adding coax lowers SWR reading on the meter?

[W5LZ]

About that comment on another forum.
"In a mobile there is no matching network..."
That's true with a typical mobile antenna. But what's stopping you from adding such a device to a mobile antenna? It's done quite often. It's never very 'simple' or easy considering the design of most mobile antennas, but that certainly doesn't make it impossible by any means.
(A 102" whip and a small coil at the feed point going to 'ground' comes to mind!)
- 'Doc

 

[Marconi]

About that comment on another forum.
"In a mobile there is no matching network..."
That's true with a typical mobile antenna. But what's stopping you from adding such a device to a mobile antenna? It's done quite often. It's never very 'simple' or easy considering the design of most mobile antennas, but that certainly doesn't make it impossible by any means.
(A 102" whip and a small coil at the feed point going to 'ground' comes to mind!)
- 'Doc

I guess we are still looking to see if changing length of coax changes the SWR and if it does, how do we fix it, and how do we tell if we fixed it?

'Doc, do you think a matching coil like you describe here would make enough difference to the match that one could tell in some way? If so, how could we tell?

 

[Needle Bender]

this comment on another forum seemed odd to me is why i asked ....

" I have found that in most all single antenna installs that I have done 15 ft of 213 was the sweet spot.

i did the math. 15 feet of 213 is an electrical 5/8 wave. curious:blink:

 

[bob85]

im not waiting eddie,
walt maxwell says changing coax length changes the impedance seen at the coax input whenever there is a mismatch at the load BUT changing coax length does not change vswr beyond any change in line loss caused by altering line length and gives the reason vswr seems to change when changing coax length,

unless somebody provides an alternative respected source to disprove or argue against what walt and others say, the mismatch at the load claim been the cause of vswr changing with line length should be read as more none factual personal opinion not backed up by any respected source as per the j-pole camps usual modus operandi,

freedom of speech/nonesense posting without ever having to prove your bs is an excellent weapon of obfuscation / misinformation,

swr meters make people stupid

 

[W5LZ]

Would adding an impedance matching device at the antenna's feed point make a difference? I would have to think it would make a difference in the impedance match between the antenna and feed line, wouldn't you? A good impedance match means that there's a better transfer of energy between feed line and antenna, more signal getting to the antenna to be radiated. The typical 25 - 35 ohms input impedance of a 1/4 wave antenna would then be closer to 50 ohms which would certainly make the transmitter 'happier'. (That holds true for any frequency, not just RF. The typical AC from those power lines for instance.) Efficient transfer of energy is the whole idea, less losses because of reactance, right? If the load is the same impedance as the transmission line and the transmitter, there's only resistive losses present because of the length of that transmission line. (Those resistive losses are nothing compared to the potential losses from an impedance mismatch.) If the load is reactive, then the amount of loss depends on just how reactive it is, and why. One of the contributors for that reactance is the feed line if there's a mismatch between it and the antenna, or it and the transmitter. That means that while the length of feed line can act as an impedance 'transformer', it also has the liability of not being designed or constructed to handle that transformation very efficiently. In other words it can be 'over-loaded' real easily and destroy it's self. Coaxial cable is just not made to handle the voltages common to impedance transformation.
So can the length of the feed line make a difference? Sure. Is there a better way of doing things? Sure. That 'itty-bitty' coil at the feed point of a mobile antenna is just one of those ways to do it 'better'. So why don't mobile antenna makers include such a coil that can be adjusted? Because then it wouldn't be 'plug-n-play' anymore. And it'd cost more.
- 'Doc

 

[hookedon6]

im not waiting eddie,
walt maxwell says changing coax length changes the impedance seen at the coax input whenever there is a mismatch at the load BUT changing coax length does not change vswr beyond any change in line loss caused by altering line length and gives the reason vswr seems to change when changing coax length,

unless somebody provides an alternative respected source to disprove or argue against what walt and others say, the mismatch at the load claim been the cause of vswr changing with line length should be read as more none factual personal opinion not backed up by any respected source as per the j-pole camps usual modus operandi,

freedom of speech/nonesense posting without ever having to prove your bs is an excellent weapon of obfuscation / misinformation,

swr meters make people stupid

thats pretty much "spot on" (I couldn't resist):tongue:

an easy way to PROVE that, is to use 2 VSWR meters, one at the feed point (where it should be), and the other one 1/2 elect wl away at the SAME time. the 2 meters "should" read about the same.

then take the one 1/2 wl away and add a 1/4 wl between it and the antenna.

did the vswr reading at the feedpoint change?..... NO
did the vswr reading at the remote 3/4 wl change from the 1/2 wl away reading? perhaps yes, perhaps no. notice I said vswr READING and not VSWR.

bottom line: only antennas produce VSWR.
feedline does not produce vswr (unless it is damaged).

 

[mackmobile43]

thats pretty much "spot on" (I couldn't resist):tongue:

an easy way to PROVE that, is to use 2 VSWR meters, one at the feed point (where it should be), and the other one 1/2 elect wl away at the SAME time. the 2 meters "should" read about the same.

then take the one 1/2 wl away and add a 1/4 wl between it and the antenna.

did the vswr reading at the feedpoint change?..... NO
did the vswr reading at the remote 3/4 wl change from the 1/2 wl away reading? perhaps yes, perhaps no. notice I said vswr READING and not VSWR.

bottom line: only antennas produce VSWR.
feedline does not produce vswr (unless it is damaged).

