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1-10 of 24 messages
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Asking Elmer's for how to use SWR meters!?
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by KI4CFS on August 9, 2004
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Asking Elmer's for how to use SWR meters!?
I have yet to find something written to my satisfaction on using low price (Under $200.00 ) SWR meters. So if you will help me I will help the community by putting it into a short article. I have looked at the ARRL books and all do not answer these questions well. Even the dummy book does not. So I will be the first dummy to ask here!
How do you use SWR meters with a 10% error, accurately? (Lets say they are a cross needle unit).
How do you use it to tune an antenna?
How do you use it with a dummy load?
How do you use it to determine if your feed line is good or bad?
How can you determine if it is accurate "enough", How can you determine if the 20W range is accurate compared to the 200W range with out expensive equipment to 'test against?
How do SWR meters tend to go bad, how would you know it, if it did?
All the books say a dummy load is good to have, but none say how to use it, how do you use it,what are the different ways?
Thanks in advance to any of you that choose to join me in this exploration!
Martin Brossman
KI4CFS@arrl.net
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RE: Asking Elmer's for how to use SWR meters!?
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by K5LXP on August 9, 2004
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Well thought out question, Martin.
> How do you use SWR meters with a 10% error,
> accurately?
Well, if it's specified as 10% accuracy, you'll never have more than a 10% confidence in the reading. However, rarely do you need to know the exact SWR value. Most of the time you're only interested in finding the minimum value (say, when adjusting a tuner), in which case even very inexpensive SWR meters will do this very well.
> How do you use it to tune an antenna?
In the case of a dipole, you would check the SWR of the antenna under test at fixed intervals (say, 25kHz) across the band it was cut for, and graph the measured SWR vs frequency. The point where SWR is minimum is considered the resonant point. Once this point is known, the length of the antenna can be adjusted to move the resonant point to the desired frequency.
> How do you use it with a dummy load?
Using an SWR meter with a dummy load is a good way to test cables. Obviously, a dummy load will always present a 1:1 match so you won't be testing it's SWR. But if you *do* measure a mismatch while using a dummy load, the cable or connectors are suspect.
> How do you use it to determine if your feed
> line is good or bad?
Using a short patch cable, transmit into the dummy load and set the forward reading for full scale. Insert the cable under test and see that the needle still deflects the needle to almost full scale. If it's significantly lower, there's a problem. If there's any reflected power at all, there's definitely a problem. If the SWR meter has a wattage scale on the forward reading, you can see just how much attenuation the cable has at the frequency you're testing at.
> How can you determine if it is accurate "enough",
You can connect some known mismatches to the antenna port and see what the readings are. Of course, a 50 ohm load will read 1:1. A 25 or 100 ohm resitive load will read 2:1, and a 12.5 or 200 ohm resistive load will present a 4:1 SWR. No load at all, or a direct short should read infinity to one. Usually, especially with inexpensive meters, the accuracy goes down as the SWR goes up. Pretty much any meter will tell you 1:1, beyond about 5 or 6 to one is tough to accurately measure with a simple instrument.
> How can you determine if the 20W range is accurate
> compared to the 200W range
Simply sending the same forward power into a dummy load should result in the same readings between the scales, albiet with a bit less resolution on the higher scale. For instance you could set your transmitter for a full scale 20W forward power, then switch to the 200W range. It should still read 20W. The actual power may or may not be exactly 20 watts depending on the accuracy of the meter's calibration. Verifying the actual accuracy is going to require some known standards and an accurate means of measuring voltage or current, or a known calibrated wattmeter.
> How do SWR meters tend to go bad, how would you
> know it, if it did?
A second one is handy to have to compare readings against and is inexpensive. The dummy load and known mismatch standards are a good means of finding a bad meter. The way most VSWR meters work it's not often they suddenly give inaccurate readings. If anything, they will fail completely in one direction or the other due to bad diodes.
> how do you use it,what are the different ways?
The most common application is to indicate a good match between a transmitter and feedline. Testing coax is probably the second most. If I need to know any more about a tuned network, antenna, or matching section I'll usually resort to my MFJ-259 analyzer that will tell me inductance, capacitance and a few other parameters that SWR meters cannot indicate. Perhaps some of the real older timers here can pass on some measurement tricks that can be done with basic VSWR meters. One that I know if is how to use an inexpensive 'CB' SWR meter on VHF. One connects it in the usual fashion between the transmitter and the load, and you set the meter full scale per usual. But instead of throwing the switch to reflected to get the SWR, you disconnect the cables on the meter and reverse the connections, and now the forward reading becomes the reflected reading. What this does is use the same diode for both forward and reflected, eliminating the error incurred by the second reflected power diode, which for an inexpensive meter won't be a well-matched pair.
