REC LPFM Advisory Letter #11 - LP250 Upgrade: Technical Planning Considerations for LPFM Stations
With the recent public notice by the FCC, placing the latest REC LP-250 Petition for Rulemaking, “Simple250” to public comment as RM-11909, there has been an increased excitement within the community of the prospect that someday, many LPFM stations would be able to increase to 250 watts ERP (or its >30 meter HAAT equivalent).
It is important to remember that at this time, we can only speculate that the FCC will take action on REC’s petition as there are many more regulatory hurdles the petition will have to cross before it can become reality. This process may take months, if not years, based on how excited the Commissioners are and the influence they may get from the Executive Branch. LP-250 will not happen over night.
With all of this excitement, we must also take into consideration some of the technical issues that many upgrade eligible LPFM stations will face in implementing LP-250. For many LPFM stations, upgrading to LP-250 is not as simple as just turning up the power at the transmitter. Instead, it would require physical changes being made, which would involve expense.
How the signal gets from here to there
I think we all know that the broadcast signal originates in the transmitter, goes through a cable (feedline) and then is radiated by an antenna. Through this process, the power from the transmitter will slightly diminish in the feedline and depending on the design and the configuration of the antenna, it may diminish or improve when it is radiated. This needs to be taken into consideration.
Certified transmitters
Prior to the creation of LPFM in 2000, broadcast transmitter manufacturers were not required to obtain lab certification for their broadcast band transmitters. Lab certification is normally used for consumer devices and other equipment that may not be under the direct supervision of a qualified engineer. FM broadcast transmitters could be used by licensed stations at the time through a process called Manufacturer’s Declaration of Conformity, also known a “type accepted”. This was a carve-out exception for the broadcast industry as full-service stations are required to designate a chief operator who would be responsible for the operation of the equipment.
When LPFM was created, the thought process of the FCC at the time was that there would be more inexperienced people using broadcast transmitters and the Commission did not want to burden these new stations with a chief operator requirement. Also, while not “on the record”, there was also a fear at the time that LPFM stations may use unstable and kit-built cheap transmitters, similar to those used by pirate radio stations at the time.
Seeing new potential opportunities in LPFM, some transmitter manufacturers had their lower power transmitters (such as those 300 watts or less) certified through a testing lab and issued a FCC ID number which would be on a label affixed to the shipped transmitter.
Antennas, on the other hand, do not require any kind of FCC certification to be used in the LPFM service.
Single bay circularly polarized antennas
Many LPFM stations are equipped with a single bay circularly polarized antenna, such as the Nicom BKG-88 and the Shively Labs 6812 series or one of the “penetrator” designs like the Nicom BKG-77 or the SWR FME-C series.
A single bay circularly polarized antenna takes the signal being fed to it and “splits” it between horizontal and vertical polarization, thus radiating half of the power horizontally and the other half vertically. Exclusive of the losses in the feedline, which we will discuss, you would need twice the power from the transmitter to drive the desired effective radiated power (ERP) at the antenna (e.g. 200 watts transmitter power output (TPO) needed to get 100 watts ERP). With feedline losses, 100 watt ERP LPFM stations with a single bay circularly polarized antenna may have TPOs around 250 watts.
Feedline Losses
Another consideration you must take is the feedline. No two types of feedline are really created equal. Feedline is rated through loss per 100 feet (or 100 meters). The feedline with the lower loss is more effective.
Determining the station’s LP-250 maximum effective radiated power
For LPFM stations at 30 meters height above average terrain (HAAT) or lower, that are currently authorized at 100 watts ERP, the LP-250 ERP will be 250 watts.
For stations above 30 meters HAAT, the maximum ERP will be based on what ERP and HAAT combination would make an average 60 dBu service contour of 7.1 kilometers. In the REC proposal, some higher HAAT LPFM stations may be capped at 100 watts ERP due to intermediate frequency (+/- 10.6 and 10.8 MHz) protection requirements. For now, you can pretty much estimate that your LP-250 authorized ERP will be 2.5 times your station’s current ERP. The check.lp250.com tool will show your station’s ERP at the LP-250 level.
REC proposed that minimum ERP by a station in the LP-250 class be one watt higher than that allowed for LP-100 stations. This is consistent with the commercial broadcast rules.
