what is your vswr at the antenna?

[Robb]

you won't find any of that 1/2wave coax voodoo on there rob, not if maxwell & cebik have looked it over and given it the thumbs up.

Well; no it wasn't found on that page and neither am I defending it.
Don't exactly remember where I read about it either.

But it does made sense that at 0, 180, and at 360 degrees of a wave that it is neither phased positive or negative. Just neutral points when looking at a graph of a single wave cycle.

Since all coax has some difference in velocity factor; then a 1/2w of coax with a .66 vf should have a different length than a piece of coax that has a velocity factor of .86. IIRC, the RG-213 used in the scenario has a vf of .66

So if both principles are true; then it should be so.
Makes sense to me.

How is that voodoo; how are these premises wrong?

On another issue already brought up by another poster, the scenario you proposed gave no mention of the amount of RF power being fed into that circuit. IIRC, coax can absorb so much reflected power over a given length before it gives off the extra reflected energy as heat (I x r). Which means as power increases, that same 500 ft piece of coax would probably show a different SWR at the shack meter as well as have a rise in heat.

 

[bob85]

its not voodoo if you are using the coax to transform impedance,
the heat would be included in the coax loss, no need to mention power so long as its within the systems limits,
the calculation is on birds white paper.

 

[The DB]

In response to Robb, bob85 got a post in before this... sneaky bastard...

The problem is unless your using something more advanced than an SWR meter then a half wavelength of coax isn't going to make any difference. SWR does not change on a feedline (other than attenuation in the feedline) simply because your not at a half wavelength multiple from the load. Yes the variables X and R are subject to the transforming effect of traveling down the feedline, but the SWR does not change. This is clearly stated in multiple chapters in "Reflections Transmission Lines and Antennas" and in any number of ARRL Antenna Book versions, (both of which were used as references in the link you provided).

Now if you have something more advanced than an SWR meter then yes, a half wavelength multiple will help you in tuning, that is, if you don't have access to the base of the antenna itself as was presented in bob85's original problem.

your buddy sits in the tree & hooks his vswr meter right at the antenna feed-point,
he always measures at the feed-point, he won't have none of that coax stuff messing with his readings,

no he hooked the meter to the antenna with a double male.

The DB

 

[bob85]

sorry db

rob
have a look at page 10&11 of the mfj manual covering electrical 1/2waves

http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf

i hope this link works

 

[Robb]

sorry db

rob
have a look at page 10&11 of the mfj manual covering electrical 1/2waves

http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf

i hope this link works

Think this is what you are referring to; as I am too - from pg.10:

3.) Even if a perfect transmission line is cut to an exact electrical half-wave (or a multiple thereof ) it is a true
half-wave multiple only on one frequency in that band. On a slightly different frequency the line will not
represent the true feedpoint impedance of the antenna. The line is only “impedance transparent” when
lossless and when an exact multiple of 1/2 wl. The longer the transmission line in wavelengths, the “more
length critical” it becomes and the less accurate measurements become.

 

[The DB]

I think he was referring to something else, namely the second paragraph under antenna hints, and then point number 6.


The DB

 

[Robb]

I read the Bird article and the last page of it.
Fine read.

 

[bob85]

page 10&11 show the issue with sweeping the antenna looking through a piece of coax which eddie was talking about recently and the falacy of swr changing significantly with coax length due to transformer action of mismatched coax,

using the conversion charts i posted

loss in 500ft of rg213 = 5db

2way loss because your signal travels up and back down the coax =10db

return loss @ vswr of 1.2:1 @ the radio = 20.8278537031645dB

subtract your two way loss = 10.8278537031645dB

convert your new return loss of 10.8278537031645dB back to vswr

vswr=1.80693791462797:1 or 1.8:1 @ the antenna

i used 500ft of rg213 as an extreme example,
clearly if you are dealing with small changes lets say going from 9ft to 12ft of coax you would see virtually no change in vswr yet people with perfectly good meters do see significant changes with small changes in coax length such as swapping a 3ft jumper for a 6ft jumper,

that is an indication you have common mode currents on the coax braid.

 

[Shockwave]

While I'm usually in agreement with Bob on most antenna discussions, I'm thinking some things have been overlooked here. Since the actual VSWR is nearly a 2:1, there are significant standing waves on the 50 ohm coax because the load impedance is either close to 25 ohms or close to 100 ohms.

Doesn't coax transformation take place precisely when the coax does not match the impedance of the load and all coax impedance inline? Just because we have used all 50 ohm coax does not mean there will be no transformation of impedance if the antenna is this far away from 50 ohms.

It has been my experience that the only time random lengths of 50 ohm coax have no effect on VSWR (other than cable loss) is when the load impedance is also 50 ohms. That cannot be the case if the VSWR at the load is showing close to a 2:1 on a 50 ohm instrument. In these cases where true VSWR is high, having a 1/2 wave multiple can be useful in determining the actual load impedance.

If I've got it wrong, I'm willing to dig deeper to get to the answers.

