rotor modulation

[harryr]

I am on a program that has a vhf fm (30-88 MHz) antenna
installed on a helicopter on the vertical next to the
tail rotor and am experiencing a rotor modulation effect that causes an uneceptable amount of distortion.
any one have this experience and what can be done?

 

[GraviMan]

I presume you're talking about signal distorsion, and not airflow streamline distorsion?

Sounds like you need to mechanically fix the antenna. I can't believe the velocities to be high enough to cause relativistic modulation of the carrier signal, but have you measured the magnitude of displacement and velocity? A simple test is whether the susceptability to distorsion from different directions alters (hint: less when perpendicular to plane of movement).

Most likely the antenna movement is causing a connection to introduce noise into the signal, so good mechanical fix is the order of the day. You could put an aerodynamic foil on the antenna, or reposition it out of the slip stream. I gather the tail rotor is a puller, so the antenna is in the downwash - maybe a kind word with the dynamics guy to make it a pusher! ;-)

Mart

 

[BrianR]

When the tail rotor blades have a metal core their length is such that they will couple into RF field if the antenna is situated close by - and amplitude modulate the signal. This is a common problem on the military FM band. The best solution is to move the antenna - up front as far from the tail rotor as possible.

If moving it is not possible you could experiment with re-orienting the antenna to put a signal null onto the blades which will reduce the effect.

 

[GraviMan]

Oh yeah, I never thought of that one. Too may wood and composite hours I guess!

Mart