Tuning jumpers and coax length

[W5LZ]

highroller,
An SWR meter (any of them) compares a 'forward' voltage reading and a 'reflected' voltage reading at the point in the feed line that it's placed. The relationship between these two voltages is SWR. The greater the difference between the two means the SWR is higher. Now, the 'catch' to that is that the voltages along a mismatched system changes depending on the length of the feed line. The 'where', that you take the measurement can make a big difference. The meter is reading what it 'sees' correctly, but that doesn't always mean it's reading is correct for the whole system. That's where the 'fooling' part of fooling the meter comes from. In a properly impedance matched system there is no difference in the F/R voltages no mater where you take the reading. So, the meter can be 'fooled', it is reading what it 'sees' correctly, but that doesn't mean it's reading the overall SWR of the system.
- 'Doc

PS - Sorry if that isn't as clear as it could be. It's late...

 

[freecell]

yeah, that's as clear as mud....

"........the voltages along a mismatched SYSTEM changes depending on the length of the feed line." nope........

let's try this................the voltages along a mismatched LINE change depending on the length of the feed line. wrong........

................the voltages along a mismatched LINE change in direct response to the amount of mismatch presented by the load, REGARDLESS of the length of the feed line.

i have no idea what you mean by "SYSTEM" but let me take a wild guess........transmitter, feedline and antenna. wrong.

standing wave reflection and the voltages associated with it (emax/emin) EXIST ONLY ON THE FEEDLINE. they don't exist on the antenna and they don't exist inside the transmitter. there is NO SYSTEM in this sense. i've attempted to explain this to you once before and i can see there has been no traction.

the swr meter ISN'T LYING to you and it ISN'T FOOLING anyone with any understanding. (it's apparently fooling some of you)
the meter reading changes on a line with reflection present because when you change the length of the jumper or the line you place the power/swr/whatever meter into a different place in the line. period. if the load is mismatched to the line then either take your readings at the load or at an electrical 1/2 wavelength (or any number of multiples) away from the load towards the source for an accurate representation of the load swr.

 

[Master Chief]

Doc, you said it well! I wish I could have said it as well.

As I have already said a long time ago, the SWR is the relation between the FEEDLINE and the ANTENNA (load) at the FEEDPOINT. If you can't check it there (which is never very easy), then check it at the next electrical half wave point of the feedline. See, FC DOES agree with me! The world will never be the same!

If the SWR meter is anywhere else in the line, you will get the actual reading AT THAT POINT OF THE LINE, but it may not be the actual SWR at the FEEDPOINT. Good job Doc!

 

[freecell]

hilarious!

you forgot to confuse the issue by using the word system. even more than that, there are a multitude of swr "relationships" between numerous places in the line and a mismatched load.

 

[Master Chief]

(wrestling pigs in the mud)

 

[freecell]

even more than that, there are a multitude of swr "relationships" between numerous places in the line and a mismatched load.

if i can get a hell yeah to that i can move on. and i'd like the url for the post where you made that statement oh so long ago MC so i can see it for myself.

 

[Master Chief]

Hell yeah! Your the MAN Freecell! Hell Yeah!



No URL, but check page 2 of THIS thread, 4th post down.

(wpim)

 

[freecell]

yeah, i'm there....point # what?

1: SWR is a measurement between the FEEDLINE and the antenna at the FEEDPOINT of the antenna........

 

[Master Chief]

My GOD man, you can read! I'm sure you can find it.

 

[freecell]

VSWR: definition: The ratio of voltage across the line at the high-E points to that of the low-E points is known as the "Voltage Standing Wave Ratio", or VSWR. this is actually a more accurate definition because it includes the feedpoint and every electrical half wavelength along the line because these are the points where Emax and Emin occur.

conversely, when the load is matched to the line then Emax=Emin and of course the swr is 1:1 at all points along the line.

 

[Master Chief]

By George, I think he's got it! Exactly what I said earlier in this thread (without all the emin emax times the log of the 10 to the 18th power crap)

If changing the coax length changes the swr reading, the problem is at the FEEDPOINT. Fix that, and coax length does not matter and your SWR reading will be the same "at all points along the line" as you stated.

I have the utmost respect for your knowledge FC, I just can't stand your sarcasm and belittling. Its one thing to educate and a whole other to rip at a person's character. Thank you for your understanding in this matter.

