I found that all the Heliax cables landing on the bottom of 1Y2 were too loose.
Due to this loose connection the RF power at 55 MHz varies from -34 dBm to 3 dBm, depending on the angle of the Heliax's head.
The looseness basically comes from the fact the black plate is too thick for the Heliax cable to go all the way. It permits the Heliax's heads to rotate freely.
What we should do is to make countersinks on the black plate like this: