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Checking Standing wave.

M

Metal Head

Guest
This has allways bugged me and I'd like an answer. When you put a meter inline lets say a 3' jumper from the radio to the meter and 18' from the meter to the ant. are you measuring the standing wave of the 3' + the 18'? Or are you messuring the 18' after the meter. If your measuring the 18' after the meter what about the 3' jumper? Won't that effect the standing wave by adding more coax from the radio to the ant.? If so wouldn't you need to add the 3' without the meter to obtain the correct standing wave? :?:
 

That really confuses me. That would mean I'm checking the SW on the 3' jumper from the meter to the ant. and not the entire length of the coax.
 
your right M-Head ....you are surely thinking up a storm there (-: Look dude ,if you got a great match on your meter using the 3 foot jumper from the radio (we have to use some kind of jumper) I really wouldn't worry about it ,at least I haven't in a few decades anyway and I've yet to burn up a radio or amp as of yet . If you really want to know what's go'in on with your stuff ? You could take it to a shop with a good rep and they could put a couple hundred dollar or so antenna analizer on your system that would surely tell you the truth if that is what your looking for here ? ....but as I said before ,I do understand where your coming from with this (well wouldn't that 3 foot jumper become a part of the system and the SWRs there after?) I would guess so or then again maybe not ? I will say this much though .....I was told a long time ago that just because I have a 1.2 match going through my amp through my SWR meter doesn't mean that it's a perfect situation )-: looks good yes ..but whats really going on ? there's reflective power and what not that I can't see without a really good decent antenna meter .....so I suppose over the years ,I've been a pretty lucky guy because to tell you the truth ,to this day ,I've never known ? ....so if it works the way I got it ? I really try not to fix it very much ,because if it were truly that bad ? chances are something would stop working one way or the other ,and in my case ,I've never ran monster watts a day in my life and I've only run run of the mill type cb radios ...I will say this much though ,this stuff can get pretty deep if one really wants it too.......but interesting all the same.
 
nomadradio said:
The lower your SWR really is, the less difference coax length will make.

The higher it is, the more that reading will be affected by coax length.

73
Click to expand...

How can you say that when the lengh of your coax is what determains the SWR?
 
the transmitter output impedance is fixed at 50 ohms. the characteristic impedance of the feedline is also 50 ohms. the only thing in question is the input impedance at the feedpoint of the antenna. since a figure OTHER than 50 ohms at the feedpoint of the antenna would be necessary to create reflection and/or standing waves on the feedline between the antenna and the transmitter, the swr measurements should be taken at the feedpoint of the antenna for best results. the only exception to this would be the use of a tuned 1/2 wave line with the swr meter inserted at the transmitter *using a double ended male PL-259 connector for example so that the mismatch present at the feedpoint can be seen directly at the transmitter end of the feedline.

depending on the match at the feedpoint of the antenna the feedline is in one of two different states or conditions.

Nonresonant Line. A nonresonant line is a line that has no standing waves of current and voltage. It is either infinitely long or is terminated in its characteristic impedance. Because there are no reflections, all of the energy passed along the line is absorbed by the load (except for the small amount of energy dissipated by the line).

Resonant Line. A resonant transmission line has standing waves of current and voltage. The line is of finite length and is not terminated in its characteristic impedance. Reflections are present. A resonant line, like a tuned circuit, is resonant at some particular frequency. The resonant line will present to its source of energy a high or a low resistive impedance at multiples of a quarter-wavelength. Whether the impedance is high or low at these points depends on whether the line is short- or open-circuited at the output end. At points that are not exact multiples of a quarter-wavelength, the line acts as a capacitor or an inductor.

in either case, a perfectly non-resonant line will only occur at a single operating frequency or a very narrow range of frequencies under the most favorable conditions. as the frequency of operation is moved from that frequency or range of frequencies where the line is non-resonant, the line will assume its tuned circuit characteristics and behave as a resonant line.

nomadradio is correct: when working with a flat, non-resonant line (1:1 SWR) the length of the feedline has no bearing on the match presented at the feedpoint of the antenna. only when the match presented at the feedpoint is something other than 1:1 does the length of the feedline have any effect. to understand why this is so one needs only to look at how the match between the transmitter and the input to the feedline is altered when reflections or standing waves are present in the case of the resonant line.

so in response to the original question, toss the jumper and insert the swr meter *directly at the output of the transmitter using a tuned 1/2 wave line or make your swr measurements directly at the feedpoint of the antenna for best results. *all interconnecting jumpers should be kept to a length of less than 1/20 of a wavelength or approximately 16" or less if the PL-259 barrel connector is unavailable.
 