So you're changing impedance at every connection or addition of another connection? causing the meter to show different vswr? not the meters fault not being able to read faults in connections.

Sorry to clarify the meter's inability to compensate for faults in feed line interruptions.

 

[Marconi]

thats pretty much "spot on" (I couldn't resist):tongue:

an easy way to PROVE that, is to use 2 VSWR meters, one at the feed point (where it should be), and the other one 1/2 elect wl away at the SAME time. the 2 meters "should" read about the same.

then take the one 1/2 wl away and add a 1/4 wl between it and the antenna.

did the vswr reading at the feedpoint change?..... NO
did the vswr reading at the remote 3/4 wl change from the 1/2 wl away reading? perhaps yes, perhaps no. notice I said vswr READING and not VSWR.

bottom line: only antennas produce VSWR.
feedline does not produce vswr (unless it is damaged).

Hookedon6, I don't get the distinction you're trying to make between, vswr and VSWR, but that 1/2 wave you describe using at the feed point will not necessarily show a correct reading at both ends as you suggest...unless the antenna (load) is purely resistive (a perfect match) and the coax is 50 ohms too, in addition to the comments Mack posted. With antennas...close is good enough and to chase the perfect match can be like chasing your own tail.

 

[Needle Bender]

bob85 i dont think anybodys trying to be a bad guy. i just thought it was curious that he sees a electrical 5/8 wave of coax work the best in mobiles where its real hard to get close to 50 ohms with no reactance
i wonder what matching qualites 5/8 of coax has?
bob what does this mean? "the mismatch at the load claim been the cause of vswr changing with line length should be read as more none factual personal opinion not backed up by any respected source as per the j-pole camps usual modus operandi," i know your speaking english but could you interperet into american?:blink: i thought a mismatched load being the antena would show a better match by cutting coax

 

[hookedon6]

that 1/2 wave you describe using at the feed point will not necessarily show a correct reading at both ends as you suggest...unless the antenna (load) is purely resistive .

"correct" reading????? what do you mean "correct"???

if a meter @ the feedpoint reads "X" vswr, and part of "X" is due to a non-resistive load" then a meter 1/2 e wl away will also read "X" vswr. meters are "dumb", all they do is make a ratio.


I already talked about non-resistive loads at the feedpoint in a prior post.

the POINT is that, in the example I gave, is that the VSWR at the feedpoint will NOT change due to any changes in the feedline prior to it.

add or remove coax all you want before the meter @ the feedpoint, and the meter at the antenna feedpoint should not change.(unless you have a coax issue).

therefore , addition of feedline does not change the ACTUAL VSWR , you may get a different reading if you have the meter in the WRONG location, but, the ACTUAL VSWR still hasn't changed.

you guys missed the most important part of the post:

bottom line: only antennas produce VSWR.
feedline does not produce vswr (unless it is damaged or it is designed to radiate).

 

[hookedon6]

...bob what does this mean? "the mismatch at the load claim been the cause of vswr changing with line length should be read as more none factual personal opinion not backed up by any respected source as per the j-pole camps usual modus operandi," i know your speaking english but could you interperet into american?:blink:

I "think" he is sayin that the idea that coax length changes the actual VSWR is bullsheet

 

[bob85]

hookedon6, read the links i posted further up the thread,
you can find the same info in the arrl antenna book and other respected sources.

you can find the same info in the arrl antenna book and other respected

walt maxwell REFLECTIONS 2, chapter 2 ( countdown for a journey from mythology to reality )
sec 2.3 ( true or false ) 21) 22) 23)

and REFLECTIONS 2, chapter 21 ( some aspects of the balun problem )
sec 21.3 ( should SWR change with line length )sources

 

[mackmobile43]

hookedon6, read the links i posted further up the thread,
you can find the same info in the arrl antenna book and other respected sources.

you can find the same info in the arrl antenna book and other respected

walt maxwell REFLECTIONS 2, chapter 2 ( countdown for a journey from mythology to reality )
sec 2.3 ( true or false ) 21) 22) 23)

and REFLECTIONS 2, chapter 21 ( some aspects of the balun problem )
sec 21.3 ( should SWR change with line length )sources

Sounds like a bible to me

 

[hookedon6]

one last time and i'm dun:mellow:,.......

triming a coax cable has nothing to do with the ANTENNAS VSWR.

the ANTENNAS VSWR is FIXED (unless it can be varied like a screwdriver/stepIR/ect) due to the length/resistance/height/ect. changing the feedline does not change any of these factors.

ONLY antennas have VSWR!!! feedline (*unless it is damaged or designed to radiate) does not have VSWR!

Trimming the unbalanced feedline will change the impedance the transmitter sees if the transmission line repeats standing waves caused by the ANTENNAS VSWR, but, it will not change the antennas actual VSWR.

if the antenna a resistive load, there will be NO SWR on the coax*.

if your VSWR changes with the length of feedline, then you either have your meter in the wrong place, or you have feedline issues.

 

[bob85]

I "think" he is sayin that the idea that coax length changes the actual VSWR is bullsheet

well spotted,


"translation"
a few dogmatic people constantly post bs that is in argument with basic physics, smith charts, arrl antenna books, walt maxwell and other smart folk,

they disrupt threads with technobullshit without ever having to reveal or post any links to sources/books to look at that explains why they don't agree with what is widely published and feel obliged to spread the manure,

the worst offender only posts bs on cb forums where he can get away with it, he does not post the same nonesense on the ham forums he frequents where that shit won't fly.


i hope that is plain enough english.

merry christmas