Overheard yesterday.....

[PA629]

Stopped by the CB shop at a local truckstop yesterday to pick up a couple of PL-259's and overheard a driver asking the tech:

"I was told to get my SWR's checked. How many should I have in about 12 foot o' antenna wire?"

Didn't have the time to hang around for the answer/explanation. Just thought it was amusing, some of the things techs get in the course of their day. This shop does antenna checking and tuning, so they probably got the guy taken care of w/o a problem.

 

[C W Morse]

I often ask the question: How many "SWR-zzzzzzzzzzzzzzzzzzz" are there in ONE feedline?"

Answer: ONE!


73

CWM

 

[Dave_W6DPS]

I often ask the question: How many "SWR-zzzzzzzzzzzzzzzzzzz" are there in ONE feedline?"

Answer: ONE!


73

CWM

Actually, I count two, but only one you can easily measure.

There is the impedance transition between the tranceiver and feedline (the one you can measure) and the one between the feedline and the antenna (the one you adjust).

If your feedline is really 50 ohm impedance and your tranceiver output impedance is really 50 ohms, you should only need to deal with the feedline/antenna impedance junction.

Unfortunately, there are always variations, and you can have degraded performance due to the reflected power right at the tranceiver output.

I bought an HF tranceiver, that was built from a kit, very cheaply once because it constantly exhibited loading issues, and it turned out that some components in the final circuit were the wrong value and caused a mismatch with the feedline. When it barely had any plate adjustment left while running into a dummy load, I pulled it apart and identified the cause.

It can be helpful to keep in mind that when you are adjusting antenna impedance, you are measuring feedline reflected power. If you can't adjust the antenna for a low SWR, no matter what, then look for problems closer to the tranceiver.

As for the original situation, I wonder if the tech found an insufficient number of SWRs and was able to sell the customer some more??? :twisted:

Dave_W6DPS

 

[C2]

S11
S12
S21
S22

Four?

 

[freecell]

the only set of conditions under which one might find only one standing wave ratio (VSWR = EMax/EMin) present anywhere along the feedline (50 ohms) is when both the generator and the load are equal to 50 ohms.

if the generator and/or the load represent anything other than 50 ohms then the feedline no longer exhibits its characteristic impedance. under these conditions multiple voltage standing wave ratios can be found at regular intervals up and down the line in direct proportion to EMax/EMin, with the largest difference between those two values establishing the max vswr and in direct corrolation the wide range of impedances present at various points up and down the line. every length of feedline is an impedance transformer when the load is anything but 50 ohms resistive, assuming a generator output impedance that matches the characteristic impedance of the feedline.

mismatch

 

[ken white]

Actually, there is only one VSWR within a given length of feedline.

The magnitude of the VSWR is a function of the transmitter ouput impedance, coax impedance, and antenna feedpoint impedance. Since VSWR is defined as Vmax/Vmin, Imax/Imin, or Zmax/Zmin. None of these values will change unless the transmitter output, or antenna input impedance changes, or the length of the coax changes when impedances are not matched.

So, the VSWR measured at the transmitter will read the same as the VSWR measured at the antenna, which will be the same as the VSWR measured anywhere along the transmission line when the overall length of the coax stays the same.

Even though the magnitude of the standing wave voltage, current, and/or impedance changes at varying lengths/points of the coax, the ratio of the max/min value will remain constant.

 

[freecell]

"Actually, there is only one VSWR within a given length of feedline. "
and only when generator, feedline and load are all equal to 50 ohms. under these conditions line length is irrelative.

"So, the VSWR measured at the transmitter will read the same as the VSWR measured at the antenna, which will be the same as the VSWR measured anywhere along the transmission line when the overall length of the coax stays the same."

and only when generator, feedline and load are all equal to 50 ohms. under these conditions line length is irrelative.

"the ratio of the max/min value will remain constant."

and only when generator, feedline and load are all equal to 50 ohms.

 

[ken white]

"Actually, there is only one VSWR within a given length of feedline. "
and only when generator, feedline and load are all equal to 50 ohms. under these conditions line length is irrelative.

"So, the VSWR measured at the transmitter will read the same as the VSWR measured at the antenna, which will be the same as the VSWR measured anywhere along the transmission line when the overall length of the coax stays the same."

and only when generator, feedline and load are all equal to 50 ohms. under these conditions line length is irrelative.

freecell, VSWR doesn't change just because it is measured somewhere else. When the VSWR is 1:1, or 2:1, or whatever the VSWR condition happens to be, it will measure the same anywhere within a given length of coax. If this wasn't true, then it would be a pretty useless metric.

 

[freecell]

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?

 

[ken white]

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?

I don't know why you are seeing this problem - maybe your coax is real lossy?

I measure VSWR with a power meter and I get the exact same forward and reflected power whether I measure it at the transmitter output or antenna input - ignoring any losses.

VSWR = (1 + (P_ref/P_fwd)^0.5) / (1 - (P_ref/P_fwd)^0.5)

As far as I know, the forward power and reflected power stay constant within a given length of coax which means the VSWR is constant too...

 

[freecell]

can't be lossy feedline. at runs of 3/4 wl or less at 27mhz. using feedline rated at 3.8db. loss per 100' @ 100mhz. is hardly what i would describe as lossy.

 

[ken white]

The the VSWR should stay the same since the forward and reflected power stays the same... Energy cannot be created or destroyed, only moved around, so the power isn't varying within the coax unless there are losses...

 

[freecell]

given a load with an input impedance that is approximately one third of the required 50 ohms to match the feedline and a resulting swr of roughly 3:1, when looking into this load from the input end of a 5/8 wave line the swr at the load is observed at the input to the line. if the line is replaced with a 3/8 wave line the swr now seen at the input to the line is 1.2:1 or less.

tell me why.

 

[ken white]

given a load with an input impedance that is approximately one third of the required 50 ohms to match the feedline and a resulting swr of roughly 3:1, when looking into this load from the input end of a 5/8 wave line the swr at the load is observed at the input to the line. if the line is replaced with a 3/8 wave line the swr now seen at the input to the line is 1.2:1 or less.

tell me why.

What does this question have to do with "where" VSWR is measured within a given length of coax and if it changes within a given length of coax? If you read my posts closely, I stated that VSWR measures the same anywhere within a coaxial transmission line that stays the same physical length...

Actually, there is only one VSWR within a given length of feedline.

The magnitude of the VSWR is a function of the transmitter ouput impedance, coax impedance, and antenna feedpoint impedance. Since VSWR is defined as Vmax/Vmin, Imax/Imin, or Zmax/Zmin. None of these values will change unless the transmitter output, or antenna input impedance changes, or the length of the coax changes when impedances are not matched.

So, the VSWR measured at the transmitter will read the same as the VSWR measured at the antenna, which will be the same as the VSWR measured anywhere along the transmission line when the overall length of the coax stays the same.

Even though the magnitude of the standing wave voltage, current, and/or impedance changes at varying lengths/points of the coax, the ratio of the max/min value will remain constant.

Now to answer your question... Changing the length of a piece of coax that is connected to non-matched loads will change the VSWR due to the coax's varying impedance properties. So the coax is now part of the tuned output port of the CB, or stated differently, the coax and antenna are part of a tuned system. I have never stated that varying lengths of coax will not change VSWR if the transmitter/antenna/coax are not matched .

 

[W5LZ]

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