Considering that the oil film in a well lubricated ball bearing is only
a few microns thick, it doesn't take much voltage to bridge that gap.
However, it is likely there are wear particles and worse rust added to
the mix. Then there is the additional problem of metal to metal contact
and fretting corrosion when the balls don't rotate enough to be
lubricated. Which happens when rotators sit on a heading and move a bit
in the wind. So the noise could be rectification or just arcing.
All lubricated bearings should have a ground lead jumper around them. I
learned this long ago making disk drives and used rotating contacts of
various types. Grounds are also needed in inverter driven motors where
high (RF) frequencies couple to the armature and arc thru the well lubed
bearing to the frame. It kills the bearings. google it.
It might be informative to model a free space mast with a top yagi at
QRO and see what the voltage would be at the rotator end.
Grant KZ1W
On 2/8/2020 03:42, SM0HRP Kari Gustafsson wrote:
Hi,
I have discovered what it seems like rectification in my HD Alu Tower where
on the top there is antenna ball bearing thrust (SKF) below a 61D Prosistel
rotor. I used various techniques to pin poin to the tower.
The harmonics is growl sounding on 20 m (TX on 40 m) and is some -120 dB
below the fundamental (40 m) after double low- and high as filters (giving
some 100 dB of combined out of band suppression) in TX and RX chain.
I have read that some have encountered harmonics rectification in thrust
bearings. Anyone has any experience from ball bearings and rectification? Or
rectification in Prosistel rotors (in the potentiometer?).
Being a physicist I wonder how the mechanism is with this kind of
rectification in bearings and grease which I assume is isolating. Or?
I am close to getting a skylift and do the shortening of the bearing with a
jumper and see if it helps bit it is winter her. Brr.cold.
73s Kari SM0HRP
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