No, it does not mean that circularly polarized antennas like the Penetrator are illegal. LPFM stations are allowed to use circularly polarized antennas, as specified in §73.816(a) of the FCC rules. This regulation permits LPFM stations to use antennas with horizontal-only polarization, vertical-only polarization, circular polarization, or elliptical polarization. Therefore, you can use circularly polarized antennas like the Penetrator without violating the rules.
If your construction permit only specifies an effective radiated power (ERP) on the horizontal plane and does not provide a value for vertical ERP or specify circular polarization, it typically means that the FCC has authorized you to use a horizontally polarized antenna.
The use of circularly polarized antennas, such as the Penetrator antenna, may be subject to specific authorization or additional requirements. To use a circularly polarized antenna or any other type of antenna not specified in your construction permit, you should seek guidance from the FCC or a broadcast engineer experienced in FCC regulations. They can provide information on the necessary steps, if applicable, to obtain the appropriate authorization for your chosen antenna type.
It's important to ensure that your station's equipment and operations comply with FCC regulations to avoid any potential issues or violations. Consulting with experts in the field can help you navigate these requirements effectively.