100 feet of RG-58A/U SWR effects shown visually.

[hookedon6]

SWR would appear to be 1:1 as in a theoretical infinite length of "perfect" no loss coax there would be no far end, and the signal would continue to travel down that coax ad infinitum, thus causing no reflected signal to measure.

If there is a far end as you describe it would in theory show an SWR that approaches infinity, assuming you waited long enough for the signal to get to the end of the near infinitely long "perfect" no loss coax and make the return trip to make the measurement. Until that signal makes its way back to the measuring device it will appear to be a perfect SWR match.

Am I right?


The DB

no

once again,.... the meter is in the wrong place.

 

[bob85]

well, which one is it?
my money is on #2

both are correct

 

[RADIOOMAN]

no

once again,.... the meter is in the wrong place.

In this situation it would not matter where the meter is.
It is just a theoretical situation used for the purpose of sheding light on a real situation.

 

[The DB]

no

once again,.... the meter is in the wrong place.

So, where would the "right" location be on this theoretical "perfect" lossless coax, and why.


The DB

 

[northern35s]

There's a problem with that explanation. The returning current will be lesser because of any resistive losses but the ratio (SWR) will stay the same or have a very, very slight change, not really noticeable. The same 'standing wave' is still there and is being constantly 'refreshed' with 'new' current because the transmitter is still transmitting. When that transmitter quits transmitting then the standing wave dies so no SWR. There isn't a gradual lessening of that standing wave (actually there is but the rise/fall time isn't going to be measurable except by calculation or some -very- expensive equipment). For all practical purposes it's instantaneous.
- 'Doc

Here's a couple of things for you to consider, if we send 100W toward the load (antenna) and a percentage of that is returned to the source (the rig) let us say 10W, when this is then re-reflected forward it adds to the original 100W thus increasing this figure to more than 100W, let us for instance say 110W, we now have 110W forward and 10W reflected, if we now add line loss to the equation we begin to alter this ratio, and the more line loss we have eg. longer coaxial line, then the more this figure changes

 

[W5LZ]

Ah, but doesn't that re-reflected power also change phase/timing? And wouldn't that also change the point along which that re-reflected wave add to and subtract from the forward power? And then there's the 'instantaneous' factor. After that first instant, the ratio between forward and reflected power 'evens out', stabilizes, and is then steady. I've never seen an SWR meter that's quick enough to show that. That first 'instant' is just too quick. You may see a slight 'bump' when that meter begins to read SWR but that 'bump' settles down to a constant after that first 'instant'. Now, if you could slide that SWR meter to various spots (nodes and nulls) along that feed line you would see a change in the SWR reading. Or better yet, slide a light bulb configured to read that standing wave's nodes and it would be very noticeable (hell of a lot easier with parallel feed lines, but still the same is going to be happening with coax). Right?
- 'Doc

(Oh, and that 're-reflected' wave is always decreasing so would add less to the forward power each 'bounce'. That's typically from resistive losses.)

 

[northern35s]

Ah, but doesn't that re-reflected power also change phase/timing? And wouldn't that also change the point along which that re-reflected wave add to and subtract from the forward power? And then there's the 'instantaneous' factor. After that first instant, the ratio between forward and reflected power 'evens out', stabilizes, and is then steady. I've never seen an SWR meter that's quick enough to show that. That first 'instant' is just too quick. You may see a slight 'bump' when that meter begins to read SWR but that 'bump' settles down to a constant after that first 'instant'. Now, if you could slide that SWR meter to various spots (nodes and nulls) along that feed line you would see a change in the SWR reading. Or better yet, slide a light bulb configured to read that standing wave's nodes and it would be very noticeable (hell of a lot easier with parallel feed lines, but still the same is going to be happening with coax). Right?
- 'Doc

(Oh, and that 're-reflected' wave is always decreasing so would add less to the forward power each 'bounce'. That's typically from resistive losses.)

I know Bob posted it earlier, but I'll repost what Maxwell has to say about changes in VSWR:

If SWR readings change significantly
when moving the bridge a few feet one way or the
other in the line, it probably indicates "antenna"
current flowing on the outside of the coax, or else
an unreliable instrument, or both, but it is not
because the SWR is varying with line length.

Here's another gem from Maxwell:

Low SWR is not proof of a good-quality
antenna system or that it is working efficiently. On
the contrary, lower than normal SWR exhibited
over a frequency range by a straight dipole or a
vertical over ground is a clue to trouble in the form
of undesired loss resistance. Such resistance can be
from poor connections, poor ground system, lossy
cable, and so forth.

Remember we are only talking about indicated VSWR, not the VSWR at the load

 

[The DB]

Ah, but doesn't that re-reflected power also change phase/timing? And wouldn't that also change the point along which that re-reflected wave add to and subtract from the forward power? And then there's the 'instantaneous' factor. After that first instant, the ratio between forward and reflected power 'evens out', stabilizes, and is then steady. I've never seen an SWR meter that's quick enough to show that. That first 'instant' is just too quick. You may see a slight 'bump' when that meter begins to read SWR but that 'bump' settles down to a constant after that first 'instant'. Now, if you could slide that SWR meter to various spots (nodes and nulls) along that feed line you would see a change in the SWR reading. Or better yet, slide a light bulb configured to read that standing wave's nodes and it would be very noticeable (hell of a lot easier with parallel feed lines, but still the same is going to be happening with coax). Right?
- 'Doc

(Oh, and that 're-reflected' wave is always decreasing so would add less to the forward power each 'bounce'. That's typically from resistive losses.)

The MFJ-259B manual disagrees with this.

