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Tuning SWR
- Thread starter ctvanover
- Start date
You ready for this??
SWR isn't something that's directly useful. It's only a 'sort of' measurement of the things that are able to be changed and thereby make tuning an antenna system possible. There's no really cheap or common way of measuring the thing that determine how good an antenna is matched to the feed line and the transmitter. That thing is called impedance, which is composed of two other thingys called reactance and resistance. To make all this just a little bit more complicated, that reactance thing can be either positive or negative (which can also be called inductive or capacitive reactance). When that reactance is combined with resistance and called impedance, it can be more than, less than, or the same as, the impedance at the input of the antenna. If the impedances are the same at the antenna's input as that of the feed line and the output of the transmitter, then things are good and you have an SWR of 1:1, or, very dammed close to it which since that transmitter, feed line, and antenna are all man-made and so subject to errors. SWR tells you if the impedance match is close, but, if it isn't close, that SWR meter has no idea which direction the impedance/reactances are off, can't tell positive/inductive from negative/capacitive reactance from each other. Just that they aren't 'close' and sort of how far 'off' they are. It can't tell you which direction, or how much things have to be adjusted.
There's another part of this that you should be aware of. Some reactances combined with some resistances appear to an SWR meter to be just what you are looking for, a 1:1 SWR. The only problem with that is that reactances do not contribute to radiation or dissipation of power. Only reisitance does that. So, if that combination of reactances and resistance doesn't equal 'zero' reactance and 50 ohms resistance (what most transmitters are looking for), then some power just doesn't get to the antenna to be turned into a radiated signal. That's the so-called 'loss' from SWR. That's sort of like thinking there's a problem with how you're measuring tire air pressure, when the actual problem is a leak some where. Fix the leak and no more problems. Fix the impedance mismatch and there's no more SWR problems. That air pressure gage and an SWR meter can tell you there's a 'leak', but not where or how to fix it. That's when knowing something about tires/antennas comes into play.
And if all that isn't bad enough, there's almost always more than one way of fixing a 'leak'/mismatch. Some better than others. And from here, this analogy breaks down real quickly, so time to start learning. "Holy 'shirt'! More books!" ... yeah but it's a lot easier than making all them @#$ mistakes.
- 'Doc
SWR isn't something that's directly useful. It's only a 'sort of' measurement of the things that are able to be changed and thereby make tuning an antenna system possible. There's no really cheap or common way of measuring the thing that determine how good an antenna is matched to the feed line and the transmitter. That thing is called impedance, which is composed of two other thingys called reactance and resistance. To make all this just a little bit more complicated, that reactance thing can be either positive or negative (which can also be called inductive or capacitive reactance). When that reactance is combined with resistance and called impedance, it can be more than, less than, or the same as, the impedance at the input of the antenna. If the impedances are the same at the antenna's input as that of the feed line and the output of the transmitter, then things are good and you have an SWR of 1:1, or, very dammed close to it which since that transmitter, feed line, and antenna are all man-made and so subject to errors. SWR tells you if the impedance match is close, but, if it isn't close, that SWR meter has no idea which direction the impedance/reactances are off, can't tell positive/inductive from negative/capacitive reactance from each other. Just that they aren't 'close' and sort of how far 'off' they are. It can't tell you which direction, or how much things have to be adjusted.
There's another part of this that you should be aware of. Some reactances combined with some resistances appear to an SWR meter to be just what you are looking for, a 1:1 SWR. The only problem with that is that reactances do not contribute to radiation or dissipation of power. Only reisitance does that. So, if that combination of reactances and resistance doesn't equal 'zero' reactance and 50 ohms resistance (what most transmitters are looking for), then some power just doesn't get to the antenna to be turned into a radiated signal. That's the so-called 'loss' from SWR. That's sort of like thinking there's a problem with how you're measuring tire air pressure, when the actual problem is a leak some where. Fix the leak and no more problems. Fix the impedance mismatch and there's no more SWR problems. That air pressure gage and an SWR meter can tell you there's a 'leak', but not where or how to fix it. That's when knowing something about tires/antennas comes into play.
