Keep in mind that while Steve's suggestion may improve your coverage, it also voids the Part 15 certification for the transmitter and will possibly place your transmitter out of compliance with Part 15.239 limits.
Part 15 rules, were originally intended for experimenting and home builders; part 15 FM rules specify a signal level at a set distance under laboratory conditions. They do not dictate the materials the antenna can be made of; or the power output of the transmitter. Most homes built today, are heavily shielded structures. Depending on the materials; greatly determines how much signal is allowed to escape or enter the building.
Part 15 rules do not require a homemade transmitters be certified to be operated legally; otherwise home transmitter builders and experimenters would not allowed to broadcast with their creations. The FCC only requires assembled transmitters sold in the USA; be certified.
There are no rules on antennas period. If the transmitter and antenna of my choosing; was used in an RF shielded structure where reception outside was impossible even at 30 watts; it would still comply with the part 15 rules. The measurement would be 250ft past the private structure; not 250ft from the transmitter. If I have a 1000 unit apartment complex on 40 acres; an the signal can't be heard 250ft beyond the property line; then it is compliant under part 15 rules. We still have private property rights here in the USA! There are plenty of private apartment and condo complexes here in the west that are spread out on large private acreage of 40-100 acres or more. I have learned through various means to keep the signals within the property line of the complexes; we even take steps to keep the signal from radiating vertically.
C.Crane transmitters may be certified, yet their power outputs are all over the place. I have had C.Cranes fresh out of the box, very in range from 20ft to 700ft; without ever cracking the cover open. Does that mean each and every certified C.Crane is legal to use out of the box? Depends where you use it; if I use it on a 40 acre ranch or apartment complex and the signal stays within the property line, it would be safe to use. If I used a poorly adjusted C.Crane in a residential development where the agent could measure the signal out on the sidewalk and it exceeded the limit; a NOUO may be forth coming. In other words; just because the transmitter is certified; does not mean you are protected from a NOUO or NAL. In reality; a certification, only means the transmitter can be legally sold in the USA; it does not necessarily mean it is safe and legal to operate.
The insistence that only certified transmitters are allowed to be discussed here; could leave the end user the false impression that all certified transmitters are safe and legal to use; that is simple not the case.
My home brewed FM transmitter with a 1/2" copper pipe antenna; could be completely legal to use. While the certified C.Crane could be illegal. It all depends on how the end user operates the device and is solely responsible for its usage.
Since experimentation is discouraged and free discussion of hobby broadcasting is not allowed; a name change maybe in order. Perhaps to "Certified Transmitters Forum"; would better describe the site and its stated purpose. Hobby Broadcaster implies its a forum for all things related to hobby broadcasting; which to me, would include experimenters, testing new ideals on antenna and transmitter design. The part 15 AM & Part 15FM in smaller type suggested it is included in the discussion. Not just confined to manufactured certified transmitters. That leaves a lot of great transmitter kits and part suppliers out of the loop
The part 15 AM & Part 15FM in smaller type suggested it is included in the discussion, but not exclusively and not just confined to manufactured certified transmitters. That leaves a lot of great transmitter kits and part suppliers like Sstran, EDM, Ramsey, and Carl's custom AM antennas out of the discussion. Each and everyone of those products can legally be operated under part 15 rules when properly adjusted. Sstran, EDM, Ramsey products are all great for teaching electronic assembly, transmitter construction, engineering and operation. There is not much to learn from or talk about a manufactured transmitter; except how to plug it in and adjusting audio levels. Everything else is pretty much going to be predictable; with the exception of the price and quality of construction.
this information removed as it may result in a violation of Part 15 regulations
Fact is turning on a C.Crane, Decade or Rangemaster may do the same thing.