Checking Antenna Systems

WB5CXC

7.035 50 ohms Coax

Antenna50 ohms

Ideal System – everything matched at 50 ohms

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).

7.035 50 ohms Coax

No Antenna – all power is reflected back

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).

SWR =ZO

ZLOR

ZO

ZL

SWR =PRPF +

PRPF -

Standing Wave Ratio (SWR) - Formulas

(Positive Number)

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

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 SlugsNotice 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.

LP-100A Antenna Analyzer

Controller Coupler

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.

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

Top of Tower - 450' AGL440 Mhz 10dB Omni

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.

XXXX

• Some