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

[bob85]

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

yes we have been through this several times, the link you posted was just one of them,
the article used information found in walt maxwells "reflections",
the arrl guys and mfj are happy enough with the explanation as are other sources,
yet we still have folk posting about 1/2wave & 1/4wave multiple nodes or a need to place the swr meter at 1/2wave multiples from the load to obtain a correct reading which is untrue and serves only to confuse anybody reading this forum,

the reasons why somebody could observe changes in swr reading with different length lines and come to the wrong conclusion as to why that is are explained in black and white on this forum multiple times with reference to the sources of information,

i have asked multiple times for anybody in disagreement with what maxwell the arrl mfj and the other guys write to post the source of their information that refutes what is claimed as i would like to read it but references have not been forthcoming,
im always happy to look at alternative sources of reference material if its exists,

we have several problems

1. people that don't or won't read technical articles from respected sources
2, people stuck in their incorrect assumptions
3, people who make up their own laws of physics AMPOWER style
4, people are allowed to frequently post technobull without having to post reference to the source of their twaddle when asked to do so

im not disheartened, its clear this time around the tide is turning in favor of transmission-line theory over mythology

 

[hookedon6]

yes we have been through this several times,...
the reasons why somebody could observe changes in swr reading with different length lines and come to the wrong conclusion...

we have several problems...

... and the reason is that you don't know where to place a meter in the transmission line!

you are measuring tx output / load output,......not load INPUT power/load OUTPUT power, or feedline impedance/load impedance, (or ... whatever , as long as the units are the same).

scenerio #1
100 watt xmitter thru (lossless) coax into a meter @ the 50 ohm load,... what is the VSWR? =100 watts input & 0 reflected watts = 100 watts load input divided by 100 watts radiated = 1/1 = VSWR 1.0:1

scenerio #2 same 100 watt xmitter thru coax with (say) 25 watts loss into a meter@ the same 50 ohm load. what is the VSWR? 75 watts input & 0 watts reflected = 75 watts load input divided by 75 watts radiated = 1/1= 1.0: VSWR<gotproof>

????? but, but, but,..... you say line loss causes VSWR,... HOW can it be that the VSWR in both scenerios is the same??????

your MISTAKE is that you put your meter in the WRONG place (at the transmitter) so you get these INCORRECT numbers:

100 watt xmitter thru a meter and then thru coax with (say) a 25 watt loss into the same 50 ohm load= 100 watts xmiter output divided by 75 watts radiated = 100/75 = 1.33:1 VSWR.:thumbdown:

once AGAIN, transmission line loss has NOTHING to do with VSWR!
the "load" doesn't "know" or "care" what "loss" is before it.

if you don't measure at the load INPUT (or a 1/2wl mult), then you are wrong!

 

[northern35s]

... and the reason is that you don't know where to place a meter in the transmission line!
the DEFINATION of VSWR, it is NOT tx output / load output,...... it IS load INPUT/load OUTPUT

scenerio #1
100 watt xmitter thru (lossless) coax into a meter @ the 50 ohm load,... what is the VSWR? 100 watts load input divided by 100 watts radiated = 1/1 = VSWR 1.0:1

scenerio #2 same 100 watt xmitter thru coax with (say) 25 watts loss into a meter@ the same 50 ohm load. what is the VSWR? 75 watts load input divided by 75 watts radiated = 1/1= 1.0: VSWR<gotproof>

????? but, but, but,..... you say line loss causes VSWR,... HOW can it be that the VSWR in both scenerios is the same??????

your MISTAKE is that you put your meter in the WRONG place (at the transmitter) so you get these INCORRECT numbers:

100 watt xmitter thru a meter and then thru coax with (say) a 25 watt loss into the same 50 ohm load= 100 watts xmiter output divided by 75 watts radiated = 100/75 = 1.33:1 VSWR.:thumbdown:

once AGAIN, transmission line loss has NOTHING to do with VSWR!
the "load" doesn't "know" or "care" what is before it.

if you don't measure at the load INPUT (or a 1/2wl mult), then you are wrong!

You appear to regularly declare evidence based reasoning as wrong, I really would like to know what you are basing your theory on, can you provide a peer reviewed source?

