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RE: Asking Elmer's for how to use SWR meters!?
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by N7DM on August 9, 2004
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Unless I missed it, none of the comments suggested telling you why a SWR is bad. I have a SWR of 15 to 1 on the feedline to my 80 meter vertical Ground Plane. It doesn't bother a thing.
I believe the heart of the issue lies in understanding that all feedlines have loss..some more than others. Then, you should remember that at each and every impedance CHANGE along the line, you will have a SWR generated...more importantly, a Reflection. So...if you have a 30 ohm antenna [as I do], fed with a 450 ohm twinlead [as I do], I have one heck of a big Reflection Cooefficent AT THAT POINT. OK, some of the 'RF' happily does it's thing on the antenna. A fair amount of it is reflected back down the line to the rig area. I say 'area', 'cuz at my rig, I have a very simple impedance matching network [ read that Antenna Tuner], which conjugately matches the feedline and sends that reflected RF back up the feedline to have another try at radiating! It is the 'back and forth' of the RF on the line that is the problem... each 'pass' has it's loss. One of the most important but little read graphs in The Book is the one that is called "Additional Loss Due To VSWR"... If you look at that graph, you will find that if you have a very low loss feedline, SWR (VSWR....ISWR...all same) has extremely little effect on your transmitted power...and the ability to have that tiny little induced current from Itchey-Crotchey's signal, get to your receiver.
Only when lossy feedline is used..and in fact over 'fair' lengths does SWR become a factor.
So...what you do is run a low loss feedline to your antenna, using some form of 'Antenna Tuner'... with your SWR METER in the coax (?) between the Matching Network and your Transmitter. You adjust your matching to 1:1 in that little chunk of coax, meaning all your RF is going into the SYSTEM of Network/Feedline/Antenna...and you are in business.
"CQ DX".........
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RE: Asking Elmer's for how to use SWR meters!?
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by KI4CFS on August 9, 2004
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I think your question of why high SWR is bad and what it really means is good too?
KI4CFS
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RE: Asking Elmer's for how to use SWR meters!?
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by X-WB1AUW on August 10, 2004
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Any ARRl handBook has a great sections on SWR, feed lines, power lost in feed lines, etc.
Congrats on your first ticket.
Have FUN
Bob
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RE: Asking Elmer's for how to use SWR meters!?
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by KI4CFS on August 10, 2004
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You said, "Any ARRl handBook has a great sections on SWR, feed lines, power lost in feed lines, etc." I have to say as a newer Ham I personally that have been refered to ARRL books for SWR & dummy load and think if you look form 'beginner eyes' would agree that they do a VERY poor job in even offering what is in this current post. They all say a "must used tool of every ham is a SWR & dummy load" . You look for the next paragraph explain how they work, how to use them, and so forth and you will NOT find it! AND thanks for your posting!
Another question: What is the relationship between SWR and Field Strength?
Also, what is the relationship between AVG & PEP ratings? I have it on both meters, one uses power and the other does not.
KI4CFS, Martin
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RE: Asking Elmer's for how to use SWR meters!?
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by N7DM on August 10, 2004
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Good point. Martin. Short of a Electrical Engineering Course in Feedlines..[and there are many, to cover all the varieties]...it IS rough to 'get going'. As a Ham, my most sincere suggestion will always be to get one of the ARRL Antenna Manuals, and start reading...from the beginning. You will NOT understand it all; I did not, nobody does. But slug your way through. Then, think about how it ties in with what you read here, or find, operating. The Re-Read it. Again. Keep it on your night stand instead of Tom Clancy. Each time you do, something will make more sense... you will find yourself saying. "So THAT is how it works!". I fancy myself as able to create a decent antenna system, but it didn't come over-night, and it didn't come from only reading the ARRL book.
There is no direct relationship between SWR and Field Strength. Field strength basically is a direct function of the the amount of current flowing in an antenna. WHERE that field is 'encouraged' to go is dependant on other things...height, ground conductivity, obstructions, other driven elements or parastic elements...etc.
SWR is simply 'ONE' way of numerically stating how well 'something' is matched to something else. There is an SWR on your feedline, IF your antenna feedpoint impedance is not EXACTLY the characteristic impedance of your feedline. There is another SWR on the tiny little piece of coax in your rig that goes from the output filter to the Antenna Output connector, IF the input to your feedline is not EXACTLY what your rig's output impedance is...50 ohms? The only way either of these SWR's can effect your 'Field Strength' is if either one of them permits losses so that some of your output power [causing current in the antenna to flow]...is lost before it gets to the antenna.
