When a RF transmitter is tuned to a certain frequency, it is not only transmitting on this exact frequency.
It is also producing harmonics, e.g. it is transmitting stray signals on other frequencies.
There are various factors affecting the frequency and the strength of those harmonics.
When the stray signals happen to be produced on the carrier frequency on another transmitter, the reception of this signal can be distorted.
The following video shows a spectrum analysis of 4 parallel video transmissions:
Summary of the video:
- When you place two video transmitter 25 MHz apart on the band, they will produce harmonics in 25Mhz steps up and down the band.
- Additional transmitters on the same band will also produce harmonics in 25 Mhz steps, even if their own frequency different from the first two transmitter is no product of 25MHz.
Take away knowledge bite:
Instead of spreading 4 transmitters in equal intervals across the band, it makes sense to put them in two pairs, on the upper and the lower end of the available band.
The frequency different between the two transmitter in each pair should be as close together as possible to avoid wide-spread inter harmonics, but they should be so far away from each other that direct (adjacent) interference is avoided.
In above example, the frequencies 5740MHz, 5.765MHz were allocated to channels 1 and 2.
5865MHz and 5880MHz were allocated to channel 3 and 4.
The distance of 1 and 2 is 25MHz, and the difference of 3 and 4 is 15Mhz.
15MHz is as close as possible, without creating adjacent interference.
Channel 1 and 2 are 25MHz apart, just outside of the 15MHz interharmonics induced by 3 and 4.
Since Im conducting my experiments in Australia, Im not allowed to transmit outside of 5740 and 5880.
If I would have more bandwidth available, I would move the two transmitter pairs further apart on the band.
If you know more about this topic than me, please contribute to this thread!
If you are testing the above channel allocations in your FPV race, please comment with your experience below.