Thanks for the site, it will be useful.

I have no problem receiving broadcasts from FM station that are within a reasonable distance and having the transmitter on or off makes no difference.
It is set to transmit a signal at 88.1 MHz - there's no station within my listening area broadcasting on or near that frequency.

Since I posted the question I found that mounting the antenna on the tower - not on a mast but on the actual tower - effectively detunes the antenna.
So I suppose the solution is to find a better mounting spot away from the metal of the tower - or get it higher and mount it above the yagi that's on the mast now but that presents a problem with the cable length being overly long.

Thanks for your input and sorry to hear about your crystal ball.