The good part of what I just told you is that YOU can find out the exact same info by Reading and Re-Reading, AND RE-RE-READING that Antenna Manual...while physically 'checking out' what you just read. This part of Ham Radio is truly 'a lab class'...
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RE: Asking Elmer's for how to use SWR meters!?
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by N7DM on August 10, 2004
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I will not tackle Average and Peak Envelope Power...someone else with more knowledge than me can do that one....I know the basics, but not enough to teach it.
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RE: Asking Elmer's for how to use SWR meters!?
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by AA4PB on August 10, 2004
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Also,remember that that there is a difference between accuracy and precision
---------------------------------------------
A very important point - especially when it comes to modern frequency readouts. The display may read down to 1Hz (very good precision) but the accuracy may only be +/-100Hz. You can't count on the actual frequency being any closer than 100Hz to the display reading. The 1Hz precision can be very useful for returning to a frequency during the short term, but you only know the actual frequency within 100Hz.
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RE: Asking Elmer's for how to use SWR meters!?
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by WILLY on August 10, 2004
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by KI4CFS on August 9, 2004
"Thanks for all the wonderful comment. And the offer to come over for some hands on! ..."
You got many good answers here, with lots of info.
In my opinion, far and away the best answer was KZ1X's offer to visit his shop and antennas. Take him up on it! Nothing beats show-n-tell. And if you have a Handbook or whatever books, take them with you. Whatever he shows you will also serve to clarify what you are reading there.
Suggestion: Before you go, find out if he likes doughnuts or beer or pizza or whatever. :) If you ply him, you'll get to spend more time drooling over his antennas and other equipment!! That'll just generate more questions, and who knows what all else you might learn.
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RE: Asking Elmer's for how to use SWR meters!?
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by KI4CFS on August 11, 2004
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KZ1X, thanks you so much for your time! What a great set up and knowledge you have!
The get together with youe was very exciting and informative. I appreciate uour wonderful generosity. true to the "Code of Ham". So lets see I learned something and any are ok to respond. If I have missed something KZ1X, it does not mean you are not a great teacher, I am just a slow student!
Q. Does SWR tell you how much radiating energy is coming from your antenna?
Not Necessarly. SWR is relevant only when transmitting for the most part. It represents the amount of power coming back into the radio. You of course do not want a lot coming into the radio for you do not have an efficient transfer of energy AND you may be putting a strain on the 'finals' (components in the radio) of your radio, also may increase RF in the Ham Shack. BUT low SWR does not mean you are transmitting a strong signal. For example a Dummy Load almost transmits nothing for it is converting all the electrical energy into heat showing a 1:1 or Low SWR . With out something that is testing the actual radiating energy from the antenna and how it changes with changes in SWR you are not getting a total picture.
A RF meter like the 'Zapper' lets you know the relative about of RF energy coming to the meter and if you set that at a low threshold, keeping it in one location, then lower the SWR and see a increase in RF then that is GOOD and you are radiating more energy.
A more effective tool to do this is a Antenna Analyzer for it hooks directly up to the antenna cable and as you move or adjust the antenna it will show if you have reduced the SWR and increased the efficiency of the antenna.
One more thing happened and I would like to know about:
Q What happens if you get a very high power reading, higher than you know the radio is rated for, and a high SWR, what could be causing that?
A friends has a mobile inside and said the antenna connection was getting real hot. So I saw this as an opportunity to test out my SWR meter (skills). We hooked up the UHF/VHF meter between the radio and the indoor antenna (a mag mount on a small pizza tray). We put it on a non used frequency and he pressed the mic on. The Power went up to a reading of 75 Watts and an SWR of 5:1. Took the same antenna down to the car and put it on the roof, got a SWR of 4:1 and a power of 25 WATTS (unit is rated to be 30 WATTS). Found out that the antenna he though was just like my 2meter/70CM antenna is NOT, but about 5 inches shorter. So what happened?
KI4CFS, learning in process.
Martin Brossman
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RE: Asking Elmer's for how to use SWR meters!?
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by N7DM on August 11, 2004
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Another interesting thing about Dummy Loads. Often you will hear or read of someone working across the State or Town...from a Dummy Load, or in one QST article, a Light Bulb. I wouldn't bet YOUR life on it, but I am pretty much sure that the actual radiation was not from the Dummy itself, but from the shield of the COAX feeding it. I have run tests...receive only of course...feeding my old Heathkit Dummy with both coax and twinlead. With twinlead, no length will let me hear a thing. With coax, I CAN hear... and the longer the coax run, the better. So I am pretty sure it goes back to the old Unbalanced Feed to a Balanced Load 'thing'..
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