Checking Antenna Systems
WB5CXC
Antenna
50 ohms
7.035
50 ohms Coax
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).
ZO
SWR =
ZL
OR
ZL
ZO
(Positive Number)
SWR =
PF
+
PR
PF
-
PR
Standing Wave Ratio (SWR) - Formulas
SWR =
PF + PR
PF
-
100 +
10
=
=
100 -
PR
10
10 + 3.16
= 1.92
10 - 3.16
10% Reflected Power
SWR =
50 Ω
25 Ω
= 2.0
Impedance Ration 2:1
SWR =
100 Ω
= 2.0
50 Ω
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.
Controller
Coupler
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.
SWR READING
% OF LOSS
ERP*
1.0:1
1.1:1
1.2:1
1.3:1
1.4:1
1.5:1
1.6:1
1.7:1
1.8:1
2.0:1
2.2:1
2.4:1
2.6:1
3.0:1
4.0:1
5.0:1
6.0:1
7.0:1
0.00%
0.20%
0.80%
1.70%
2.80%
4.00%
5.30%
6.70%
8.20%
11.10%
14.10%
17.00%
19.80%
25.00%
36.00%
44.40%
51.00%
56.30%
100.00%
99.80%
99.20%
98.30%
97.20%
96.00%
94.70%
93.30%
91.80%
88.90%
85.90%
83.00%
80.20%
75.00%
64.00%
55.60%
49.00%
43.80%
WATTS
AVAILABLE
100
99.8
99.2
98.3
97.2
96
94.7
93.3
91.8
88.9
85.9
83
80.2
75
64
55.6
49
43.8
Top of Tower - 450' AGL
440 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