FM Boosters for LPFM
Updated June 23, 2023.
What is an FM booster?
FM Boosters are FM transmission facilities that operate on the same channel as the primary station that it is broadcasting. FM boosters are used to fill in gaps of coverage that are normally created by terrain.
FM boosters require the programming to be fed to it through other means. The optimum method of feeding a booster is microwave. This allows for the best possibility of synchronization between the primary station and the booster transmitter (usually done by delaying at the primary station's transmitter). Using a Barix or other method of internet/Wi-Fi feeding will cause significant delays and make for a very bad listening experience.
Where can I put one?
The 60 dBu protected service contour of the FM booster must remain within the 60 dBu protected service contour of the primary station. This means that the FM booster will use a directional antenna. The intention of an FM booster is not to extend the coverage of a station but instead to fill in a gap in coverage in a service (contour) area that you are already entitled to. In the full-power world, FM boosters are permitted to operate an ERP of up to 20% of the maximum power authorized in the class of service (for class-A, the maximum is 1.2 kW). For LPFM, the maximum ERP is 20 watts.
The primary thing you need to be aware of is that a booster is a transmitter that is on your same channel. There's nothing different about a booster transmitter it would no different than having a FM translator or other transmitter operating on your channel. In other words, the ability for FM to discern the strongest signal (capture effect) will work in areas where one signal is much stronger (15 dB) than the other. This means that an improperly designed FM booster is going to cause interference and make the experience worse for some of your existing listeners while perhaps it makes things better for others.
For the purpose of the FCC rules for LPFM, a booster is counted like a translator and an LPFM can commonly own up to two translators (or two boosters, or one of each) In very extreme situations, the FCC may enterain a request for more than two boosters. If your organization is in this situation, please contact REC for assistance.
Protections to other FM stations
There are not many rules that boosters have to folllow for protection of other facilities as their primary stations are properly spaced to not only prevent interference to other stations but also to receive interference from that other station. As a secondary service. LPFM is much different. LPFM stations can come inside the interfering contours of full-power stations. In addition, subsequently authorized full power authorizations can bring full power stations closer to the LPFM to the point where the LPFM's interfering contour crosses into the protected contour of the full-power station. In the full-power world, FM boosters have a rule that states the signal of any first-adjacent channel station must exceed the signal of the booster station by 6 dB at all points within the protected contour of the first-adjacent channel station. In the full-power world, intermediate frequency (IF) would also need to be protected (for proposed facilities 100 watts or more) however since LPFM boosters would not run more than 20 watts, we don't have to worry about IF.
The interference remediation rules that apply for FM translators (§74.1203(a)) also apply to FM boosters.
Planning a booster system, especially where there is not a case of obvious hard terrain is an extensive task and in some cases, is even beyond my skill set and is not for everyone. I still believe that for over 98% of the LPFM stations out there, a booster will not work effectively. If you think your station can possibly benefit from a booster to help a major coverage gap in an area of your 60 dBu contour, please contact REC before attempting any filing. REC can do the filings if we feel that it may be a good candidate. Remember, REC does have reasonable fees for LPFM licensees even for the more complex stuff. Also, unless you have access to V-Soft, CommStudy2 or similar software that can create contours and do studies, please do not attempt this application on your own.
Also remember, if you get a booster and then decide to move your primary station, you will likely to have to either modify or cancel your booster. Even if you move, your booster still has to remain entirely within your primary station's 60 dBu contour.
Analysis of FM Booster System Configurations (Stanley Salek) (excellent reference ~ discusses placement of stations and issues around syncronization)
Other than during an application freeze, getting an FM booster can be done at anytime without having to wait for a filing window.