Evaluation of Antenna Tuners and Baluns–An Update
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Evaluation of Antenna Tuners and Baluns–An Update How to have high confidence in your measurements. By Frank Witt, AI1H n a two-part article in April and May 1995 QST,1 I lossy when they got very hot or when a component failed described a simple method for evaluating antenna (if running high power). QRPers had no way of knowing Ituners. Application to the evaluation of baluns was de- whether or not their antenna tuners handicapped them. scribed the following year.2 The method involves a resis- Some manufacturers’ claims for their antenna tuners were tance-load box and a low-power analyzer, and it works (and still are) unreliable, and that is being kind. Much equally well for evaluating the performance of equipment light was shed on this matter by the programs written by with balanced as well as unbalanced loads. A simple ex- Dean Straw, N6BV, TLA and its successor TLW,4 which tension was described that allows the equipment evalua- compute antenna-tuner performance. The indirect method tion with complex-impedance loads as well. complements this analysis tool by providing a very acces- Amateurs around the world have since used the method, sible measurement tool. which has been dubbed the “indirect method,” the “AI1H From the source in Note 1 (May 1995, page 37): “This method” and the “Witt method.” It provides—at moderate new application of low-power SWR testers is a demanding cost—a simple means for evaluating antenna tuners and one, since the accuracy must be excellent for valid results. baluns. The method was used to evaluate four antenna tun- Perhaps we will see even more accurate SWR testers in the ers for a Product Review in March 1997 QST.3 future, and maybe antenna tuner manufacturers will be Most hams had been ignorant about the performance inspired to improve their designs.” That day has arrived. of their antenna tuners. They only knew the circuits were Improved SWR analyzers and antenna tuners5 are now available. 1Notes appear on page 14. The purpose of this article is to show how measurement instruments that are now available provide improved ac- ARRL Technical Advisor curacy for the simple characterization of antenna tuners 41 Glenwood Rd and baluns. It summarizes a better understanding of any Andover, MA 01810-6250 inherent limitations of the evaluation method. Finally, it [email protected] provides a comparison with other measurement methods. Sept/Oct 2003 3 SWR, Reflection-Coefficient Magnitude and that measures SWR, reflection coefficient magnitude, |ρ|, Return Loss or return loss, RL, in decibels. The measurement is car- Before getting into the subject of improving the measure- ried out as follows: ment accuracy, it is appropriate to discuss the quantities to 1. Set the geometric resistance box to the desired load be measured. The relationship between reflection coefficient, resistance, RL. ρ ρ, the impedance of the device to be tested, Z , and the refer- 2. Adjust the antenna tuner so that SWR = 1:1, | | = 0 L or RL is maximized. ence resistance of the analyzer, RREF, is as follows: 3. Switch to the next lower load resistance, RL/2, and record the SWR, S , or |ρ | or RL . ZL − RREF (Eq 1) 1 1 1 ρ = 4. Switch to the next higher load resistance, 2RL, and ZL + RREF ρ record the SWR, S2, or | 2| or RL2. The value of R for the analyzers we consider here is 5. Calculate the antenna-tuner loss, L, in decibels and REF percentage of power lost, P , from: around 50 Ω. For our purposes in this application, SWR, LOST reflection-coefficient magnitude (|ρ|) and return loss (RL, in decibels) are equivalent in the sense that we can mea- ()()S1 +1 S2 +1 RL1 + RL2 L=5log = −5log()9 ρ1 ρ2 = − 4.77 dB sure any of them and find the antenna-tuner loss. The re- 9()()S1 −1 S2 −1 4 lationships between them are: (Eq 5) 1+ ρ (Eq 2) L SWR = − 1− ρ P = 1001−10 10 LOST SWR −1 (Eq 3) ρ = ()()S −1 S −1 SWR +1 = 1001− 3 1 2 (Eq 4) (Eq 6) RL = −20log ρ ()()S1 +1 S2 +1 ρ For a discussion of the nature of SWR, | | and return = 100()1− 3 ρ1 ρ2 loss and how SWR is sloppily used, see my discussion of RL +RL “SWR Bandwidth.”6 − 1 2 = 1001− 3⋅10 40 I’ve included these relations because some instruments measure |ρ| more accurately than SWR, whereas others might measure return loss more accurately. So, to achieve the highest accuracy, the correct parameter must be used. These measurements are carried out the same way for unbalanced and balanced loads. The Indirect