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Third Order Intermodulation (3IM) -
What is it?

What can it do to me?

by Andy Low


3IM is the "stray image signal" produced when the radio frequency (RF) signals from 2 transmitters combine in the input stage of a receiver. Basically, 3IM is produced by the mixing of the 2nd harmonic (or overtone ) of one Tx signal with the fundamental (1st. harmonic) of the 2nd Tx.

When any two RF signals "mix" or combine, they produce two "stray" frequencies, namely a "sum" frequency and a "difference" frequency. This is not restricted to RC, it is a physical fact of nature.

Now, using an example, with RC38 and RC39, 72.550MHz and 72.570MHz.

The first 3IM product comes from 2 x RC38 and RC39...

"Sum frequency"

2 x 72.550 + 72.570 = 217.67MHz (too high for the RC Rx's to worry about)

"Difference frequency":

2 x 72.550 - 72.570 = 72.530MHz (argh! This is RC37!!)

The 2nd. 3IM product comes from RC38 and 2 x RC39...

(In general, we can safely ignore the "sum" freq), so...

2 x 72.570 - 72.550 = 72.590MHz (aieee... this is RC40!!)

So... your 2 Tx's, operated simultaneously and close together, potentially generate "strays", in our example, at RC37 and RC40. These are your 3IM products.

In general, any two RC frequencies will produce two 3IM strays, one above the higher of the two, and one below the lower of the two, spaced apart by the difference between the two.

What a mouthful! In simple terms take the two RC channels, subtract the channel numbers to find the difference. Add it to the upper channel, thats 3IM #1. Subtract it from the lower channel, thats 3IM #2.

Using another example, RC40 and RC56 Difference = 56 - 40 = 16. Your 3IM strays will be at RC24 (40 - 16) and RC72 (56 + 16). Of course, RC72 doesn't exist, being outside our band, so in this case, only RC24 is in jeopardy.

Notice that it did not matter whether you are PCM, FM or AM... 3IM's are produced by the radio carrier frequencies, not the modulation method!

Why does it matter that the Tx's are "close together" and "strong"? When Tx's are in close proximity and at high output levels (measured at the Rx), the Rx input stage is driven into non-linearity, which causes the 2nd. and higher harmonics of the Tx signals to be produced, which can mix like we calculated. Also, Tx's in close physical proximity can mix their signals inside each other, re-transmitting the mixed 3IM frequency! Whoa!

Note, however, while 3IM strays can cause shoot-downs, IT DOESN'T ALWAYS... the conditions have to be "just right". Tx's have to be physically in close proximity, the Rx's have to be "close enough" to the offending Tx's, etc...

Simple precautions such as keeping Tx's separated per AMA recommendations (minimum 10-foot separation), and using a good-quality Rx greatly reduce or virtually eliminate the 3IM hazard.

What does this mean in practice?

Any 2 Txs potentially produces 2 stray channels. On a flight line with 6 radios running, you potentially can generate as many as 30 distinct stray signals!

Imagine what can possibly happen at a big meet!

Consider what mayhem can possibly be generated as a pilot walks behind the flight stations to get out to his assigned station, along a walkway less than 5 feet away from active Txs, generating strays whenever he gets close a pilot thats flying!

Worse yet, whats happening in the pits, and startup areas with fully extended antennas on Txs less than 5 feet apart?

Recommendations:

  • Always, but always observe the 10-foot minimum spacing rule, for the sake of your fellow pilots.
  • Set up a walkway behind the flight stations at least 10 feet away from the active pilots. Dont loiter behind your buddies-in-flight with your Txs active in close proximity.
  • Keep at least 10 feet away from other Txs in the pits and startup areas.
  • If at all possible, keep your Tx antenna collapsed or extended only one section while in the pits or working on your airplane.

Be a good fly-in citizen, dont be the cause of someone else getting hit!

Third Order Intermodulation (3IM)
De-Mystiflying NiCd's Part 1
De-Mystiflying NiCd's Part 2
De-Mystiflying NiCd's Part 3
Installing RC Equipment in Larger Aircraft pdf file (250kB)
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