Mark K5LXP
Albuquerque, NM
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RE: Asking Elmer's for how to use SWR meters!?
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by KZ1X on August 9, 2004
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All good questions. The questions you have asked all focus on the meter and dummy load devices, rather on what they do and how they work, in general.
If your article talked about how these devices work and what they are for, the questions you ask get answered automatically.
If instead, you try and answer these questions from the point of view of the device itself, then you have essentially written an instruction manual for one particular meter (or range of similar types) which can be had from the manufacturer.
I think what you are asking is for more of an explanation of how they work and what they do. This is an example of something that is best taught in situ rather than by reading something in a book or on the web. 30 minutes in my shop and another 30 in my backyard (which is only a short ride away from you, btw) can equal $75 in books and a week to read them all.
Seems like a good deal to me!
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RE: Asking Elmer's for how to use SWR meters!?
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by K0RFD on August 9, 2004
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I'll try to answer a few of your questions. I'm sure others will jump in.
how do you use a 10% meter accurately? You don't really need to. 10% isn't enough change in reflected power to worry about in most cases. Most meters detect the zero or near-zero reflected power condition pretty well. That's really what you care about. With a cross-needle meter, I generally don't try to read the actual SWR. I just tune for minimum reflected power.
How do you use it to tune an antenna? well, ideally you want to tune the antenna so that the minimum SWR is at or near the part of the band where you operate. Measure the SWR at several frequencies within the band. If you want to move the minimum SWR higher in frequency, make the antenna a bit shorter. If you want to move it down in frequency, make the antenna a little longer. This assumes that the minimum SWR occurs in a range where you are allowed to transmit. Often when you build a new antenna, it's pretty out of whack and the resonant frequency will be outside the range where you are allowed to transmit. (DO NOT TRANSMIT OUT OF BAND) In that case, you measure the SWR at each end of the band. If the SWR is lowest at the lowest frequency of the band, chances are your antenna is too long, make the antenna a bit shorter and see if the minimum swr is within the band. If the lowest SWR is at the high frequency end of the band, then make the antenna a bit longer, again hoping you can get the true minimum inside the range where you can transmit. This is just a rule-of-thumb -- sometimes you can't really tell. In that case, borrow an Antenna Analyzer or Receiver Noise Bridge from somebody and find where the resonant frequency REALLY is.
How do you use it with a dummy load? See the answer to the next question for one example.
How do you use it to determine if the feedline is good or bad? Connect the dummy load directly to the meter and measure the SWR. It should (ideally) be 1:1 or close. Then connect the feedline to the meter and connect the dummy load to the other end. If the SWR is very high, then chances are the feedline, one of the connectors, or (most commonly) one of the solder joints in the connector is bad. Basically, you are just taking the antenna out of the equation and measuring the SWR of a known impedance with and without the feedline in the system.
How can you tell if a meter is accurate "enough" without expensive test equipment? Depends on what you mean by "enough". I really don't pay much attention to absolute SWR values so long as my antenna is close enough to avoid going into power foldback, which most solid-state transceivers do at pretty low SWRs. I just use my meter to find the SWR minimum when I am tuning a new antenna, and to see if anything has changed drastically from last time. The absolute accuracy isn't important, and the precision is usually good enough to tell me if my antenna fell down or the coax came unhooked in the last windstorm.
SWR meters can go bad in a lot of ways. If it's a D'Arsonval meter, the movement can go bad. You can put too much power into it and burn it out. Connectors can go bad, solder joints can go bad, just about the same things that can go bad with any other piece of electronics. How do you tell? Well, if you can't get a reading on the "calibrate" setting, or if nothing moves under any circumstances, or if you get readings that make no sense at all, test with a dummy load, or borrow a known good meter from somebody else and compare.
What is a dummy load used for? Well the "classic" use was to tune the tank circuit of a transmitter or amp with tube finals without going on the air and QRMing everybody. But a dummy load is useful whenever you want to test or adjust something into a known impedance without going on the air.
If you want to learn more, go to your local library and see if you can find an ARRL Handbook or ARRL antenna book.
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RE: Asking Elmer's for how to use SWR meters!?
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by KA5N on August 9, 2004
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The primary use of an SWR meter is to determine the amount of reflected power in an antenna system. The accuracy of inexpensive meters is relatively unimportant and 10% is good enough. Whether the SWR is 1.1: or 1.3:1 isn't critical. The SWR number accuracy is determined by how closely matched the measurements of the forward reading and the reverse reading are. So in any SWR meter the SWR readings should be pretty close. Where accuracy enters the picture is the forward power indicated. If the meter reads 100 watts is the actual power 90 watts, 100 watts, or 110 watts? For all practical purposes 10% accuracy is good enough. Even the best SWR meters are only about 5%.