LPFM stations at HAATs higher than 30 meters
Depending on the transmitter originally purchased, LPFM stations with lower ERPs because of high HAAT may be able to achieve an upgrade to LP-250 using their existing equipment and it would only require an increase of the transmitter power. For example: an LPFM station that is at 53 meters HAAT and is authorized 31 watts ERP as an LP-100 and the station uses a single bay circularly polarized antenna, such as a SWR FME-C and is running 70 feet of less expensive LMR-400 cable. The transmitter is currently operating at 77 watts transmitter power output (TPO) in order to achieve 53 watts at the antenna. In this case, the LPFM station had previously purchased a 300-watt class certified transmitter such as a BW TX-300 or a Nautel VS-300. If this station can upgrade to LP-250, their authorized ERP would be 82 watts. Since the transmitter they already have is capable of up to 300 watts TPO, they can turn up their power to 203 watts at the transmitter to complete the upgrade.
What about getting a higher-powered transmitter?
Breaking the 300-watt barrier has some substantial challenges. Of course, there is the expense of the new equipment purchase, which could greater than $5,000. Another thing to consider is the selection of equipment that is certified may be reduced as some manufacturers, who are otherwise not required to seek certification, went through the expense of certification just to reach the LPFM market. The LP-250 service is expected to have the same certification requirements as LP-100.
Finally, you also have to take electrical power into consideration. Not only would a higher power class transmitter result in increased energy consumption (thus meaning higher electric bills), your station may need to have electrical work performed at the transmitter site. This is because some 600 watt class transmitters, such as the BW TX-600 V3 will require a 220 volt connection, unlike their 300-watt counterpart that can be plugged into a standard 110 outlet.
Changing the antenna would be a better option
As we mentioned before, certain antenna configurations, such as single bay circularly polarized will experience a loss at the antenna. If the station has the ability to use more space on the tower (which may not be the case in some cases involving leased/shared tower space), upgrading the antenna to two or more bays would be a more efficient means of upgrading to LP-250. Using two properly configured circular polarized antennas spaced one wavelength apart (approximately 10 feet at 99.9 MHz) will result in near-“unity gain” at the antenna. In other words, there would be very minimal (less than 0.2 dB) loss at the antenna. The majority of the loss would be in the feedline. In this case, you still have to have a better quality feedline. For example, if you are running a 300-watt transmitter with a 2-bay Nicom BKG-77 or 88 circularly polarized antenna at full-wave spacing, and you are running 70 feet of LMR-400 cable, you will only get about 242 watts at the antenna. If that feedline was a higher quality, such as Andrews LDF4-50A, which has a lower insertion loss, you could run the transmitter at 288 watts to get a full 250 watts ERP at the antenna. If the feedline is too long, then consider going to at least 3 bays where you can start experiencing gain.
For two bays, configuration is also important. As we mentioned, two bays at one wavelength apart will result in a near-unity gain, however, if space is limited and the antennas are only spaced half wavelength apart (about 5 feet at 99.9), the antenna will still experience loss, but not as substantial as the single bay configuration. For example, the same configuration as above with 70 feet of LMR-400 into a ½ wave spaced Nicom BKG-77 or 88, you would have to run about 439 watts TPO.
An interesting thing in the Nicom specification for the BKG-77 and 88 antennas is that if you place the antenna bays in a 0.85 wavelength configuration (about 8.5 feet spacing for 99.9), then you will actually experience 2.14 dB of gain. This would mean that the station running 70 feet of LMR-400 would only need to operate 190 watts TPO in order to reach 250 watts ERP, even lower with the LDF4-50A feedline (174 watts TPO).
More information on the gain specifications of multi-bay antennas can be found on REC’s Antenna Gain Table.
To do a preliminary check to see what transmitter power output may be required to upgrade to LP-250 based on the station’s current antenna configuration, you can use REC’s LP-250 Antenna Check tool.
Vertical-only antennas
There has always been a long running debate about antennas operating only one polarization over the other. In full-service radio, the FCC, in most cases, requires stations to operate at least with horizontal polarization. LPFM stations, on the other hand, have the flexibility to operate horizontal only, vertical only or both. Some have said that horizontal polarization is good to reach those with radio receivers in homes as many times, those built-in antennas a horizontally polarized while vertical polarization is better for reception in vehicles. Running a single polarization does involve some sacrifices in performance, but for your station’s individual situation, those sacrifices may be minimal compared to the flexibility offered by these antennas.
