FAQ: Why should a second-adjacent short-spaced LPFM proposal be as close as possible to the second-adjacent channel station(s)? That seems highly illogical. I thought it was, the further, the better.

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All LPFM proposals need to provide minimum distance separations to domestic FM facilities (full-service, FM translator and LPFM) and vacant FM allotments on co-channel and first-adjacent channels.  They must also meet a minimum distance separation to full-service FM, FM translator and vacant FM allotments on second-adjacent channels.

However, LPFM stations that can meet the co-channel and first-adjacent channel minimum spacing, but cannot meet the second-adjacent channel minimum spacing may be able to request a waiver of the second-adjacent channel minimum spacing.  This is done through a technical study that shows that there will be no interference.

To understand how this all works, you need to have a basic understanding of field strength contours.  

Each broadcast facility has a designated service contour.  This contour is based on a calculation that takes into consideration the facility’s effective radiated power (ERP) and height above average terrain (HAAT) in each direction.  Through a “curves” calculation, the distance from the facility to the appropriate field strength contour is determined.  The field strength that is used for this calculation is 60 dBu for all facilities except for stations in the 92.1~107.9 “non-reserved” band where the field strength is 57 dBu for Class B1 stations and 54 dBu for Class B stations.  The contour distance is based on an estimation of the area where the station will be received with at least that field strength on 50 percent of the receivers, 50 percent of the time.

Each broadcast facility also has several designated interfering contours.  The contour distances are calculated in a manner similar to the service contour already described, but uses a different formula based on that field strength on 50 percent of the receivers, 10 percent of the time. The interfering contour field strength is based on the channel relationship between the LPFM and other facility.  If the other facility is on co-channel (95.5 if the proposed LPFM is also on 95.5), then the field strength will be 20 dBu less than the service contour (e.g. 40, 37 or 34 dBu based on channel and service class).  If the other facility is on a first-adjacent channel (95.3 or 95.7 if the proposed LPFM is on 95.5), then the field strength will be 6 dBu less (54, 51 or 48 dBu).  If the other facility is on a second-adjacent channel (95.1 or 95.9 if the LPFM is on 95.5), then the field strength is 40 dBu higher (100, 97 or 94 dBu). 

The higher the “dBu”, the stronger the signal is.  Therefore, a signal will be stronger the closer you are to a transmitter site and weaker as you move farther away.  Let’s say the full-power station is a Class A and operates at 6 kW (6,000 watts) at 100 meters above average terrain in a particular direction, the station would have a 60 dBu field strength 28.3 kilometers away, a stronger 70 dBu field strength only 16.1 km away and an even stronger 80 dBu field strength at 9.1 km away, and so on.  In other words, the higher the dBu number is, the smaller the contour area is.

The protection method is also called an “undesired to desired” (or U/D) ratio.  In our context here, the “undesired” station is the proposed LPFM and the “desired” station is the existing broadcast facility.  For second-adjacent channel, we use a U/D ratio of 40 dBu.  This means that if the existing station is protected out to its 60 dBu service contour, our interfering contour must be 40 dBu higher, hence 100 dBu. 

In some rare cases, a short-spaced second-adjacent station may be so far away from the proposed LPFM station where the LPFM’s interfering contour would not overlap into the existing station’s service contour.  This can be caused by several factors including the existing facility not operating at the maximum facility for its service class, the existing facility using a directional antenna and in some cases, the intervening terrain between the existing station and the proposed LPFM facility. 

However, in a large majority of second-adjacent channel short-spacings, there is overlap between the service contour of the existing facility and the interfering contour of the proposed station.

In the case of second-adjacent channel short-spacing, the FCC does allow a technical showing where if in the area where there is greater than a 40 dBu U/D Ratio, there are no occupied structures or major 4-lane highways, a showing can be made that there is no population in that area, then you can say that the proposal, although short-spaced, will not cause any interference with any listeners or potential listeners of the short-spaced station. 

