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434 Adjusting, installing, and troubleshooting antennas and transmission lines
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22-1 VSWR-vs-frequency plot
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22-2 VSWR-vs-frequency plot for three frequencies
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so it makes a very good match to 75- coaxial cable But the actual impedance of a real dipole may vary from a few ohms to more than 100 For example, if your antenna exhibits a feedpoint impedance of (say) 25 , using 75- coaxial cable to feed it produces a VSWR 75/25 3:1 Not too great Measuring the feedpoint impedance is therefore quite important to making the antenna work properly There are any number of instruments on the market that will aid in making antenna measurements Some of them are quite reasonably priced (or can be built), while others are beyond the reach of all but the most ardent and well-endowed en-
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VSWR analyzers 435 thusiasts In this chapter we will look at a newer breed of more universal instrument called the SWR analyzer
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VSWR analyzers
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One basic premise in this chapter is that the instruments used for measuring VSWR must be accessible to people who don t have a commercial or ham operator s license, as well as those who do (we must remember our SWL and scanner friends) Some of the instruments used by hams meet that requirement, but a relatively new breed of instrument called the SWR analyzer provides a lot of capability to the SWL, scanner operator, and ham radio operator alike It uses a low-power RF signal generator and some clever circuitry to measure the VSWR of the antenna One model also measures the feedpoint resistance The MFJ-259 VSWR analyzer (MFJ Enterprises, Inc, PO Box 494, Mississippi State, MS, 39762) is shown in Fig 22-3 This instrument combines a VSWR analyzer with a digital frequency counter, and operates over the range 18 to 170 MHz A band switch is set to the desired band, and then the tune control is set to the desired frequency The meter will then read the VSWR at the design frequency Alternatively, you can adjust the tune control until the minimum VSWR is found This frequency is the actual, versus the desired, resonant frequency of the antenna The front panel of the MFJ-259 has two meters, SWR and RESISTANCE The SWR meter is calibrated up to 3:1, with a little uncalibrated scale to indicate higher SWRs The resistance meter is calibrated from 0 to 500 , which is consistent with the SWR range Two controls on the front panel are tune and frequency (a band switch) The MFJ-259 has a digital frequency meter to measure the operating frequency of the internal oscillator This frequency counter can also be used to measure the frequency of external signal sources (Do not connect the counter to the output of a transmitter; the instrument will be destroyed) The top end of the MFJ-259 has a number of controls and connectors An SO-239 UHF-style coaxial connector is provided for the antenna connection A BNC coaxial connector is provided to apply external signal to the frequency counter, while a push button input switch is available to switch the counter from internal to external signal sources Another push button switch is used to set the gate timing of the counter (a red LED on the front panel blinks every time the gate is triggered) The tuning is from 18 MHz to 174 MHz, while the counter will measure up to 200 MHz The MFJ-259 will work from an external 12-Vdc source, or from an internal battery pack consisting of eight size-AA standard cells MFJ recommends that either alkaline or rechargeable batteries, rather than ordinary zinc-carbon cells, be used in order to reduce the possibility of leakage that can damage the instrument (this is good practice in all battery-powered instruments) Unlike many lesser SWR meters, this instrument is not fooled by antennas that have impedances consisting of both resistance and reactance elements An example in the manual demonstrates an impedance of 25 j25 (ie, R is 25 and reactance X is also 25 ) When connected to a 50- load one might be tempted to think the VSWR is 1:1, and some cheaper meters will so indicate But the actual SWR is 26:1, which is what the MFJ-259 will read
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