Post History

It sounds like you are doing the measurements right.  However, I expect the problem is in the space around the antenna.

At 434 MHz, the wavelength is 690 mm or 27 inches.  Everything out to about a meter or 1½ m should be considered near field, and can effect the antenna directly.  This includes whatever table it is sitting on.  Even with a table that's transparent to RF, it needs to be a meter or more above the floor.  Usually dry light wood is a good material for such a table.

Even then the surroundings can still matter.  If there is something near by, like a flat piece of metal, that reflects the radio waves, it can still matter even if it's out of the near field.  This is because the reflection has a fixed phase offset from the transmitted signal.

If you have access to an anechoic chamber, try your tests there to see if they become more repeatable.  If so, then you know your environment is the problem.

If you don't have access to an anechoic chamber, try an open field test.  See if you can suspend the antenna several meters above the ground.  If possible, put some RF-absorbing material on the ground below the antenna.  Failing that, move the antenna up higher so that the reflection off the ground is more attenuated.

Local transmission, especially at ISM frequencies, are probably not much of an issue.  The signal you are feeding the antenna should be many many times larger than anything the antenna might pick up from elsewhere.