Now, notice we said 40 dBu U/D Ratio and not, say, the 100 dBu contour?  The FCC will accept U/D ratio based on the field strength of the short-spaced facility at the LPFM transmitter site.  For example, let’s say the short-spaced facility places an 83 dBu field strength contour at the proposed LPFM site.  This means that simply add 40 to 83 to determine the required interfering contour and in this case, it is 123 dBu.  Therefore, we must calculate the distance to the 123 dBu interfering contour.  Using the REC or FCC Curves Calculator, enter the ERP of the proposed LPFM facility in kilowatts (0.1 = 100 watts), always enter 30 for the HAAT and enter the previously calculated interfering field strength needed, in our example, 123.   In this case, it returns a distance of 50 meters.  This means that in the area 50 meters surrounding the proposed LPFM antenna (looking at it as a sphere), there cannot be any occupied structures or major 4-lane highways within 50 meters of the antenna. 

Not everyone is lucky to have their short-spaced second-adjacent channel stations that close.  Let’s say your second-adjacent short-spaced station places a 68 dBu contour at the LPFM site. Again, we add 40 to that and get 108 dBu and then run that in the Curves Calculator.  At 100 watts, the interfering contour where the LPFM station must provide protection has increased now to 279 meters surrounding the LPFM antenna.  

These contour distances are measured from the radiation center of the antenna. Therefore, in some cases where the short-spaced second-adjacent channel station comes in so strong, it may be possible that the resulting interfering contour may not reach the ground.  Let’s say you are proposing operation on a stand-alone tower with a radiation center of 20 meters above ground level and the short-spaced second-adjacent channel station places a very strong 97 dBu service contour at the LPFM site.  Again, we add 40 and we get 137 dBu.  At 100 watts, the 137 dBu interfering contour only extends to 10 meters.  Since the antenna is at 20 meters above ground level, the 137 dBu contour does not reach the ground and remains above all occupied areas.  Therefore, it cannot cause interference to listeners or potential listeners of the existing station.  We always provide a 2-meter allowance for a single-story building (as a person’s height can be 6 feet tall).  So, in the 97 dBu case, we still had 8 meters to spare.  For multi-story buildings, we normally determine the height in meters of the actual floor of the highest occupied floor and then add 2 meters to that.  Achieving second-adjacent waivers on rooftops is very challenging unless you are extremely close to your second-adjacent channel station.

If your LPFM station will be reduced in power because of height above average terrain, this will result in your interfering contours to be smaller.  You may also elect to operate at less than the fully authorized power in order to achieve a second-adjacent waiver.  Normally, you can not propose a power of less than one-half of the ERP allowed based on your HAAT. 

If your station is short-spaced to more than one facility, then you must base the field strength study on the weaker station at the LPFM site (the lower dBu number).  The interfering contour for the weaker station will fully encompass the interfering contour of the stronger station.  For example, if the short-spaced station on 95.1 places an 83 dBu contour at the LPFM site but the short-spaced station on 95.9 places a 68 dBu contour at the LPFM site, your second-adjacent study must be based on protecting the station on 95.9 as that is the weaker of the two stations and the station on 95.1 would have an interfering contour completely inside the interfering contour related to 95.9.

Different antennas and configurations have different horizontal (outward) and vertical (downward) patterns. The manufacturers may provide those numeric values in their documentation.  An antenna with multiple bays is designed to suppress the power going downward and focus more on directing power outward.  With the right antenna and a small enough interfering contour, it could be demonstrated that the interfering contour in the directions towards the ground will not penetrate into any occupied structures thus reaching no population.  The more bays you have, the more downward suppression you have.    Tower height also plays a role here.  The closer the antenna is to ground level, the more downward suppression is required. 

Bottom line: A proposed second-adjacent short-spaced facility that is closer to its second-adjacent channel short-spaced stations (based on field strength) does not have to “work as hard” to protect listeners of the short-spaced station as compared to a facility with a second-adjacent channel facility further away and thus weaker at the LPFM site.

More about Second Adjacent Channels at:

Also note, in areas near the Canadian and Mexican borders, LPFM stations are also required to protect foreign allotments.  Those protections are on co-channel, first-adjacent, second-adjacent and third-adjacent channels as well as intermediate frequencies (+/- 10.6 or 10.8 MHz).  Second-adjacent (or third-adjacent) channel waivers are NOT available towards Canadian and Mexican short-spaced facilities.

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Answer Date: 
Saturday, November 11, 2023