We are testing a field strength meter and decided to measure a low
power (100mW) FM transmission using an unused frequency. On the first
test, we used a dipole antenna connected to a balance-unbalance device
before being connected to the field strength meter. The second test we
utilized an FM telescoping antenna.

Using the dipole, when we turned off the transmitter, the reference
field strength reading remained pretty steady. But when we used the
monopole, the reference reading jumped up and down by a few microvolts
even with the FM transmitter off.

What would be an explanation to this?

Thank you!