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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