Directional Antennas for LPFM
Updated April 26, 2020.
Reflects recent rule changes due to the adoption of MB Docket 19-193. Until the rule is published in the Federal Register, the rule does not go into effect until the date that is included in the Federal Register.
With the Report and Order in MB Docket 19-193, the Federal Communications Commission now permits the use of directional antennas by LPFM stations for any purpose. The question is, does your station really need a directional antenna?
For almost all stations, the answer is a resounding, NO.
Those who come from an amateur radio background may know that directional antennas provide additional “gain”, which then would allow your signal to be stronger. For example, let’s say that you take into consideration the loss of the feedline and the gain of the antenna results in a 3 dB net gain, a 100 watt 2-meter transmitter will actually provide 200 watts at the antenna. In this case, the 100 watts is the transmitter power output (TPO) and the 200 watts is the effective radiated power (ERP).
Broadcast radio works different than amateur radio. This is because the power that is authorized (like the standard 100 watts for LPFM) is the ERP, not the TPO.
With that said, in order to obtain a 100 watt ERP in an antenna and feedline combination with 3 dB of gain, the transmitter would need to have a TPO of 50 watts.
In other words, you can’t use a directional antenna to “increase your power” like you can in amateur radio. A directional antenna in broadcast actually requires you to reduce your transmitter power to assure that the proper ERP is maintained.
Unlike in amateur radio, a directional antenna in broadcasting does not increase your coverage but instead, diminishes your coverage by blocking out certain areas from receiving the broadcast signal.
Why would an LPFM station want to use a directional antenna?
Like we mentioned, there are very few reasons to do so:
Second adjacent channel waiver
LPFM proposals that are short-spaced to a second-adjacent channel station which has a service contour very close to but not over the LPFM transmitter site can propose a directional antenna along with a contour study that shows that the 100 dBu interfering contour of the LPFM station does not overlap the 60 dBu protected contour of the protected station. (For commercial Class B1 stations, those contours are 97 and 57 dBu and for commercial Class B stations, they are 94 and 54 dBu).
In some cases, the directional characteristic of the antenna coupled with the elevation pattern of the antenna could demonstrate that due to the lower power going into a particular direction, the actual interfering contour does not reach a nearby occupied structure. (This is a very tricky showing. REC has been successful with these showings in the past.)
While it would be difficult for a 100 watt LPFM station to create an interfering contour to violate the Canadian agreement, the circumstances are different on the southern border.
LPFM stations that are within 125 kilometers of the Mexican border are limited to 50 watts in the direction of Mexico. In the past, these stations were given 50 watt ERPs even if they would have otherwise been eligible for more. A directional antenna can now be used to limit the radiation in the direction of Mexico to less than 50 watts ERP while allowing more than 50 watts in other directions.
More information on this can be found in LPFM Near Mexico.
Station operating on solar
Some solar systems may not accommodate a very high TPO due to high power consumption. These systems may need to be operated at a very low TPO. If coverage to the entire “non-directional” service contour is not important (as in, the station is on the side of a mountain), the station can operate a directional antenna to limit power in a particular direction. For example, exclusive of feedline loss, a solar station needing 100 watts ERP can operate into a highly directional, single polarization antenna with a gain of 7 dB and only need 20 watts from the transmitter. In this case, you are sacrificing a lot of your coverage area.
Resolving interference to another station
Because of distance separation rules, some stations, even though properly spaced, will have a form of contour overlap with another station. One or both stations could incorporate directional antennas to reduce the interference between the two stations.
Protecting TV channel 6
With the recent changes, the FCC has allowed LPFM stations to request a waiver of §73.825 of the rules by using a contour study similar to FM translators to demonstrate that a LPFM station won’t interfere with the Channel 6 TV station. Notification to the Channel 6 station is required.
Proof of Performance/Construction Verification
Under the new rules, LPFM stations seeking to use a directional antenna for purposes other than the exempted uses following must obtain a proof of performance from the antenna manufacturer. The proof of performance is a report that states how the antenna is designed to operate. The installation will also require verification of the antenna installation by a licensed surveyor. This is to assure that the directional antenna was constructed at the right height, is the right antenna and is pointed in the right direction. This information is to be included when the application for license to cover the construction permit is filed.
If a directional antenna is proposed by an LPFM station solely for the following exempted purposes, a proof of performance and verification are not required:
- Second-adjacent channel waiver request.
- International agreement (such as being within 125km of Mexico).
- Local, county, state and other governmental entities operating the LPFM for public safety purposes.
So, as you can see, in most cases, a directional antenna is not needed for an LPFM station. It may be more hassle and expense than its worth.