This unit (or any other impedance measuring device) displays the antenna's impedance, 50 ohm SWR, and resonant frequency as modified by transmission line "transformer" actions of the feedline and other components between the antenna and the MFJ-259B. If the line is at 50 ohms, this unit will always display the antenna's true SWR, with the exception of a slight reduction in SWR with longer or more lossy feedlines.

Then further down the same page.

5.) If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving the analyzer to another point in the line will NOT change SWR reading. Impedance and resonant frequency might change from line transformation effects, but the SWR will not change.

6.) If SWR changes with coaxial line length, line placement, or line grounding (any distance away from the antenna) changes, the feedline has one or more of the following shortfalls:
a.) The feedline is carrying common mode current and radiating.
b.) The feedline is not a 50 ohm line.
c.) The feedline has high loss.

Also from an aforementioned publication mentioned in this thread earlier:

21) SWR indicators need not be placed at the feed-line/antenna junction to obtain a more accurate measurement. Within its own accuracy limits, the indicator reads the SWR wherever it is located in the line. The SWR at any other point on the line may be determined by a simple calculation involving only the SWR at the point of measurement, the line attenuation per unit length (available in a later installment), and the distance from the measured point to the point where the SWR is desired.
22) SWR in a feed line cannot be adjusted or controlled in any practical manner by varying the line length (ref. 7, p. 51).

And the earlier quoted passage.

23) If SWR readings change significantly when moving the bridge a few feet one way or the other in the line, it probably indicates "antenna" current flowing on the outside of the coax, or else an unreliable instrument, or both, but it is not because the SWR is varying with line length. Some writers insist the bridge must be placed at a halfwave interval from the load to obtain a correct reading. This is incorrect. All readings are invalid if they change significantly along the line, even though they may repeat at half-wavelength intervals (ref. 2, pp. 101, 106, and 132).

I located Reflections in .pdf format after Bob85 mentioned it, it is a very good read so far. Only about a third of the way through the .pdf.


The DB

 

[HomerBB]

How does this fit into this thread?

"I once placed a dummy load at the end of a 100' RG58 common brand coax. I placed the SWR/Power meter at the radio end and keyed 10 watts with a 1:1 SWR.
Next I put the meter at the far end of the 100' RG58 coax with the dummy load beyond the SWR/Power meter. I keyed the same wattage out of the radio, 10 watts, but measured only 5.5 watts on the meter. There was a 45% power loss through that coax. I bought something else to use."

 

[The DB]

How does this fit into this thread?

"I once placed a dummy load at the end of a 100' RG58 common brand coax. I placed the SWR/Power meter at the radio end and keyed 10 watts with a 1:1 SWR.
Next I put the meter at the far end of the 100' RG58 coax with the dummy load beyond the SWR/Power meter. I keyed the same wattage out of the radio, 10 watts, but measured only 5.5 watts on the meter. There was a 45% power loss through that coax. I bought something else to use."

I was trying to show what coax length can do to SWR, your quote shows another aspect that is related, namely direct loss in power before the signal even gets to the antenna. Both are caused by the same thing, namely losses due to the coax itself.


The DB

 

[W5LZ]

None of those negate what I said. The same principle holds for mechanical vibration.
- 'Doc

 

[bob85]

i think some folk get confused between the impedance looking into different length mismatched lines,
where line length effects the impedance seen looking into the line and 1/2wave electrical multiples of line length reflect the load impedance / odd 1/4wave multiples invert the load impedance

and

the swr along a mismatched line that has no currents flowing on the outer shield,
where line length only effects swr due to loss in the line ( the further from the load you measure the lower the reading ), 1/2wave & 1/4wave electrical multiples play no part in swr readings ,
significant changes in swr reading with small changes in line length would indicate high coax loss, a faulty/uncalibrated meter or the wrong impedance coax,

but but

if you have currents flowing on the outer shield, the electrical length of the outer shield and it been connected to ground or not connected to ground effects common mode impedance seen in parallel with the load causing a situation where coax length effects the load impedance and real swr,

if changing the coax length by a few feet changes the swr reading significantly as many people have observed you need to call Houston.

 

[Lil'Yeshua]

i think some folk get confused between the impedance looking into different length mismatched lines,
where line length effects the impedance seen looking into the line and 1/2wave electrical multiples of line length reflect the load impedance / odd 1/4wave multiples invert the load impedance

and

the swr along a mismatched line that has no currents flowing on the outer shield,
where line length only effects swr due to loss in the line ( the further from the load you measure the lower the reading ), 1/2wave & 1/4wave electrical multiples play no part in swr readings ,
significant changes in swr reading with small changes in line length would indicate high coax loss, a faulty/uncalibrated meter or the wrong impedance coax,

but but

if you have currents flowing on the outer shield, the electrical length of the outer shield and it been connected to ground or not connected to ground effects common mode impedance seen in parallel with the load causing a situation where coax length effects the load impedance and real swr,

if changing the coax length by a few feet changes the swr reading significantly as many people have observed you need to call Houston.

I've adjusted the coax length before to equal the swr readings between channels 1 and 40 on 11 meters. That was with a 1980's Radio Shack 1/2 wave dipole base antenna I once had. I used to cut the coax a little on the long side for my application and then adjusted the length to get the swr right. I used to measure out my coax in 9' multiples. But now I will measure it out in half wave multiples. I bought Coleman RG 8x with a .78vf

 

[hookedon6]

didn't we go thru this before on this forum? SWR Meters make You Stupid!

one more time,.... coax/ladderline/waveguides/ect, do NOT create VSWR

 

[HomerBB]

, but significant line loss affects SWR meter output readings.
Not to be mistaken for the actual condition of the load, if there is one, on the other end from the source.