Overheard yesterday.....

[freecell]

"I stated that VSWR measures the same anywhere within a coaxial transmission line that stays the same physical length"

and i'm telling you that other conditions are required for that statement to be true. a 3:1 mismatch exists at the load and a 1.2:1 swr is seen at the feedline input. here is a set of conditions where your statement is not true. swr here is clearly not the same regardless of where it is measured and you have yet to define just what physical length needs to stay the same. just exactly how do we determine the length required to validate your statement as true? what physical length of line is required under these conditions so that swr is the same no matter where it is measured?

 

[ken white]

And to carry that just a tiny bit further. If the 'matching' is done at the input of the antenna instead of in the feed line, the length of feed line doesn't, and will never, make any difference. That makes the feed line do only the job it was designed to do, and lessens the risk of ruining the feed line. Seems simple to me, not sure why it doesn't to others.
- 'Doc

Doc, the VSWR measured at the antenna is the same as the VSWR measured at the transmitters output, so the coax will still be part of the tuning process unless the coax is cut to a 1/2 wavelength multiple of the tuning frequency. The coax will always be part of the tuned system if the transmtter output, antenna input, and coax are not exactly matched at the same frequency. If a 1/2 wavelength multiple is used, then the impedance measured via the coax will be the antenna impedance without the coax impedance transformation properties occuring.

 

[ken white]

"I stated that VSWR measures the same anywhere within a coaxial transmission line that stays the same physical length"

and i'm telling you that other conditions are required for that statement to be true. a 3:1 mismatch exists at the load and a 1.2:1 swr is seen at the feedline input. here is a set of conditions where your statement is not true. swr here is clearly not the same regardless of where it is measured and you have yet to define just what physical length needs to stay the same. just exactly how do we determine the length required to validate your statement as true? what physical length of line is required under these conditions so that swr is the same no matter where it is measured?

freecell, tell me how the forward and reflected power is changing at different points within a length of coax. If this is true, what power is the correct one and at which point?

SWR is defined as the ratio of Vmax/Vmin, or Imax/Imin, or Zmax/Zmin, and is typically calculated using power measurements. At any point measured along a length of coax there will be a standing wave value that is different from another point, but only within a 1/2 wavelength section of line. However, the maximum and minimum measured values within this 1/2 wavelength section of line will not change and neither will the SWR since it is derived from these values...

So, your argument is the maximum standing wave and minimum standing wave values aren't fixed for a given power. Is this correct? If so, what is the definition of a standing wave and what causes it?

How exactly did you measure the VSWR in your example? Did you use a power meter at the CB output and then move the power meter to the antenna input and did the overall coax length stay the same? Did you stand next to the antenna and change its properties? Did the power readings change?

:?

 

[C2]

this is better than a presidential debate! Wish we could get this on film with the opponents at their respective podiums.

:bash

 

[ken white]

this is better than a presidential debate! Wish we could get this on film with the opponents at their respective podiums.

:bash

 

[C W Morse]

This argument will last 'til the end of time---or the end of your coax. Whichever comes last. ...........or first.............or....something.


CWM

 

[rfoverlord]

wow get them both a new mfj-269pro and they will be busy testing the accuracy of that and forget all about it.

 

[W5LZ]

Ken,
Ah, but that's the point! If the matching is done at the antenna input, then the antenna is the same impedance as the coax, as the radio's output. Yeah, I know that's at only one frequency, but considering the normal variations in normal coax, radio outputs, and antenna inputs, it's about as 'practical' as you can get.

C2,
Color me as in Ross Pirot. NOT that inventor of the internet!

CWM,
I'll take the 'something else', as in ladder-line, then it just doesn't make a lot of difference.

- 'Doc

 

[ken white]

Ken,
Ah, but that's the point! If the matching is done at the antenna input, then the antenna is the same impedance as the coax, as the radio's output. Yeah, I know that's at only one frequency, but considering the normal variations in normal coax, radio outputs, and antenna inputs, it's about as 'practical' as you can get.

- 'Doc

Doc, standing wave ratios greater than 1:1 occur because of an impedance mismatch between the transmitters output and coax, or antenna input and coax, or both. When this condition occurs, the signal sees a transition point discontinuity, which creates a reflection, and increases the SWR. The SWR that is measured only resides within the coax and there is no way to determine which end is causing the the increase when tuning the antenna with a power/swr meter. Because of this mismatch, the coax is changing properties from being the ideal constant 50 ohm transmission impedance to a varying impedance based on wavelength - it acts like a tuned filter. The only way to try and take the coax out of the equation when tuning is to use a 1/2 wavelength multiple piece of coax. This wavelength allows the coax to appear transparent to the transmitter output when tuning the antennas feedpoint so there is minimal coax interaction. Of course this works when using an antenna analyzer as well.

