are part of the antenna?
Put all of this together and you will see what I was referring to.


The DB
 
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The key is its grounded at the source end Eddie,
low impedance ground inverted through odd 1/4wave multiples to a high impedance at the antenna end,
disconnect it from ground & your odd 1/4waves have maximum CMC like the grounded 1/2wave,
what joe is claiming only applies to a certain set of conditions rarely found in mobile installs,

Next we will have people arguing the virtues of cutting coax to electrical lengths of the outside of the braid to minimise CMC vs cutting electrical lengths of the inside to minimise impedance transformation.
 
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The key is its grounded at the source end Eddie,
low impedance ground inverted through odd 1/4wave multiples to a high impedance at the antenna end,
disconnect it from ground & your odd 1/4waves have maximum CMC like the grounded 1/2wave,
what joe is claiming only applies to a certain set of conditions rarely found in mobile installs,

Next we will have people arguing the virtues of cutting coax to electrical lengths of the outside of the braid to minimise CMC vs cutting electrical lengths of the inside to minimise impedance transformation.

Bingo.

The problems with applying said theory in a mobile environment, are:
  1. The radio is rarely directly connected to ground, making the connection to ground longer than the physical length of the coax as it has to travel up an additional wire.
  2. The velocity factor of the outside of the coax is unknown unless you have measured it, and is almost certainly different than the velocity factor listed for the inside of the coax, generally higher.
  3. In addition to the above, the interaction between the metal body of the vehicle and the outside of the coax shielding will also affect the physical length of coax needed.
Is is possible that their is someone out their that can account for these issues? Sure, but I wouldn't waste my time with something like this when other methods are much easier and generally more reliable.


The DB
 
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I depend on my models for information, but I'm not sure about these mobile objects I use.

Well DB, to this point I have little to no confidence my mobile objects being anywhere near accurate, and that is why I posted the comment above. I would leave it to the reader to go back and reread the article and pay attention...if they missed that important distinction concerning the length of the ground side of the line.

To give a visual of this effect in modeling...it is to be noted that when the mast touches the ground in the model "O" with a little circle inside...the feed line length for both the shield and the center conductor would be identical, and that is a difference that must be understood. As Bob will probably note...such a situation is likely to not be possible in our antenna work at home and in our back yard. The center conductor and the shield length to ground will not likely ever be the same length.

W8ji is correct, however, that is not a complete explanation of what is happening in the stated situation, namely a mobile environment. A few questions for you.

Yes IMO W8JI is right, but there is a lot more to consider that the words here cannot likely explain easily. I too will put mobiles in a different category in this case, but I'll leave that to you to explain.

Personally I never noticed the ill-effects on performance when CMC were manifest in my CB system. Maybe my antennas did not have bad enough issues with CMC. However, I did think I was able to tell there were CMC on the feed line, but I wouldn't bet my life on the fact. IMO there is likely little to no way to really tell how or if these CM currents help or hurt performance, even though I've heard a lot of symptoms being solved immediately upon stopping these currents from manifesting themselves locally. Similar stories to accounts about earth grounding and antenna responses come to mind. IMO a lot of that is likely in the imagination of the operator.

Sometimes my models show enough CMC on the mast that I see it ill-effect the pattern produced and sometimes I see the CMC seem to help the far-field gain and pattern produced.

So, if there is any truth in my observations and models here...then I doubt there are too many out there that will be able to really do anything...except possibly minimize the CMC from manifesting locally to there station. But again, I don't see how they can determine if eliminating these currents helped performance or not, I never have seen it happen.

On two occasions with antennas that acted very badly in this regard...I got local reports the antennas performed with a bit stronger signal than other antennas I had up an working at the same time and that suggest to me that CMC can also be of benefit in some situations.

The key is its grounded at the source end Eddie,

Yea Bob, that is why I called attention to the specific sentence in W8JI's article where he made the same claim. I even highlighted it by underlining it, because I think most CBr's overlook the very important distinction you just made.
 
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Well not necessarily, I tune the coax for a perfect 1/4 wave length to reduce common mode current instead of using a balun. Its a perfectly acceptable way. Mobile antenna installation are most often slightly imbalanced.
Since purely resistive 50 ohm mobile quarter wave antenna doesn't really exist, how are you getting 50 ohms to the transmitter with the quarter wave impedance Transformer ?
Are you using a tuner or some other matching method?
 

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