Dumb Questions about Dummy Loads & SWR Meters
By Martin Brossman - KI4CFS
People said I was a Ham all my life and in fall 2003 I became one. Loving the hobby I wanted to really learn how things work and especially how to test and trouble shoot my own problems. I read in 4 books the importance of an SWR Meter and a Dummy load but the paragraph stopped their. It looked like they cut and pasted the paragraph about these two devices from one book to the next. So I started searching the web for answers on just how do you use SWR Meters & Dummy loads. I found out how you can build them, the problems with low price ones, which models were used to test which equipment but very little on how to really use them. I was clear I was not going to spend $1100.00 on a meter but when I asked which were good ones they referenced the real expensive ones. So I explored the wisdom of Ham's on eham.net in required to using SWR Meters & Dummy loads that were under $200.00 and here is what I learned. I found this very useful and wanted to thank all the ham's that contributed. You will see some redundancy in this article and that is because I am offering different Ham's perspective on the same topic. Sometimes I need to read something in a slightly different way to understand it. May you find these questions and answers helpful to you also!
Question: What is an SWR meter & a Dummy Load AND how do they work?
KA5N: 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.
K0RFD: How does an SWR meter work? Depends on the meter. There's a pretty good explanation of one type of very simple circuit here:
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.
N7DM 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'..
Question: How do you use SWR meters with a 10% error, accurately?
K5LXP: 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.
K0RFD: 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.
W4TYU: 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.
Question: How do you use an SWR meter to tune an antenna?
K5LXP: 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.
K0RFD: 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.
Question: What is a dummy load used for?
K0RFD: 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.
Question: How do you use the SWR Meter with a dummy load?
K5LXP: 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.
N8UZE: Dummy loads can be used for several purposes. One of these is to tune the old tube type radios so that their output impedance is 50 ohms. On the moder non-tube radios, this is not needed. Another use of the dummy load that does apply to modern radios is adjusting the mike gain. Instead of doing this on the air, you can do it transmitting into the dummy load.
Question: How do you use the SWR Meter / Dummy Load, to determine if your feed line is good or bad?
K5LXP: 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.
K0RFD: 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.
Question: How can you determine if the SWR meter is accurate "enough"?
K5LXP: 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.
K0RFD: 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.
Question: How can you determine if the 20W range is accurate compared to the 200W range?
K5LXP: 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?
Question: How do SWR meters tend to go bad, how would you know it, if it did?
K5LXP: 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.
K0RFD: 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.
Question: How do you use and SWR Meter, what are the different ways?
K5LXP: 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.
Question: What is the relationship between SWR and Field Strength (radiating energy)?
N7DM: There is no direct relationship between SWR and Field Strength. Field strength basically is a direct function of 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.
KI4CFS: SWR does NOT tell you how much radiating energy is coming from your antenna.
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. Without 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.
Question: What is the difference between the Peak Envelope Power (PNP) and Average Power?
N8UZE: The study guide "Now You're Talking" has a fairly thorough explanation. When you were originally studying, there was so much to learn that this probably didn't sink in. Go back and reread it now. However basically as you talk into a microphone on sideband, the amount of power output will vary as you talk. Most meters will give you an Average Power reading. The reading on this type of meter will be and should be less than the max capability of your radio. If it reads full power then you are overdriving the radio and splattering all over the band. If you have a meter that reads Peak Envelope Power instead, it will jump up to max at the points where your voice is max. On FM, AM, and CW both types of meters will give the same reading as it will read the carrier for these modes.
Question: Are their other test tools worth having or having access to (barrowing)?
KI4CFS: From ham's I talked to on the radio the other test tools that seem worth having are a volt-ohm meter, an antenna analyzer, a field strength meter, and a frequency counter. An antenna analyzer hooks directly to the antenna and lets you know information about your antenna with out having to use or endanger your radio. The field strength meter lets you know that RF is radiating out of your antenna and the relative strength of it. The frequency counter tells you the frequency that is dominate in its environment (which would be YOUR antenna if it was radiating well). Last the volt-ohm meter is used to measure AC & DC voltage and DC resistance.
I want to thank all the good natured ham's that have contributed to this resource. My intention is that others find this useful as well. This is a wonderful hobby where both mentoring and community support are richly alive! I truly feel more knowledgeable with my SWR meter from the comments I received and have used them more.
Author: Martin Brossman, KI4CFS is a TRUE Ham in many ways and as a profession is a Executive & Life Coach ( www.CoachingSupport.com ) . He can be reached at: KI4CFS@toinquire.com , (919) 847-4757 and his Ham resource page is: www.toinquire.com/ham