Does different coax affect swr?

[tba02]

There is no reactance at resonance. That is the definition of resonance.......the frequency at which Xc completely cancels Xl or vice versa.

I probably should have quoted the post that referenced the "resistance". It might have made a bad joke mediocre.

 

[Captain Kilowatt]

I probably should have quoted the post that referenced the "resistance". It might have made a bad joke mediocre.

Apparently now the joke is on me. I misunderstood something.

 

[tba02]

It's OK, I am like a bad dipole, too obtuse to provide a good match.

 

[JoeDirt]

Tuning with coax lengths as Joe dirt basically suggests so an amp will see 50 ohms isn't the correct way to do it either. That's the Mark Sherman way to do it. The correct way if you really want to be precise is to add some inductance right at the antenna feed point. A simple home made shunt coil will do the trick.

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.

 

[fourstringburn]

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.

Where did you learn that about coax lengths reducing CMC?

1/4 wave mobile antennas aren't slightly unbalanced, they are unbalanced. A dipole is a balanced antenna. A typical dipole is 2 equal lengths.

 

[JoeDirt]

Where did you learn that about coax lengths reducing CMC?

1/4 wave mobile antennas aren't slightly unbalanced, they are unbalanced. A dipole is a balanced antenna. A typical dipole is 2 equal lengths.

It doesn't reduce common mode current a 1/4 wave feeder the common mode current is at a minimum, 1/2 wave feeder the common mode current is at a maximum.

 

[fourstringburn]

It doesn't reduce common mode current a 1/4 wave feeder the common mode current is at a minimum, 1/2 wave feeder the common mode current is at a maximum.

You still didn't answer, where did you learn that any coax lengths reduces or adds to CMC?

 

[bob85]

altering coax length can effect CMC magnitude because you are changing the electrical length of the outside surface of the coax braid which changes the common mode impedance of the path to ground down the outside of the coax seen in parallel with the load,

Don't know where the 1/4wave is minimum 1/2wave is maximum came from, some cber made that up,
There are no optimum lengths, either length could give maximum or minimum CMC or something between depending on what's connected to the radio end and its electrical length to ground,

 

[fourstringburn]

But it doesn't get rid of it or alter it enough to make much difference.

The proper way is to choke it off at the source, anything else is merely a band-aid if that.

It's still there no matter if you lengthen or shorten the coax.

 

[JoeDirt]

You still didn't answer, where did you learn that any coax lengths reduces or adds to CMC?

I remember doing it as a teenager, picked it up from an older radio operator at the time.

Here is a site that I found that talked a little more about it
https://www.w8ji.com/common_mode_current.htm

 

[JoeDirt]

But it doesn't get rid of it or alter it enough to make much difference.

The proper way is to choke it off at the source, anything else is merely a band-aid if that.

It's still there no matter if you lengthen or shorten the coax.

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.

 

[The DB]

Don't know where the 1/4wave is minimum 1/2wave is maximum came from, some cber made that up,

It is based on an incomplete understanding of theory. Essentially, his statements presumes certain assumptions that rarely exist in a mobile installation.

Their are certain other aspects to the hobby that work because of the same principle, such as the 1/4 wavelength coaxial transformer, stub tuning in general, and why OCF dipoles have a higher feed point impedance than their center fed counterparts.


The DB

 

[JoeDirt]

altering coax length can effect CMC magnitude because you are changing the electrical length of the outside surface of the coax braid which changes the common mode impedance of the path to ground down the outside of the coax seen in parallel with the load,

Don't know where the 1/4wave is minimum 1/2wave is maximum came from, some cber made that up,
There are no optimum lengths, either length could give maximum or minimum CMC or something between depending on what's connected to the radio end and its electrical length to ground,

1/4 wave is capacitive a longer than a 1/4 wave is inductive. 3/4 wave will also work, its not ideal but it works better than most crappy baluns people buy or build.

Edit: You are correct there is no optimum length for a properly terminated transmission line. However when there is common mode present the correct length transmission line can cancel out the common mode current via the series capacitance and inductance of the transmission line.

Edit: sorry I read that wrong

 

[fourstringburn]

It seems that the consensus with many CB shop techs is that 1/2 wavelength coax feedlines is the magical cure-all for all tunings and RF issues in mobile antenna installations.

 

[JoeDirt]

It seems that the consensus with many CB shop techs is that 1/2 wavelength coax feedlines is the magical cure-all for all tunings and RF issues in mobile antenna installations.

I think they stuck with that length because the truck stop 18' foot half wave isn't even close to the real electrical half wave. Depending on the velocity factor of the coax the real half would be a great recipe for burnt lips.