Method The indirect method involves connecting a resistance load Antenna-Tuner Loss in Decibels versus box with switchable resistors to the output terminals of the Percentage of Power Lost antenna tuner. The load box has been named the “geometric The loss of an antenna tuner is presented above in two resistance box,” because the values of resistance follow a ways: as power loss, L, in decibels, and percentage of power geometric progression. For each load, the resistances of the lost, PLOST. PLOST has an advantage in the antenna-tuner adjacent loads are twice and half the load resistance. loss application. We all know approximately how much The input of the antenna tuner is connected to a meter power our transmitter delivers when it is feeding a 50-Ω Fig 1—In A: Measured SWR for resistive loads using a calibrated MFJ-259B. The circles are for R > 50 Ω and the squares are for R < Ω Ω L Ω L 50 . In B: Error in percentage of power lost. The solid trace is for RL > 50 and the dashed trace is for RL < 50 . These measurements apply over the entire HF band. 4 Sept/Oct 2003 load. What we want to know is how much of that power is esting to notice from Eq 6 that percentage of power lost is being absorbed or radiated by the antenna tuner and not linearly related to the geometric mean of the reflection- reaching the antenna system. Percentage of power lost tells coefficient magnitude readings. us this directly. For example, a kilowatt transmitter feed- ing an antenna tuner with a 20% of power lost figure means Accuracy of the Analyzers that 200 W are lost in the antenna tuner. Most of that power is usually heating components in the tuner. MFJ-259B On the other hand, if the loss is expressed in decibels, When I first discovered the indirect method, I used low- we must make a mental translation to percentage of power power SWR testers. These were the Autek Research Model lost in order to know what is happening. Some familiar RA1 and the MFJ Model MFJ-259. These units displayed decibel-loss quantities are 0, 1, 3 and 10 dB, which equate SWR, so the SWR reading was used for loss calculations to 0%, 21%, 50% and 90% of the power lost in the antenna using Eqs 5 and 6. These instruments do not display |ρ|, tuner, respectively. Other quantities of decibel loss are far although it is the basic quantity measured. SWR is calcu- less familiar to most of us. So, the preferred loss-charac- lated internally from the value of |ρ|. This calculation is terization method is percentage of power lost. It is inter- a source of error. Fortunately, some newer instruments Fig 2 — In A: Measured reflection-coefficient magnitude for resistive loads using a calibrated MFJ-259B. The circles are for R > 50 Ω Ω Ω L and the squares are for RL < 50 . In B: Error in percentage of power lost. The solid trace is for RL > 50 and the dashed trace is for RL < 50 Ω. These measurements apply over the entire HF band. Fig 3 — In A: Measured return loss for resistive loads using a calibrated MFJ-259B. The circles are for R > 50 Ω and the squares are Ω Ω L Ω for RL < 50 . In B: Error in percentage of power lost. The solid trace is for RL > 50 and the dashed trace is for RL < 50 . These measurements apply over the entire HF band. Sept/Oct 2003 5 display |ρ| and return loss directly. Further, SWR, |ρ| The error for return loss is around 8%, however. These dif- and return loss are displayed on a LCD, which is easier to ferences arise from deficiencies in the algorithm that con- read reliably than an analog meter. An instrument for verts the basic |ρ| measurement into return loss and the measuring the loss of antenna tuners is the MFJ-259B, poor resolution of return-loss measurements in part of the the successor to the MFJ-259. Some features of other can- desired region, which is discussed later. didates for this application are discussed later in the ar- For all calculations, it was assumed that the reference ticle, but here we will primarily answer the question of resistance, R , for this particular MFJ-259B is 50.1 Ω. whether or not the MFJ-259B will perform adequately in REF This is the value of RREF that gives the lowest mean-square this application. error (0.77%) for percentage of power losses from 0 to 100%. Although the MFJ-259B impedance analyzer measures R is nominally 50 Ω, but this method of finding the ac- many properties of the unknown connected to its termi- REF nals, we will focus here on the antenna-tuner evaluation tual value provides a better evaluation of the MFJ-259B. application.