 

[RADIOOMAN]

scenerio #1
100 watt xmitter thru (lossless) coax into a meter @ the 50 ohm load,... what is the VSWR? 100 watts load input divided by 100 watts radiated = 1/1 = VSWR 1.0:1

In this scenario you specify 100 watts thru (lossless) coax into a meter with the 50 ohm load immediatly after the meter; And since the load matches the transmission line there is no reflected power so you are correct, the VSWR is 1.0:1.

scenerio #2 same 100 watt xmitter thru coax with (say) 25 watts loss into a meter@ the same 50 ohm load. what is the VSWR? 75 watts load input divided by 75 watts radiated = 1/1= 1.0: VSWR

In this one you indicate that we now have a 'real word' coax that has loss and show again the correct answer.

100 watt xmitter thru a meter and then thru coax with (say) a 25 watt loss into the same 50 ohm load= 100 watts xmiter output divided by 75 watts radiated = 100/75 = 1.33:1 VSWR.

Now in this one you have real coax and you have finally put the swr meter where it should be. ( I don't know why you insist on putting the swr meter where the antenna tuner should be )
But in this case your answer is wrong.
50 ohm coax and 50 ohm load is a matched condition and there will be no reflected power and the VSWR will be 1.0:1.
You are using transmitter power to the ratio of the radiated power to calculate VSWR and that is incorrect.
That is not the way it is done.
You are trying to use coax loss to create VSWR.
Transmission line loss does not create VSWR.
You appear to be disagreeing with your own logic.

 

[Captain Kilowatt]

Question: If line length does not affect SWR when the load is not a pure 50 ohms then why can we use a specific length of cable to create a transmission line impedance transformer?

 

[northern35s]

Question: If line length does not affect SWR when the load is not a pure 50 ohms then why can we use a specific length of cable to create a transmission line impedance transformer?

In this case there is a mismatch, and the greater the mismatch between the load and the line then the greater amount of energy will be reflected, using a 1/4WL of line can act as a transformer when the characteristic impedance of the transmission line is equal to the square root of the load/line impedance product.

Any kind of transmission line can be used as the transformer

From the ARRL antenna handbook:

Yagi Driven Elements

Another application for the λ/4 linear transformer
is in matching the low antenna impedance encountered
in close-spaced, monoband Yagi arrays to a 50-Ω transmission
line. The impedances at the antenna feed point
for typical Yagis range from about 8 to 30 Ω. Let’s
assume that the feed-point impedance is 25 Ω. A matching
section having Z0 sqrt 50x25 = 35.4 Ω is needed.
Since there is no commercially available cable with a Z0
of 35.4 Ω, a pair of λ/4-long 75-Ω RG-11 coax cables
connected in parallel will have a net Z0 of 75/2 = 37.5 Ω,
close enough for practical purposes.

 

[bob85]

i posted where i got my info from, you have seen what the arrl and mfj say, how about you posting your source <gotproof> its starting to sound like AMPOWER'S little black book and im sure you don't mean to come across that way

... and the reason is that you don't know where to place a meter in the transmission line!

you are measuring tx output / load output,......not load INPUT power/load OUTPUT power, or feedline impedance/load impedance, (or ... whatever , as long as the units are the same).

scenerio #1
100 watt xmitter thru (lossless) coax into a meter @ the 50 ohm load,... what is the VSWR? =100 watts input & 0 reflected watts = 100 watts load input divided by 100 watts radiated = 1/1 = VSWR 1.0:1

scenerio #2 same 100 watt xmitter thru coax with (say) 25 watts loss into a meter@ the same 50 ohm load. what is the VSWR? 75 watts input & 0 watts reflected = 75 watts load input divided by 75 watts radiated = 1/1= 1.0: VSWR<gotproof>

????? but, but, but,..... you say line loss causes VSWR,... HOW can it be that the VSWR in both scenerios is the same??????

your MISTAKE is that you put your meter in the WRONG place (at the transmitter) so you get these INCORRECT numbers:

100 watt xmitter thru a meter and then thru coax with (say) a 25 watt loss into the same 50 ohm load= 100 watts xmiter output divided by 75 watts radiated = 100/75 = 1.33:1 VSWR.:thumbdown:

once AGAIN, transmission line loss has NOTHING to do with VSWR!
the "load" doesn't "know" or "care" what "loss" is before it.

if you don't measure at the load INPUT (or a 1/2wl mult), then you are wrong!

