FCC101: Tropospheric Ducting and FM Radio
There ain't no cure for the summertime blues. Summer is when FM interference increases. Here's why it happens and what LPFM stations can and cannot do about it.
Summer. A time for family barbecues, kids out of school, laying out in the sun doused with Hawaiian Tropic and increased FM interference.
This is the time of the year that REC receives an uptick of inquiries from principals of LPFM and other stations regarding the strange distant station that suddenly appeared on their channel. More this year as the new 2023 Third Generation LPFM Filing Window stations start coming on the air.
I have heard things like, “the other station has jacked up their power”, yet there is no FCC application activity that would suggest a facility change. Even for LPFM stations that are inside the interfering contour of a nearby full-service station, the interference is not coming from that station, but from a station located hundreds of miles away.
This interference is not intentional, it is natural.
Capture Effect
First, we need to touch on capture effect. LPFM stations inside of interfering contours may be very well familiar with this. Capture effect is when an FM receiver is detecting more than one signal from different sources. When one arriving signal is significantly stronger than the other, that is the station that will be heard (e.g. it “captures” the receiver). When two signals are coming in about equal strength, you will hear a “flutter” where the receiver may go back and forth between the two signals.
This is why when an LPFM station starts transmitting on an FM channel where before, there was a very weak FM station heard on the channel will suddenly “take over” the channel. Even though coverage may be limited in the direction of that distant station, the local LPFM station dominates the area surrounding the transmitter site.
Now that we understand how an FM receiver handles a “competing” incoming signal, let’s understand why an unexpected interfering station comes in at times, especially in the month of June.
Tropospheric Ducting
FM radio is normally line-of-sight. The signal will continue until it either weakens out or gets stopped by hard terrain. Even though the signal goes “outward”, it also goes “upward”.
Tropospheric ducting (or “tropo”) can happen on days with stable weather with little wind. It is caused by temperature inversions where a mass of colder moist air (which some may call the “June gloom”) meets up with warmer dry air in the troposphere. The signal will then reflect at that point and find its way to distant locations hundreds (and in some cases, thousands) of miles away. If the incoming distant signal is strong enough, the listener may hear flutter or the distant station taking over their receiver.
Tropo is the most pronounced in June. There’s also a weaker chance of it also happening in December. Even those these are the peak months, it can happen any time of the year. In the United States, Tropo is the most prominent in coastal areas as well as in the southeast part of the country.
The lower the frequency, the more vulnerable it is to tropo. Back in the days of analog TV, it was fairly common to see very distant stations on TV Channels 2 and 3 that were coming in through tropo. Likewise, an LPFM station on 88.3 is more vulnerable to tropo than one on 107.5. Since LPFM stations operate at a significantly reduced effective radiated power, compared to other broadcast stations, low power facilities, such as LPFM and FM translators are the most vulnerable to any interference coming in by tropo. LPFM stations with a §73.807 predicted interfering station are even more vulnerable because a listener in a fringe area to the LPFM may encounter three different FM signals attempting to capture the receiver.
There are sites, such as the DX Info Centre operated by William R Hepburn that provides forecasts of tropospheric activity. This is a good report to look at if you want to see the potential of it happening today.
I have provided just a general simplified description of the phenomena, if you want to get down and dirty with the science of tropo, expanding on things that I did not discuss here (and honestly some stuff not in my own skill set), there is a really good explanation of tropo at the IEEE website.
Sporadic E (E-skip)
Another propagation phenomena is called Sporadic E (or “E-skip”). E-skip is caused by small ionization patches in the E region of the atmosphere (about 60~75 miles up). Signals reflect off of those patches and then come down over longer distances causing impacts to FM stations similar to tropo. Like with tropo, E-skip is the most pronounced in the summer months, peaking in early June.
Therefore, if your station is receiving interference from a more distant station, it could be either tropo or E-skip and overall, it does not really matter which one it is.
In some coastal areas, we see some paths where propagation is more persistent. A good example of this is the path between San Diego and Los Angeles, especially on 101.5 MHz, while growing up in the Los Angeles area, it was very common to hear KGB-FM booming in from San Diego.
HD Hijack
“HD Hijack” is a term coined by REC Networks. HD Hijack happens when those with an HD receiver will hear the audio from a distant station, even though there is a very strong local station on the same channel. This happens because an FM signal with HD is “wider” than a standard analog signal with no HD. The two subcarriers of the HD FM signal that are used to carry the digital information is detected by the receiver and the receiver switches to the digital audio instead of the analog.
HD Hijack is not limited to just tropo and E-skip, but can be more consistent, especially for LPFM stations and FM translators that are not operating HD but are inside the interfering contour of a full-service FM station that is operating HD. Licensed LPFM stations can use the channel search tool in REC’s myLPFM portal to determine the potential for HD Hijack.