There is also a SWR that occurs within the antenna and its elements, but these values cannot be measured directly since there is no easy access point to connect equipment on the other side of the discontinuity, and since the antenna impedance is not anywhere near 50 ohms when measured at various points. When tuning the antenna, the overall electrical wavelength of the transmission line and antenna is being varied such that the impedance at the coax's connection point at the antenna will reduce the impedance mismatch between the transmitters ouput and coax, however, at the antenna it may not be anywhere near 50 ohms when matched. When the antenna is tuned, it is not making the antenna any more efficient since the physical length of the antenna determines this, not the tuning. Tuning makes the antenna and coax appear as 50 ohms to the transmitters output.

This is one reason why I get a chuckle out of people that say a tuner is really tricking the transmitter, while tuning an antenna by varying inductance or capacitance does the same thing. However, if the antenna is tuned by trimming and is truely resonant, then the antenna is working at a high level of efficiency.

The key thing to understanding VSWR is that it resides inside the coax, that it is constant if there are no losses and nothing else is changed, and its magnitude is a function of the external variables that are connected at its ends which define whether the the coax is a varying impedance or a constant impedance.

I'm not trying to change anyones mind about VSWR or coax length, I know that can be futile in some instances, but rather I'm just trying to open up some dialog so we can all learn and share our experiences.

 

[Marconi]

It is just fortunate for us all that we don't have to be perfect, ain't it?

 

[ken white]

It is just fortunate for us all that we don't have to be perfect, ain't it?

The discussion isn't about perfection, it is about trying to understand and learn...

 

[GRTS]

if it is i don't see it. for example, why am i able to see a 3:1 mismatch at a given load and see 1.2:1 or less at the transmitter input looking into the same load with a given length of line?

If your coax line was an odd multiple of a half-wavelength for the testing frequency you would have seen 3:1 at both ends.

Changing coax length can never change SWR at the antenna but it can make the SWR meter give false readings. SWR can only be changed at the antenna.

This is the main reason old school hams adjust mono-band antennas with the SWR meter at the feed point.

 

[Master Chief]

why am i able to see a 3:1 mismatch at a given load and see 1.2:1 or less at the transmitter input looking into the same load with a given length of line?

You already know the answer to this question. The given length of line transforms the 3:1 to 1.2:1 AT THE TRANSMITTER.

 

[bob85]

lets have a guess,
the impedance at the feedpoint will be mirrored periodically every electrical halfwavelength of coax from the antenna/load and swing to a maximum deviation every 1/4 electrical wavelength from the load,
somewhere inbetween a certain number of electrical degrees from the load will be your 1.2:1 impedance match also repeated periodically as you move away from the load, this impedance looking into the coax will be reactive regardless of the load been reactive or not whenever the load is different to the lines own characteristic impedance,

will the vswr measure the same anywhere on the line ?,
at this point i believe the poor vswr is there and is constant along the line as that is what i have read, yes books and the internet can be wrong too and i am always willing to learn lol,

i have never tried to move the meter and keep the same electrical length of feedline under these exact conditions to see what happens to the indicated vswr at another point in the feedline,
im pretty sure i have seen adding longer patchleads between meter and radio untill the radio is happy with the transformed impedance does not make the meter now part way down the line show a good vswr but i would not put a large wager on it,

i dont think cutting coax gets rid of standing waves but it can make a vswr meter read a low vswr at the input end of coax terminated with a load other than its characteristic impedance,

maybe the different vswr readings is due to how a typical vswr meter samples the signal and displays the results,

what ken is saying is what i have read too but i have also seen that cutting coax can give a good vswr indication at the transmitter,
does this also mean that the good indicated vswr is constant along the line?
some sources claim that cutting coax only transforms the impedance but the standing waves remain,
do they change in magnitude when you tune the coax? i dont know,
i have never seen anything specifically about the vswr along a mismatched line that has been cut to match the load to the transmitter but im sure somebody will have written it down somewhere,
what i have read and what i think i have seen do not tally unless a vswr meter is not actually measuring vswr.

 

[freecell]

"You already know the answer to this question. The given length of line transforms the 3:1 to 1.2:1 AT THE TRANSMITTER."

my question was why and how and what effect does this have on the swr previously seen at the antenna input.