Notes on the use of RF1CHAR Dan Maguire AC6LA email: bagardn@ibm.net RF1CHAR will lead you through the steps necessary to use the Autek RF-1 to measure various characteristics of a piece of transmission line. The data calculated includes velocity factor, characteristic impedance (resistive component), and matched line dB loss. The dB loss numbers are normalized to dB per 100 feet at 10 MHz for easy comparison with published sources, as well as being given for the length of line under test at various ham band frequencies. The line to be measured may be coax or balanced line, although there may be some inaccuracies introduced when using balanced line because of the stray reactance of the coax connector on the RF-1. To use the program you have to know the physical length of the line. Measure it if possible with a tape, but if that isn't practical just use your best guess. The velocity factor and dB per hundred feet calculations will be off by whatever percentage your guess was off, but loss per length will still be accurate. The longer the line being measured the better the accuracy of the results. About 30 feet is the minimum you can use and still get good numbers. You have to be able to terminate the far end of the line with either a short or open circuit. If the far end already has a PL-259 plug, you might want to make up a matching SO-239 with a short piece of heavy wire between the center pin and the shell for a short circuit. For an open circuit you can just leave the PL-259 in place but disconnected, perhaps with the outer barrel unscrewed and slid back by a few inches. Supposedly an open PL-259 will make the transmission line appear slightly longer electrically because of the end effect of the open pin, but I have not been able to verify that with the accuracy of measurement provided by the RF-1. In addition to a short or open circuit termination you'll also be asked to use a termination of approximately the line Zo, such as a 50 ohm or 75 ohm resistor for the various standard types of coax. This is optional, used only if you are not sure about the Zo of the line in question. Running the program You'll first be prompted for the length of the line (in feet) and the type of termination (short or open circuit). Following that is a prompt for the approximate velocity factor. Although velocity factor is one of the items to be calculated, if you have at least a good guess then the program can do some number crunching to save you a bit of time in the next step. For regular coax use .66, for foam core coax use .78, and for ladder line use .95. If you don't know what kind of coax you have, don't enter anything (just press the Enter key). Next you slowly increase the frequency of the RF-1 until the Z (ohms) reading peaks or goes off scale (reads 'H'). If you entered an estimate for the velocity factor the program will calculate approximately what that frequency will be. If not, you'll have to start at the lowest RF-1 frequency (about 1.2 MHz) and slowly work your way up. As you progress up in frequency the Z reading may at first decrease to a low point (at or near zero) before starting up. Enter the frequency at which Z peaked for the first prompt followed by the actual Z reading for the second prompt. (You'll definitely need to develop a "touch" for the Tune and Fine knobs on the RF-1. The peaks will be very sharp, but that adds to the accuracy of the measurements.) For the first peak the Z value will probably jump around a bit. Just enter what you think is the average reading. Then slowly increase the frequency. The Z reading will fall to a low value and then build back up. When it reaches the next peak again enter the frequency and Z value. The program will calculate the approximate frequency of the next peak, but this is just a guideline to help you avoid inadvertently skipping over a peak frequency. Repeat this process until you've taken five readings or the frequency can no longer be increased (about 35 MHz). Next you'll be asked to change the termination to the approximate line Zo (eg 50, 75, or 450 ohms, not particularly critical) and take a Z reading at two distinct frequencies that the program indicates. If this is not possible (or if you want to override the program calculations), just enter your best guess for the line Zo for both of the next two prompts. Note that if you -do- change the termination and take measurements, the two measured values will -not- be the same. This is as expected. Any error in your guess for Zo (or any small error due to RF-1 inaccuracies) will have a small effect on all the computed dB values. For example, if you entered '50' for both prompts but the "true" line Zo is 52 ohms then all dB values will be 4% low. The next screen will show all the calculations. A few items to note are: Velocity factor (VF) is computed for each odd (short circuit) or even (open circuit) quarter wave point. You'll probably see the number creeping up for each higher frequency. This may be due to stray reactance in the termination and/or stray reactance in the RF-1, but I can't give a good explanation for it. Note that in many cases it is not necessary to know an exact velocity factor number. If you know that a line is a certain electrical length (wavelengths or degrees) at a given frequency, the electrical length at a different frequency (or the frequency for a different electrical length) is a simple ratio. The program shows you the electrical length in wavelengths (WL) for each measurement frequency. As mentioned above, the velocity factor numbers and the dB per 100 feet values will only be as accurate as the length (and line Zo) numbers used in the calculations. If your guess was 10% off, these numbers will be 10% off. However, the dB per ham band numbers for the given length will be accurate (as long as the Zo was approximately correct.) The dB numbers shown are the "matched line" figures. Any SWR greater than 1.0 which might be present on the line in normal use will make the actual loss figures somewhat higher. You can use the ZIZL program (or other programs) to see what the actual loss would be with a given load for a given frequency. Also note that accuracy to three decimal places is not really supported by the accuracy of measurements taken with the RF-1. Three decimals are included to let you see any trends in the numbers (perhaps when running theoretical test cases), but don't get hung up on thousandths or even hundredths of a dB. The final item to note is the column labeled 'sigma'. Sigma is the exponent used to interpolate a given dB loss figure at one frequency to another frequency. If you look at a published graph of transmission line losses, sigma has to do with the slope of the line as frequency changes. The formula is: dB at F2 = dB at F1 * (F2 / F1) ^ sigma Rearranging to solve for sigma gives: sigma = LOG(dB at F2 / dB at F1) / (LOG(F2 / F1) The program will calculate sigma values based on the current dB loss number and frequency as compared to the previous half wave point values. The calculated sigma values should in theory all be the same. Because there is some variation in the numbers, the program uses a fixed value of .56 to calculate the loss at 10 MHz and at the various ham band frequencies. This is a good estimate for the average of published sigma values for common types of coax (RG-8X, RG-58, RG-213). However, the loss numbers at 10 MHz and at the various ham band frequencies will change slightly depending on the sigma used for calculations. You'll probably want to look at the average number calculated (perhaps throwing out any obvious measurement errors) and then use that number to re-calculate. The final prompt at the bottom of the screen is just for that purpose. Sample measurements If you want to see the program in action but you haven't taken any of your own measurements, here are the numbers I got from one of my test cases. These are for a piece of Radio Shack RG-58 that I happened to have. Length, feet: 38.2 Termination: open Frequency Z at peak 8.310 1060 16.69 724 25.11 570 33.51 480 Z at 4.155 MHz: 56 Z at 8.310 MHz: 51 After initial results are shown, recalculate using .579 as sigma. Feedback If you have any thoughts on what causes the velocity factor "creep" or why the calculated sigma values are not very consistent (these may be related) I'd like to hear from you. It can't all be explained by just saying that there are inaccuracies in the RF-1 measurements. Or to put it another way, if the RF-1 numbers are inaccurate, why are they? I'd like to gain a better understanding of how any strays in the RF-1 are affecting the measurements. I've written some software to make corrections for the Z reading of the RF-1 under various conditions (which I'll gladly share), but the results are inconclusive. 73 Dan