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.

If you understand FM - you will realize that FM has many side bands - hence the center frequency of your whole house transmitter is 88.1 - but it has harmonics on other frequencies on either side of 88.1 - this is how FM works!

All signals reduce at the square of the distance away....

You either need to move your transmit or your receive antenna - have some sort of separation between the two or use a filter.

http://dsp.stackexchange.com/questio...d-in-am-and-fm

Harmonics I understand but I'm not sure how it applies to my problem.
As I said I am able to get FM broadcasts available in my area and I do get a signal at 88.1 from the transmitter in the house. The nearby commercial FM stations are clear no matter if my transmitter is broadcasting or not but when I raise the antenna above the roofline - all bets are off and not all are listenable.

I can get the transmitter's signal (and commercial FM broadcasts) as long as the antenna is on the ground but they are not noiseless. As soon as I raise it, say to the peak of the garage roof, most stations I'm interested in get very noisy to completely unlistenable depending on the antenna's orientation.

Testing, I positioned it lying on the roof and oriented in several different directions. Some positions were good for one favorite station but not the other and none of the positions I tried would receive both the two favorite stations and the transmitter.

When I bought it back down to ground level I could find a spot or two that allowed reception for all frequencies, but again not clearly.

Due, I guess, to the nature of frequency modulation, at some times of the day no amount of re-positioning the antenna would allow getting a clear signal from the more distant commercial stations, only the nearest.

Moving the equipment to another area of the garage made a huge difference so I'm thinking wiring in the garage might have affected the signal path.

Sorry I don't understand what you mean by "...have some separation between the two..." they are now 50' to 75' apart, wouldn't that be sufficient? - and what sort of filter are you referring to?

Frequency Modulation works on the principal of Capture Effect - where it receives the strongest signal and ignores the rest.

http://en.wikipedia.org/wiki/Capture_effect

You have to be able to look at what is in your first and second Fresnel Zone.

http://en.wikipedia.org/wiki/Fresnel_zone

Easier said than done for a newbie.