Checking Antenna Systems WB5CXC
Energy Transfer • You get maximum energy transferred when all impedances are the same. • Normally the two items that are fixed are the transmitter (50 ohms) and the coax (comes in different impedances – normally 50 ohms) • The antenna will change impedance with a frequency change. Can be complex impedance (resistance and reactance).
Reflected Power • If the end of the transmission line is Open or Shorted – no power is dissipated by the load (there isn’t any load). • All power will be reflected back. • The transmission line will absorb some due to the loss in the line. • We usually measure the reflected power and refer to it as Standing Wave Ratio (SWR).
(Positive Number) Standing Wave Ratio (SWR) - Formulas
10% Reflected Power Impedance Ration 2:1 Modern Transceiver SWR Limitation – SWR < 2 (Less than 10% power reflected OR 25 – 100 ohms)
Equipment for Measuring SWR • SWR Meter • Need to measure Forward & Reflected Power & not just SWR • Bird Wattmeter (need proper slug) • Antenna Analyzer • Other Specialized equipment • Each type has their place and use. You will probably end up with several of these.
SWR Meters • Older types - you have to set the forward reading to a calibration setpoint, then read the SWR • Newer types have double needle meter showing Foreword & Reflected Power (usually different scales), read SWR on another scale. • Automatic double needle meters and SWR meters.
Notice the different scales – Forward power 300 watts, reflected 50 watts
Bird Wattmeter • Power Meter that has slugs (Frequency & Power). Turning the slugs enables it to read Foreword & Reflected Power. • You compute SWR. • Bird Wattmeter was the standard for commercial radio departments for years. • Requires different slugs for the different frequency & power (i.e. 300 – 400 Mhz 10w)
Bird 43 Wattmeter Bird 43 Wattmeter Slugs Notice the arrow. Notice the direction arrow.
Antenna Analyzer • They use a low power signal generator. Signal is sent to the cable and the analyzer computes the values of R, Z, SWR. • By adjusting the analyzer frequency you can tell how far off resonance you are, and if the antenna needs to be shorter or longer.
RF-1 Analyzer MFJ Analyzer
Fixed Station Antenna Analyzer • Computer connected antenna analyzer that do many things and are laboratory grade instrumentation. Standalone for limited indications. • They are a little expensive but within the price of amateur radio.
Coupler Controller LP-100A Antenna Analyzer
Problems with High SWR • Most modern transistor transceivers now have protection against high SWR. They will limit the output so the finals are protected. Most transceivers start limiting the output at SWR of 2. • High SWR reduces the amount of energy that is radiated from the antenna.
Practical Problem • You are measuring a 440 Mhz repeater, it has 475 feet of 7/8” foam coax. • Bird Wattmeter read 45 watts forward and 4.49 watts reflected. Good or Bad
Analysis of Problem • The antenna lead has snapped off at the antenna. • 100% of signal is reflected • The line has a loss of 1.02 dB/100 feet. • Total loss of the line is 1.02 dB X 4.75 = 4.98 dB (5 dB). • What is the reflected power at the top of the line?
Analysis of Problem cont’d • 5 dB = 3.16 loss (approximately 1/3 of the power at the top) • 45 watts * 1/3.16 = 14.22 watts • This is reflected back down the line. • 14.22 * 1/3.16 = 4.49 watts (loss back down) • Line is BAD (open at the antenna) • Can’t just look at the SWR !!!! SWR = 1.92
Real Story • An Ham bought an antenna and he had an high SWR he tried to adjust it but couldn’t get it to come down within SWR < 2. • Wrote to manufacturer and they said you need at least 75’ of RG-58 and then the antenna will be good. • Add more loss to system makes the antenna have less SWR.
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