 

[W5LZ]

The cite in that last post (#36) is a typical example of conjugate matching, you can find it in several sources (the ARRL's Handbook and their Antenna series of books for just two of them). It's been a common means for impedance matching for quite some time. It's certainly not the best in the world, but usually quick-cheap-n-easy.
Depending on your method of measuring, or calculating, it's effect it can 'look' like a pretty good way of doing that impedance matching. I don't think it's 'perfect' at all for a number of reasons and wouldn't recommend using it (unless you just have to), but it does work. All it's ever been is a 'shade-tree' method of doing something to 'get by'. If you're satisfied with that, have at it! Really doing it right requires more effort.
- 'Doc

 

[RADIOOMAN]

I am beginning to wonder if maybe some are confusing "the length of the line" with the "posisition of the SWR meter" in the line

Some might visualize that the way you "change the position" of the swr meter in the line is to add or remove sections of coax.

If transmission line is terminated with a value of load that is the same as its characteristic impedance there will be no reflected power.
And in that case it does not matter where the swr meter is placed in the line and it does not matter how long or short the line is. (disregarding common mode currents, or fallible testing equipment)
But if the transmission line is mismatched at the load end then this is when things begin to get interesting.
In this case changing the length of the line will transform the impedance and subsequently the swr value, but if the length of the line does change in this case of a mismatched load, then changing the posistion of the swr meter in a fixed length of line will yield (disregarding common mode currents or fallible test equipment) the same swr value at every point.

 

[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 ran an experiment to see if I could confirm this information with my coax, or get close. Using the return loss scale on my analyzer I connected a short circuit test load directly to the analyzer, .05 dB return loss, very low as it should be.

I then added the 100' of RG58A/U coax between the analyzer and the short, it reports a 4.44 dB return loss.

As this figure is from the losses of two passes of the coax (down and back) you can get the losses present at the far end by dividing by 2, so now I'm at 2.22 dB loss for this length of coax.

2.22 dB loss is near as makes no difference to a 40% loss.

Data sheets I've seen on this type of coax (RG-58A/U) show a 2.5 dB loss at 30 Mhz, which is closer to 43.8% loss. I am not sure if there are any other standards for RG-58 type coax, I do know there are other varieties which, unlike the one I used here, does not have a copper center conduction or shielding. I do not know the specs (nor have I seen a separate datasheet) for the coax used the the quote above.

The coax in the quote above has a 45% loss give or take, which is about 2.6 dB loss give or take. I say give or take because I always add in a margin for error on analog readouts.

Note, I was not trying to disprove the above quote, I was simply trying to see how close this test coax was to that one. I would say that the one coax I have is of slightly better quality than the one in Homer's example, but mot by much...


The DB

 

[HomerBB]

Mine was from radio shack. Go figure.

 

[The DB]

Thats about as bad as it gets...


The DB

 

[northern35s]

Thats about as bad as it gets...


The DB

I'm not sure about ebay in the US, but for the absolute worst kind of cable, this is where I'd expect to find it in the UK, one chap on another forum found a VERY cheap roll of RG58 and posted it on the forum, a good few members bought the coax and all had the same experience, almost impossible to get a low VSWR on their antenna systems, it turns out the coax screen and centre conductor were copper plated steel and no where near 50 ohm characteristic impedance, there's a reason why it was half the price of anywhere else :blushing: {Cry_river}

 

[Captain Kilowatt]

I'm not sure about ebay in the US, but for the absolute worst kind of cable, this is where I'd expect to find it in the UK, one chap on another forum found a VERY cheap roll of RG58 and posted it on the forum, a good few members bought the coax and all had the same experience, almost impossible to get a low VSWR on their antenna systems, it turns out the coax screen and centre conductor were copper plated steel and no where near 50 ohm characteristic impedance, there's a reason why it was half the price of anywhere else :blushing: {Cry_river}

Copper plated steel should not have affected the impedance. Impedance is more a function of the ratio of the diameters of the inner and outer conductors as well as the spacing and dielectric. I used to have a coax in commercial service that was copper plated aluminum inner conductor and stainless steel braid and it was 50 ohms. Forget the type but it was about an inch or bigger in diameter.

 

[northern35s]

I understand how a coax gets it characteristic impedance and as I didn't buy the cheap rubbish I couldn't tell you what it's impedance was, the fact it was copper plaed steel was an indicator as to why it was so cheap, the fact it was almost impossible to get a reasonable VSWR with it in line tells me there was a potential problem with its characteristic impedance