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Intermodulation
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Intermodulation
Origin of Intermodulation Products
if two frequencies f1 and f2 pass through a non linear element (semi conductoreffect), harmonics of the original frequencies are created (2f1,3f2,...)
combinations of these harmonics result in a frequency spectrum calledintermodulation products
IM products occur in even and odd orders
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Intermodulation
IM product of even order (e.g. f1 + f2 or 2f2 - 2f1) :
uncritical because of
1. the big distance to original frequencies2. signals used in mobile communication show no static component. Products of
even orders will not occur.
2f2 - 2f1 f1 + f2
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IM products of odd orders (e.g. 2f2 - f1 or 4f1 - 3f2)
dangerous due to their closeness to the original frequencies
IM products falling into the Rx band, cannot be suppressed by filters andmay cause interferences with operating frequencies
Intermodulation
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Intermodulation
Example : GSM 900 Tx - Band Rx - Band935 960 MHz 890 915 MHz
Intermodulation products f1 f2 fIM
3rd Order 2f1 - f2 936 MHz 958 MHz 914 MHz5th Order 3f1 - 2f2 938 MHz 956 MHz 902 MHz
effects of intermodulation
- blocking of Rx frequencies
- overloading the system by creating non-existing subscribers
- higher amount of drop calls, higher bit failure rate
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GSM standard specification :
Level of 3rd order IM product below carrier : - 103 dBm
Ref. to two carriers of 20 W (43 dBm) each : - 146 dBc (103 dBm + 43 dBm)
Kathrein antennas provide a minimum intermodulation attenuation of the
3rd order product of -150 dBc (2 x 20 W carriers)
A measurement of 2 carriers for 3rd order is the worst case
Intermodulation
carrier IM - productComparison
0 dBm - 150 dBmAvera ed distance earth - sun 150 Mill. kilometerquivalent distance 0,15 mm
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Final test range for dualband antennas in an
anechoic chamber :
100 % test of
- VSWR
- Isolation
- Intermodulation min. -150 dBc
at 900 MHz
1800 MHz
UMTS
Must be measured because
IM value also depends on the frequency where measured
(Important for dual- and multiband antennas)
Xpol Antennas / Intermodulation
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How to avoid intermodulation products in passive devices like antennas, cables orconnectors ?
pure materials such as copper, brass or aluminum are recommended- mixed materials like stainless steel are forbidden
no material combination with a high chemical electrical potential
- a thin corrosion layer between the materials act like a semi conductor
contacts have to be well defined and fixed- no rivets or unlocked screws
cable connections should be soldered
External creation : Rusty bolt effect
Intermodulation