In urban and extended suburban areas typical of southern California and the northeast United States, it can be difficult to find a clear broadcast FM (87.9-107.9 MHz) channel to use for license-free transmissions. As 2020 drove many communities to outdoor activities, many organizations used license-free FM transmitters to share music, lectures and entertainment a large audience locally in their automobiles. An issue we’ve observed is poor audio quality and erratic coverage of the license-free FM system due to sideband spillover of adjacent HD Radio broadcasts. With HD Radio, the first adjacent channel spillover power is hundreds to thousands of times higher on average than with an analog FM signal alone. The usual sound of HD Radio interference is simply a hissing noise that is difficult to distinguish from ordinary noise when a station isn’t present.
The typical community member new to license-free FM would have difficulty distinguishing HD Radio first adjacent channel interference from simply having poor range. Without special equipment beyond an automobile or handheld FM radio, it’s difficult for the inexperienced person to detect the interference. A $20 RTL-SDR USB radio stick would give a spectrum analyzer view showing the HD Radio sidebands spilling within 100 kHz of the license-free carrier frequency. This energy will cover over the license-free FM broadcast. Without such equipment, look in the HD Radio database to see if the FM stations on the channel above and below have HD Radio. If so, pick a different clear license-free FM channel, if the first adjacent stations are clearly audible on your radio when on their channels.
Suppose a first adjacent HD Radio station has a carrier signal strength at the radio of about -70 dBm with a typical car radio antenna. -70 dBm is typically more than adequate for clear-sounding broadcast FM audio when in an outdoor environment where there’s not excessive interference. Take a conservative and simple estimation that the HD Radio first adjacent has a -20 dBc HD Radio injection level, and that 50% of that power (-23 dBc) falls within the license-free receiver passband. This means roughly -90 dBm is on-channel of the license-free FM station. The area to be covered was about 150 meters from a theoretical central location for the FM transmitter. This would be well in excess of that expected for a legal license-free FM transmitter. For us the license-free FM program audio was only detectable over about 80% of the event, when we were probably within about 100 meters of the transmitter. However the audio quality was only usable for about 50% of the park.
We didn’t attempt to quantify this further, but this poor radio coverage would be expected from the use of a first adjacent channel to an HD Radio station, and trying to cover too large an area with a license-free FM transmitter. Once might suppose the FM transmitter used might have more than legal FM transmit power. Even in that case, one should not use a first adjacent channel to HD Radio, as this can drastically reduce coverage. Again, a key metric is if the first adjacent HD Radio channel (say 88.9MHz) is clearly audible when tuned to that channel (88.9), then trying to broadcast license-free on a first adjacent channel (88.7 and 89.1) will be a futile effort.
Illegal broadcasts also seek clear FM frequencies that are naturally not in any official database. Before printing promotional materials for your event, it’s important to verify with a local FM receiver than the planned channel is actually clear. Also consider that another operation may choose the same FM frequency near your event. It may be better to simply instruct attendees to look for big signs near the entrance(s) of your event so that the FM frequency can be quickly changed if interference is experienced. Be sure someone on-site knows how to change the FM transmitter frequency.
Reference: FM Receiver measured performance