 

[bob85]

yes we are usually in agreement, which part do you not agree with, is it because i used such a long coax or do you not agree that the transformer action of a mismatched coax in a typical install does not effect vswr?,

i see no relation between transformer action when the load is reasonably near to the coax characteristic impedance and vswr along the same line in a typical install although that idea is often claimed on forums,

it is my understanding that common mode currents on the braid are the true cause of significant changes in vswr with small changes in coax length such as when somebody claims their reflect is high and they are told to swap their 6ft jumper for a 3ft jumper and that seems to fix the vswr issue,

the three sources i posted, bird, mfj, w2du and others don't agree the transformer action effects vswr,
w2du says in his reflections that some writers incorrectly claim it does but i have not found the writers he is aiming that comment at other than what you often see posted on forums,

i have probably posted the same idea since i believed transformer action manipulated vswr at one time although i don't recall where i got the idea from,
its likely that i observed changes in vswr with coax length as most of us have and came to the wrong conclusion about the mechanism causing the change in vswr,

i fully agree that the impedance looking into a mismatched coax from the radio end changes with coax length and that an exact electrical 1/2wave of coax with no loss mirrors the terminating impedance, i don't agree that effects vswr in the same coax if the braid is free from common mode currents.

 

[blue runner]

Near field error

I'd be concerned that a person being so close to the antenna may detune / change the SWR of it. Being within the inductance field of the antenna may cause meter errors too.

I think the most accurate reading may be done with a minumum full wave length of coax away with a choke on it.

 

[Marconi]

I'm reading all the different ideas here, and I think I'm hearing echos on echos, and it may have something to do with the idea of conjugate match.

 

[Shockwave]

yes we are usually in agreement, which part do you not agree with, is it because i used such a long coax or do you not agree that the transformer action of a mismatched coax in a typical install does not effect vswr?

I'm thinking if the load impedance is not close to the transmission line impedance that we should expect transformation to take place with different lengths of cable. The extreme length just means more loss.

i see no relation between transformer action when the load is reasonably near to the coax characteristic impedance and vswr along the same line in a typical install although that idea is often claimed on forums,

I agree however, in the example given one could easily say the load is NOT reasonably close to the 50 ohm cable impedance since the true VSWR is approaching a 2:1 ratio. Without knowing the reactive properties of the load we cannot determine if its impedance is high or low but the 2:1 VSWR reading on a 50 ohm line section can only mean it's either 25 ohms or 100 ohms.

This match is off by nearly 50%! When we use 75 ohm cable to make a "Q" section or a co-phase harness for its impedance transformation properties, the mismatch is only 33% and we still see significant change in impedance as a result. Why wouldn't a 50% mismatch produce similar results?

it is my understanding that common mode currents on the braid are the true cause of significant changes in vswr with small changes in coax length such as when somebody claims their reflect is high and they are told to swap their 6ft jumper for a 3ft jumper and that seems to fix the vswr issue,

the three sources i posted, bird, mfj, w2du and others don't agree the transformer action effects vswr,
w2du says in his reflections that some writers incorrectly claim it does but i have not found the writers he is aiming that comment at other than what you often see posted on forums,

i have probably posted the same idea since i believed transformer action manipulated vswr at one time although i don't recall where i got the idea from,
its likely that i observed changes in vswr with coax length as most of us have and came to the wrong conclusion about the mechanism causing the change in vswr,

i fully agree that the impedance looking into a mismatched coax from the radio end changes with coax length and that an exact electrical 1/2wave of coax with no loss mirrors the terminating impedance, i don't agree that effects vswr in the same coax if the braid is free from common mode currents.

I will admit I have not read the information from those 3 sources and I probably should. Without having done that, I have to assume most of this depends on the definition of the relative term "reasonably close in impedance" is? At some point the mismatch between the load and cable becomes great enough to cause impedance transformation.

Exactly when this becomes noticeable is not something I can definitively answer although we know mixing 75 ohm cables with 50 ohm loads can turn that 50 ohm load into 100 ohms. That is noticeable. I would say that when the load impedance is "reasonably close" to the cable impedance that the cable length alone will not have a noticeable effect on VSWR unless there is significant CMC present on the braid.

 

[bob85]

i have no argument with the impedance transformation whenever the load is not the same as the coax characteristic impedance,
no argument that measured resonant frequency can change with line length,
its the idea that line length effects vswr i don't agree with unless you have common mode current on the braid, then relatively small changes in line length can effect vswr significantly,

im not sure if a very bad match say 20:1 would cause vswr to change with line length since its something i have not tested but i doubt it would,

i can't find any respected source that says a line free of common mode current will change vswr with changes in line length, w2du says some writers incorrectly claim it does so i presume there must be articles that make that claim,

have a look at the three links i posted, i can post others if needed.

 

[The DB]

Think of it in terms of a Smith Chart when you think of SWR and feedline impedance transformation. You have the middle point, which is a perfect match and a 1:1 SWR. Elsewhere on the chart, you have all other combinations of impedance and reactance.

Using the match point and any other point on the smith chart you can use a compass and make a circle, that circle shows us two things. One, all of the points (or combinations of reactance and impedance) on the smith chart that have the same SWR, and two, the results of the transformer action of a (admittedly theoretically perfect lossless) feedline as you travel away from the load. These circles are exactly the same.

If you factor in feedline losses as you travel down the feedline that circle becomes a spiral, spiraling towards the perfect match point.


The DB