And if all that isn't bad enough, there's almost always more than one way of fixing a 'leak'/mismatch. Some better than others. And from here, this analogy breaks down real quickly, so time to start learning. "Holy 'shirt'! More books!" ... yeah but it's a lot easier than making all them @#$ mistakes.
- 'Doc
Excellent post Doc ! Reminds me of the time I "thought" I had a 1.1 with a cheapy Radio shack meter , although running through 400 sum odd watts , it still read good enough at 1.3 . Well , had somebody put one of there fancy antenna analizers on my system and it really wasn't as good as I thought it was or what the cheapy meter was telling me , although , it had gone up to a 1.5 on the fancy meter , I was still fairly good to go. I agree , for your average radio operator , there's a lot more then meets the eye's (and understanding) when it comes to SWR readings.(you think you have one match and in reality you have a different one) I look at it this way , if your cheapy meter says you got a good match , and everybodies telling you your signal and audio is up to stuff and you have been using the stuff for a period of time ? I'd say you really had nothing to worry about then. I understand where power is concerned it changes things when being concerned about SWR , most have said keeping it under 1.5 or lower is to be on the safe side, you don't want to back up to much power not getting out and blowing your power up. So to speak. Power surely changes things and this is when one should have a fair concern concerning ones SWR . Especially if running freak'in magmounts ! Damn I hate those things !!(they tend to change SWR from one spot to the other depending on where your at) For those that don't deal with power , I'd say you were treading on much safer waters concerning your SWR , Yeah sure , everybody whom cares would love nothing better then having a 1.1 SWR around there bans of choice , but guess what , a 2.1 will work just fine ! and I truly doubt from 2.1 to 1.1 would be noticed if any on the receiving ends of your transmission.........I would like to thank the good Lord when it comes to SWR over the last few decades , WHY ? because what I didn't know ,apparently didn't hurt me. I surely like things the best that I can personally get them , but I would have drove myself right out of the hobby years ago if I were have been a perfectionist about it all. If it works , don't fix it.
Excellent posts !
I am trying to learn here guys , how do you go about checking the reactance resisitance on your antenna / feedline ? Hooking up an SWR meter is one thing ......but, would you check this with an ohm meter ?
And, as SWITCH says "....what I didn't know appaprently didn't hurt me." I'm still here , still learning....
I am trying to learn here guys , how do you go about checking the reactance resisitance on your antenna / feedline ? Hooking up an SWR meter is one thing ......but, would you check this with an ohm meter ?
And, as SWITCH says "....what I didn't know appaprently didn't hurt me." I'm still here , still learning....
OOOOoooo, now you've come to the really fun part! Reactances aren't directly measurable. Reactance values are calculable, though, and that's sort of how it's done. Either on paper or by using a gadget like an antenna analyzer, which does that calculating electrically (MFJ's is something like $260, cheap, right?). The next 'trick' is learning to read that thingy and understanding what the readings mean. More fun, yeeeha!
It's a lot like building a house. If you don't start with a fairly good foundation (knowledge), the results aren't gonna be fantastic without lots of luck... or making lots of mistakes. Not that there's anything wrong with making mistakes and learning that way, sort of. But if you at least don't understand a little of what's going on, you might as well call it magic. You just ain't gonna live long enough to make all them mistakes (believe me, I know).
Having fun yet??
- 'Doc
It's a lot like building a house. If you don't start with a fairly good foundation (knowledge), the results aren't gonna be fantastic without lots of luck... or making lots of mistakes. Not that there's anything wrong with making mistakes and learning that way, sort of. But if you at least don't understand a little of what's going on, you might as well call it magic. You just ain't gonna live long enough to make all them mistakes (believe me, I know).
Having fun yet??
- 'Doc
I too am trying learn a thing or two! A buddy of mine sold me his used MFJ and bought a new one for himself, I got it at pretty good deal you could say. He said was easy to use but I still ain't figured it out yet!? I bought a set of Browning BR-28 High Gain Broadband 63" antenna's the other day, good looking antenna's too but that don't mean much either unless you know how to set'em up correctly and have a good standing wave!
OBTW Hello Chainsawgang.....my left hand ain't nearly as sore as was after the operation, it's still a little tender! LOL
