In a previous Radio-Guide article we touched on various types of FM Broadcast antenna systems. There is a big misconception that all antennas are the same. Many think that they all will work – which they will to some extent. The problem is an LPFM station is Low Power, 100 Watts ERP with fairly low antenna height – 100 feet HAAT. With a system like that, the antenna system, including the coax, is key to coupling your hard work into the ether. While all antennas do just that, they all do it with varying levels of efficiency. One of the most significant factors determining the success or failure of any FM broadcast station is the quality and coverage of its signal. A station that does not provide a clean, interference free signal in its coverage area cannot expect high audience cumes, regardless of programming, equipment, etc.
While I have established my clear preference for an antenna based on the “Penetrator” design, I promised to go into further detail in a future article.
There is one caveat to this improved propagation. Since we are no longer transmitting in a single plane, the gain of the system is greatly reduced. Just as receiving a transmission which was originally transmitter horizontally, receiving it with a vertical antenna would result in a much lower received signal. Since most receivers in vehicles are vertically polarized (unless they are embedded into the windshield) the obvious answer would be to transmit your signal vertically. However, this also has its own set of signal issues. In low power operations with low HAAT, this can create a unique multipath problem. The multipath is created as the angle of radiation drops quickly at low HAAT installations. Within a few miles, the signal is already running into power poles, buildings, etc. This can cause what is described as a picket fence sound when the multipath cancels the main signal briefly. If there is interference on the frequency, this problem can be greatly aggravated. The slower you drive, the longer the interference periods. Sometimes stopping within a wavelength from the pole will completely cancel your signal. Moving ten feet or so will clear it up. These are problems inherent in vertical transmission.
Additionally, reception by vehicles with antennas embedded into the windshield and home receivers is greatly reduced by using vertical transmission. (Embedded antennas and home receivers generally use a horizontal type antenna.)
We are currently waiting for the FCC to codify the new Low Power FM Radio Rules. This should occur in late September / October 2012. The filing window will be opened shortly after the rules are published in the the federal register. The filing window is expected to open in the Spring / Summer of 2013. We will keep you updated on the latest news, information and filing opportunities as they develop. Below are the latest LPFM Radio developments.