 

[freecell]

no, it is not exactly what you said.

here's what you said:
SWR is a measurement between the FEEDLINE and the antenna at the FEEDPOINT of the antenna........

when speaking of a short line less than half an electrical wavelength the statement is true. when speaking of a long line in the case of a fixed station installation at 27 mhz. this statement is false.

here's what i said:
VSWR: definition: The ratio of voltage across the line at the high-E points to that of the low-E points (E=voltage) is known as the "Voltage Standing Wave Ratio", or VSWR. this is actually a more accurate definition because it includes the feedpoint and every electrical half wavelength along the line because these are the points where Emax and Emin occur when Zl does not equal Zo.

both statements involve only mismatched lines. in the case where Zl = Zo then under these conditions SWR is 1:1 because Emax=Emin EVERYWHERE along the length of the line and no standing waves are present, no reflection exists.

furthermore, when Zl does not equal Zo the problem is not just at the feedpoint, any mismatch present at the feedpoint is also reflected back up the line to the transmitter too. a problem at the load is also a problem at the source. when Zl no longer equals Zo then Zs (the transmitter) no longer sees Zo at the input to the line, or to put it another way, when the load is no longer equal to the characteristic impedance of the line then the transmitter no longer sees the characteristic impedance of the line at the other end and power delivered by the transmitter is reduced in direct correlation to the amount of reflection generated at the load.

now if were close to anything resembling agreement at this point then i have a question.

 

[Master Chief]

I agreed with you long ago. My comments were simple and easy to understand. Your more involved data supported what I had already said.

Go ahead and ask your question; I'm sure someone out here will have an answer for you, or know where to get it.

 

[W5LZ]

freecell,
The word 'system' in relation to antennas means the antenna, and feed line, and anything else 'in-line' (as in switches, connectors, meters, etc). Your transmitter sees the whole 'system', not just the feed line, or just the antenna.
I'd be willing to bet that anyone who is 'familiar' with this stuff would know exactly what I meant (even if it was late when I wrote it). In a well designed system there are no impedance mismatches, and the length of the feed line really doesn't make any difference. In a 'not so well' designed system the length of the feed line can make a big difference in what the transmitter sees (impedance). In some instances, the length can be just 'right' and even with mismatches the transmitter 'thinks' it sees a 'good' system (also holds true for meters). Those instances are not common, and are more a 'miracle' than anything thought out. Some people would rather work up a sweat by changing feed line lengths. Then there are those that do the sweating by using what's between their ears. And ~then~ there are those who just match impedances, and don't waste good sweat on it.
- 'Doc

 

[C W Morse]

freecell,
The word 'system' in relation to antennas means the antenna, and feed line, and anything else 'in-line' (as in switches, connectors, meters, etc). Your transmitter sees the whole 'system', not just the feed line, or just the antenna.
I'd be willing to bet that anyone who is 'familiar' with this stuff would know exactly what I meant (even if it was late when I wrote it). In a well designed system there are no impedance mismatches, and the length of the feed line really doesn't make any difference. In a 'not so well' designed system the length of the feed line can make a big difference in what the transmitter sees (impedance). In some instances, the length can be just 'right' and even with mismatches the transmitter 'thinks' it sees a 'good' system (also holds true for meters). Those instances are not common, and are more a 'miracle' than anything thought out. Some people would rather work up a sweat by changing feed line lengths. Then there are those that do the sweating by using what's between their ears. And ~then~ there are those who just match impedances, and don't waste good sweat on it.
- 'Doc

Doc, I am basically out of the argument, but I only have a couple of questions.

1. WHY is feedline length soooooo important to *some*? To them, you simply MUST or BUST?

2. Why does this question come up MOSTLY in reference to
CB forums.

I am not trying to be funny or cause further argument, but I am really confused as to why it is just so important. As I said, in 1966, I was 18 years old and had very little knowledge of how to install an HF radio. I had a '61 Chevy and a friend helped me get an old Lettine 240 transmitter together for a mobile. He had been a ham since '55. The antenna was a Master Mobile with a 3' mast and a 6 foot whip. We had very little equipment save a Grid Dip meter, a milliamp meter and a florescent bulb (Remember when you set up mobiles on the same 75 Meter frequency and held up a florescent tube to the antenna/mobile across the way; if it struck as we keyed the mike on OUR car, we were in tune?). Not once did Jim, my elmer, mention the coax length thing and I never even HEARD of 'coax length' until the CB craze of the 70's. I was like, "WHAAAAT????" By that time, I had made thousands of contacts on HF, learned to build a few of my own antennas, and generally had a very successful mobile station in the intervening years. To this day, Jim (now 75 years old) STILL scoffs at coax length as a NECESSARY factor in a mobile station--or in any station, and I talk to whomever I can hear, usually, if he is in America or Albania! And I STILL never cut coax to a "certain" length except to insure it will reach from the radio to the antenna. Doc, I am sure that, in 39 years, I would have run into a problem SOMEWHERE as a result of my coax being "the wrong length"! :shock: To me, while learning much of the formulas in my journey to Advanced (that and a buck will get you a cup of coffee ), I just don't understand why it is sooooo important and why this seems to have been birthed on CB and lives on CB, and will remain there until it's dying day! What happens if I attach the antenna directly to the set? "I just gotta have 18' else it won't work!!" :x

I honestly, truly, while it may seem like I am trying to argue the point, simply don't understand why it is so important when I've been doing this since my teens without considering formulas and have had a long ham hobby without worrying about coax length.

Everybody have a great day, it's all good, and it's certainly not worth getting worked up at each other over.

Best 73 to all, and I'll try to stay quiet!

CWM