A dummy load is used to check the operation of a transmitter and also can be used to check the SWR meter. A good dummy load should show an SWR of about 1.1:1 over its operating range. If your SWR meter suddenly shows an SWR of 5:1 on the dummy load there is a good chance either the meter or the dummy load have gone bad. If your antenna SWR suddenly changes drastically then you check with the dummy load and if that reading is ok something has gone wrong with the antenna system.
Tuning an antenna with a SWR meter is easy with an antenna tuner. You simply tune for lowest SWR (lowest reflected power). Tuning an antenna where you are making an installation means to adjust the length (or coil tap, or capacitor) to give low SWR over the range you intend to operate.
An SWR meter is not a laboratory instrument and for amateur use 10% accuracy is good enough. Using the meter requires a lot of reading of the ARRL Handbook and Antenna Book (and other information sources) and a good deal of hands on use. Learning to diagnose problems in antenna systems from SWR readings requires a good understanding of antenna principles and logical troubleshooting.
Allen
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RE: Asking Elmer's for how to use SWR meters!?
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by W9PMZ on August 9, 2004
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I'm not sure I would worry about the 10% issue. A SWR measurement is a relative measurement and is not an absolute measurement.
So long as the detectors indicate a max forward power and a min reflected power; then you have the minimum SWR possible.
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RE: Asking Elmer's for how to use SWR meters!?
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by KZ1X on August 9, 2004
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I have several different power/SWR meters. The actual operation of the meter is different for each one. For example, one meter requires me to flip a switch, then with a fixed level carrier, set the meter to a known point, then flip the switch back, and read the SWR. Another meter requires me to turn a knob-like modular element 180 degrees, take a measurement with each position, then look up the SWR on a nomograph. Still another meter requires that I adjust a knob to a certain setting, then read two meters ... and yet another meter is actually TWO meters, that read simultaneously!
I have meters that are thermocouple driven on a known load, others that are in-line absorptive, and still others that recify and use part of the RF going through them to drive the meter movements. Other meters sample the RF in a similar way, and then use a series of externally-powered op-amps to drive the meter movements. Some meters work from 1.8 MHz through 30 MHz, and others have different frequency ranges. Some will provide readings at frequencies other than which they are designed, but the readings are inaccurate.
As you can see, it is likely more important to understand what the meter does, how it works, and what you are trying to do with it, than to just describe the steps needed to operate it. Armed with such understanding, one can use ANY meter and interpret the readings to arrive at useful conclusions.
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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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Thanks for all the wonderful comment. And the offer to come over for some hands on! One comment was "If you want to learn more, go to your local library and see if you can find an ARRL Handbook or ARRL antenna book. " And I have to say, I HAVE and their is better write up in the posting here than 5 MAJOR ARRL Handbooks about using these products. I have been refered to ARRL over and over and they are great for a lot of thing but to me fail miserably in this are, the assume you know this stuff, and the instructions books with the equipment are worse!!! I have learned more from the generious ham's here that took the time to answer the question than any of the reference books! Even the Dummy Guide , which is a great book, does a very poor job in this ares.
One posting said that if the meter was explained that that would explain everything. Well let me throw that question out too!
How does an SWR Wattmeter actually work?
How does a Dummy Load actually work?
Thanks again and I look forward to other Ham's comments!
Martin Brossman
KI4CFS@toinquire.com
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RE: Asking Elmer's for how to use SWR meters!?
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by K0RFD on August 9, 2004
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I don't want to hog the thread.
How does an SWR meter work? Depends on the meter.
There's a pretty good explanation of one type of very simple circuit here:
http://engphys.mcmaster.ca/~elmer101/rfpower/swrtheory.html
How does a dummy load work? It's just a big dumb resistor, non-inductive so that the load is 100% resistive. Essentially, it works by converting nearly all of the transmitter's power to heat instead of radiating it as electromagnetic radiation.
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RE: Asking Elmer's for how to use SWR meters!?
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by W4TYU on August 9, 2004
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A meter's accuracy is usually expressed as a percentage of full scale reading (5% FS). This means that the error of any reading will be that ammount in error. e.g. If full scale is 100 and the error is 5%FS then the actual error is 5 units at any reading on the meter. If the reading is 50 the actual error would be +/- 5.
Also,remember that that there is a difference between accuracy and precision. A reading can be precise but not accurate.
Ole man JEAN
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