"Ron J" wrote in message
oups.com...

Happy New Year!

If I have an FM transmitter in a vehicle, do I need to worry about the
vibration frequency? Is there such a thing as intermodulation from
vibrating objects attached to an FM transmitter?

I'm curious.



Short answers:

Worry - Probably not

Intermodulation - No.





Really long answer...



Not sure what you mean by "vibration frequency".



Here's the logic for what is called "Environmental Desensitization", or
"Environmental Desense" for short...sort of.



The type of transmitter is not important for this discussion.



The antenna produces an RF field around it and the vehicle (the desired
effect since it extends out to infinity).



Any conductor relatively near the vehicle or a conducting part of the
vehicle (call it a parasitic conductor or "PC" for short) will have RF
currents flowing in them as a result of the transmitter (this occurs whether
it is near or not - actually the distance only affects the strength of the
current, but for close spacing, the effect is easier to see & understand).

More simply stated: "Everything in the world is a receiving antenna."



The strength and relative phase of the current in the PC (compared to the
transmitted signal) will depend on many things (distance to main antenna,
shape, size, orientation, frequency, other objects and conductivity)



These PC RF currents will also produce an RF field in the region (around the
PC and vehicle). It is no different than the intentional transmitting
antenna except it is not cut to a desired length and shape, but accidental.



The PC RF field(s) will sum with the field from the transmitting antenna
thus producing a slightly modified field - I'll call the "Sum-Field".

The actual "Sum_Field" amplitude and phase depends on the relative strength
and phase of the two fields at any point and is therefore complex. These
fields sum in a vector addition manner. Sounds all pretty random up to
here...



However, we can say for sure that:



If the current in the PC changes, the resulting "Sum-Field" will change.

If the position of the PC changes, the current in the PC will change.

If the size of the PC changes due to an intermittent connection with another
near-by PC, the current in the PC will change.



And we know that:

If the PC current changes, the resulting PC field will change.



And most certainly

If the PC field changes, the "Sum-Field" will change.



Keeping in mind that:

A receiver near or far away will receive the "Sum-Field" at its location.



This means that if we have a conductor near the transmitter that, due to any
of the above changes, it can "modulate" the field in some way. The
modulation must have some relation to the change causing it. It will indeed
contain the same characteristic that causes the changing current / field.
Therefore, the "modulation" will have the same characteristic as the
original change, and it will most certainly contain that vibration
frequency or the noise which is the intermittent contact.



So, the bottom line is that the vibration (or noisy contact) will modulate
the signal at the receiver. This modulation must most certainly be a
combination of amplitude and phase modulation.



This effect is there and usually small enough to be unnoticeable, therefore
it is of little to no concern most of the time. It can be the cause of
noise in a mobile instillation from things like tail pipe vibration, hood
(bonnet) hinges and latch and the like.



P.S. this same argument explains how directional antennas work, such as the
common Yagi-Uda and all phased arrays whether parasitic or driven (phased).



73, Steve, K9DCI



P.S. The Apollo recovery and communication ships had this problem due to
the High power HF transmitters causing RF current in the Deck railing chains
which caused noise sidebands which extended up into the microwave
frequencies interfering with the radars...believe it or not - don't know if
this is on the web, but I have the NASA papers somewhere.