What not to do
The first instinct would likely be that whenever a station suddenly appears on your channel that was not there before is to assume that they have decided to “jack up the power” (without FCC authorization) to try to get listeners during certain periods of the day. Over the years, we had reports of LPFM stations contacting full-service stations, practically bullying them to get them to turn down the power. This course of action is totally inappropriate and reflects negatively on the LPFM service and strains the relationships between REC (and LPFM in general) with the full-service broadcast industry, which can harm regulatory advocacy.
If you do suspect that a full-service station is up to any kind of “funny business”, the first thing to do is check FCCdata.org to assure that the station does not have a recent construction permit that modifies their power, location and/or directional antenna pattern. If there is no evidence of a recent construction permit, then the proper action is to get a qualified engineer to take field strength readings and gather extensive evidence and then file a complaint through the proper FCC complaint channel. Remember, LPFM and FM translator stations are secondary status, they are merely “guests” on the dial. If a full-service or LPFM station does make an authorized modification and is operating with authorized facilities, there’s nothing that can be done. If the modification was made by an FM translator station, recourse may be possible in the rules.
LPFM stations are authorized under §73.870(a)(1) to file to change to any channel that meets the §73.807 minimum distance separation requirements if a technical showing can be made that the change will result in reduced interference. In most cases, the channel changes are triggered by incoming interference that the LPFM receives from other stations. The technical study is normally based on the interfering contours of the interfering station(s) and compared to the incoming interfering contours on the channel that is being desired for the change. In the case of tropo and E-skip, the contours of the interfering stations are well separated from the LPFM station. The FCC Audio Division will not accept a non-adjacent channel change from an LPFM station that claims that their interference is caused by tropo or E-skip.
What can an LPFM station do?
First, as previously mentioned, don’t bully other stations. That’s just plain wrong.
Tropo and E-skip are natural occurrences and only happen from time to time, especially in the solstice months of June and December. There’s no way to reduce the signal strength of these incoming signals.
LPFM stations can look at their facilities to make sure that they are operating at their full potential. Make sure that your transmitter is set to the correct transmitter power output (TPO) that takes into consideration the gain (or loss) your antenna system and the feedline loss of the cable. REC can calculate that for you. Also, if your station is authorized less than 100 watts effective radiated power (ERP), verify your station’s height above average terrain (HAAT) to assure that you had been authorized the correct ERP based on the actual HAAT. If it is incorrect, a modification of facility needs to be filed with the FCC. REC can assist on this. Also remember, if you do change your station’s TPO or swap out antennas or feedline, as long as you stay at the same location and antenna height, it can be done without advanced authorization from the FCC, but you must follow up within 10 days by filing a Modification of License. REC can assist with this.
LPFM stations can refer to §73.875(c) of the FCC Rules to see what changed without advance authorization from the FCC but must be followed-up with a modification of license application (Schedule 319) within 10 days of the change.
Most LPFM stations authorized at 100 watts ERP are not operating at their full potential. This is because of lower antenna heights and other obstructions which degrade the coverage of the station, regardless of any incoming interference. Trees, terrain, taller buildings and other obstructions also play a role.
There are some camps that theorize that using a multi-bay circularly polarized antenna, as opposed to a single bay antenna may also help as the elevation patterns of multi-bay antenna systems emphasize more outward emission as opposed to downward.
LPFM stations encountering HD Hijack can recommend that their listeners disable the HD reception in their receiver. This would result in the listener once again being able to hear the LPFM station but will also result in the listener not being able to pick up other HD services on their radio (including “HD2” stations).
If considering to operate on a different channel (or for new applications during a filing window), you should always “drive test” the candidate channel. A “drive test” means that you drive in the area about 4 miles surrounding the potential LPFM transmitter site location and survey the quality of the incoming signal by ear. It is very tricky to do without the right instruments, but it may give you an indication, based on the strength and stability of the incoming signal what the potential for interference will be and in what direction from the LPFM station will be impacted the most. When drive testing, make multiple trips on different days and at different times of the day and night. If you drive test in a peak period (around June and December), perhaps make a second set of tests after the peak is over.
Again, if your station needs assistance or advice, please contact REC.
In closing
Where it comes to tropo and E-skip, there ain’t no cure for the summertime blues. This is a reality that broadcast and non-broadcast facilities (as well as hams) operating on VHF frequencies need to face. This is something that is not as consistent than, let’s say, skywave propagation on the AM band, but it is something that does happen. Even though you can’t stop nature, there are things to assure that you have the best possible signal within your authorized parameters.
Hopefully, this article is helpful, especially for those of you who are just starting off down the rabbit hole of radio, on what to possibly expect, why it happens and what to do if it does. Now, let’s go out and make some great radio!