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I told you it got deeper M-H ....Nomad kept it simple and Freecell spelled it out . Now like I said earlier ,if it works in my case ? I'll more then likely leave well enough alone. (THE TRUTH CAN HURT)

In many ways M-H my head tends to think like yours does ,I had a car once (very small Honda) and I drilled a hole in the roof to install a Wilson 1000 , I sanded underneath to make sure that the ground would be a good one when the bottom of the antenna adaptor (star shaped goodie) bit into the roof nicely. OK ......when I went to check the SWRs on this I was like a 2.5 to 3.0 )-: ON A WILSON OF ALL THINGS !!! ....I heard they sold longer stingers for such cases so I went out and got one ,Guess what ? .....NO CHANGE AGAIN !! )-: The Wilson like many other antenna's come with roughly 17 feet of coax ......just for grinnies I added a 7 foot jumper with a barrel connector to my coax and low and behold the match dropped down to 1.1 around the band !!!!! and the same going through the amp on the high side !!! ......I was told a long time ago that coax size really shouldn't make a differance but that with vehicals it can to a degree .....I was also told that because I had added 7 ft to the already 17 ft of coax on the antenna that it was really all a lie and what my meter was reading )-: Now we would be talking about a antenna analizer of some kind like AM Power was talking about or a meter that will show reflected power ......IM only guessing here ? but with the adding of the coax like i did (i did just solder on a solid 24 piece of 95X from the antenna) ......there's a very good chance that I could be reflecting power very baddly ? ...IM grounded like you wouldn't believe as well ,radio/amp doors / frame ect.ect. (IM surprised I don't wear a ground strap when IM driving down the road) ......I run a 4 x 454 amp and it stays fairly cool with my big mouth chattering away .....Reflection ? I really don't know ? .....but most would say that I had alot of it , or from what I've been reading over the years.....Alrighty then ! ......I still talked all over the world and the locals told me that I sound like a base station on wheels from time to time (pat pat (-:) even if I knew the whole truth and nothing but the truth when it comes to reflect and SWRs in that car ....I really don't know what more I could have really done to make things better ? It worked all the same and as far as I was concerned ,it worked very well. Hmmmmm ........Was it truly a reflecting ,lying SWR situation in my case ? ....chances are ,that it could have been ? How much can a little Rat Shack 29.99 meter really tell us other then what it was meant to tell us ? How many people out there in this world of cb radio operators truly get deeper then that ?
 
"Was it truly a reflecting ,lying SWR situation in my case ? ....chances are ,that it could have been ? How much can a little Rat Shack 29.99 meter really tell us other then what it was meant to tell us ?"

prove it for yourself. place and support a field strength meter out some distance from the vehicle and adjust for a midscale reading with the original 17' length of cable supplied with your wilson antenna. now, add the 7' extension and take another look at the field strength reading.

common sense tells us that with reduced reflection on the line that the transmitter is able to transfer more of the available developed power to the antenna and radiate it. this results in higher charge levels in both the near field e-plane and the far field h-plane. the field strength meter will indicate the increase in the amount of energy being radiated, confirming that the reduced swr reading resulting from the added feedline is anything but a lie.
 
easy freecell ....it is the goodness or badness of the antenna situation IM referring to here . As far as the field strength is concerned ? what would one be looking for here ? let alone setting the meter itself for a field strength test ......what are we looking for here ? ...key the the radio up with the meter out side of the car , and what is it were looking for ? a lesser reading would be good or the presents of a signal would be better ? .....you see Freecell "We" try and explain ourselves the best that we can on these Forums and sometimes things just don't come out right or they or misunderstood or it's hard to read between the lines ....sometimes good knowledgable folks explain things the way they truly are ,but sometimes guys like me still dont understand ? so we do our best to ask again until we hopefully get it right. No offence to you .
 
Posted: Wed Jun 15, 2005 10:12 am by Switch Kit
I understand what yout saying here........

Posted: Wed Jun 15, 2005 10:46 am by Switch Kit
but sometimes guys like me still dont understand ?

sounds like you're confused. reread: Posted: Wed Jun 15, 2005 10:02 am

you made the comment that when you added the extra 7' of feedline to the already existing 17' that the swr dropped from 2.5:1 - 3:1 down to 1:1 and that someone told you that the lower swr reading was a lie. what i said is simply this.

prove it for yourself. if the lower swr reading was truly indicative of a better match and not "a lie" then the lower swr reading would be accompanied by an attendant increase in measured field strength. i thought i kept it fairly simple.
 
instead of echoing my words ,wouldn't it had been easier to answer them ? ...it's all good with me. I read your post twice. confused ? Maybe so , but tit for tat over the years never really did me much good. thanks for the info. all the same. SK
 

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