checking antenna systems
DESCRIPTION
Checking Antenna Systems. WB5CXC. Ideal System – everything matched at 50 ohms. Energy Transfer. You get maximum energy transferred when all impedances are the same. - PowerPoint PPT PresentationTRANSCRIPT
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Checking Antenna Systems
WB5CXC
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7.035 50 ohms Coax
Antenna50 ohms
Ideal System – everything matched at 50 ohms
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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).
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7.035 50 ohms Coax
No Antenna – all power is reflected back
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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).
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SWR =ZO
ZLOR
ZO
ZL
SWR =PRPF +
PRPF -
Standing Wave Ratio (SWR) - Formulas
(Positive Number)
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SWR =PR PF +
PR PF -
+
-=
100 10
100 10
=10 + 3.16
10 - 3.16= 1.92
SWR =50
25= 2.0
Ω
Ω
SWR =100
50= 2.0
Ω
Ω
Modern Transceiver SWR Limitation – SWR < 2(Less than 10% power reflected OR 25 – 100 ohms)
10% Reflected Power
Impedance Ration 2:1
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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.
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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.
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Notice the different scales – Forward power 300 watts, reflected 50 watts
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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)
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Bird 43 Wattmeter
Bird 43 Wattmeter SlugsNotice the arrow.
Notice the direction arrow.
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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.
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RF-1 Analyzer
MFJ Analyzer
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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.
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LP-100A Antenna Analyzer
Controller Coupler
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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.
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SWR READING % OF LOSS ERP*WATTS
AVAILABLE1.0:1 0.00% 100.00% 1001.1:1 0.20% 99.80% 99.81.2:1 0.80% 99.20% 99.21.3:1 1.70% 98.30% 98.31.4:1 2.80% 97.20% 97.21.5:1 4.00% 96.00% 961.6:1 5.30% 94.70% 94.71.7:1 6.70% 93.30% 93.31.8:1 8.20% 91.80% 91.82.0:1 11.10% 88.90% 88.92.2:1 14.10% 85.90% 85.92.4:1 17.00% 83.00% 832.6:1 19.80% 80.20% 80.23.0:1 25.00% 75.00% 754.0:1 36.00% 64.00% 645.0:1 44.40% 55.60% 55.66.0:1 51.00% 49.00% 497.0:1 56.30% 43.80% 43.8
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Top of Tower - 450' AGL440 Mhz 10dB Omni
Antenna
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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
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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?
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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
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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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