The primary advantage that a vertical can provide is gain, as power is not having to be distributed among two different polarities. Two popular vertical antenna models in the LPFM arena include the Norwalk Dominator, which specs at a 3.0 dB of gain, as well as the Comet CFM-95SL, which has a 1.26 dB of gain. For some stations with the room to place a vertical only antenna and do not have certain other issues, such as having to deal with a second-adjacent channel waiver, a vertical may be a good inexpensive method for upgrade, especially where there is a lot of in-car listening.
Second-adjacent channel short spacing
Like with LP-100 stations, LP-250 stations would be required to protect full-service and FM translator stations on second-adjacent channels using minimum distance separation. For LP-250, those minimum required distances will be about 1 to 2 kilometers higher than the LP-100 specifications. This can result in some cases where an LP-100 station that currently meets minimum distance separation to second-adjacent channel stations will suddenly find themselves short-spaced at the LP-250 distance. For stations already short-spaced at LP-100, they will also be short-spaced at LP-250.
Consistent with the policy for FM translators, it is REC’s position that if LP-250 is adopted, existing LP-100 stations on second-adjacent waivers would have to redemonstrate their ability to provide protection to short-spaced second-adjacent channel stations.
There are two ways to demonstrate second-adjacent channel protection:
For LPFM stations right on the very edge of a minimum distance separation requirement or if the short-spaced station is running a directional pattern, the LPFM could sometimes make a demonstration that their full interfering contour (100 dBu towards most stations) does not overlap the protected contour (60 dBu for most stations) of the incumbent station. This is done through a contour study.
The more common method used is called the “Undesired to Desired” (or “U/D”) method. This is sometimes referred to as the Living Way method. In this case, we use the FCC propagation curves to measure the field strength contour of the short-spaced station at the LPFM site, we add 40 dB that field strength and figure out the size of an interfering contour at the same site for the LPFM station. If there is no population (or major highways) within that interfering contour, then a showing can be made that no population would receive interference from the proposed LPFM facility. In addition to this, some antenna manufacturers provide specifications called elevation patterns that we can incorporate to show that while there may be population in the interfering contour, the design of the antenna is such that the signal is radiated in a way where it does not go as much downward as it goes outward. Thus meaning the interference will not reach near the ground but will remain above the roofs of the homes and other structures underneath. Because LP-250 uses a higher ERP than LP-100, the size of the interfering contours will be larger.
For example: Let us say an LPFM station is short spaced to a second adjacent channel station. That short-spaced station places an 82 dBu field strength at the LPFM site. This would mean that we would need to calculate the 122 dBu (82+40) interfering contour to represent the LPFM station. At 100 watts ERP, 122 dBu would be 56 meters. This would mean that there would need to be no population within 56 meters of the base of the antenna (or that an elevation pattern study shows interference does not reach the ground). Now, at LP-250 (250 watts ERP), that 56 meters now increases to 88 meters, meaning you have a much larger area to protect. For some, this may be an impediment for upgrading.
Its also important to know that the more bays on the antenna, the better chance that the antenna will be able to direct power outward instead of downward. For example, if the LPFM associated with the short-spaced station arriving at 82 dBu had a 2 bay 0.85 wave spaced Nicom BKG-77 antenna at LP-100, the 122 dBu interfering contour would not reach any structures. However at LP-250 (250w ERP), interference would reach structures. Upgrading to a 3 bay BKG-77 at 0.85 spacing would allow for 240 watts ERP, going to 4 bays would provide a full 250 watts.
The key things about second-adjacent channel waivers are that you either want to be so far from the station that contours will not overlap or to be as close to the that station as possible so the LPFM station does not have to work that hard to protect the short-spaced station.
Conclusion
Going LP-250 is not as easy as turning the knob to 11. You have many logistical challenges in your transmission plant that will need to be considered to determine if the station can be viably upgraded to LP-250 or if a different transmitter site (or channel) must be found in order to achieve the higher power.
It is also important to remember that LP-250 is purely speculative at this time. We still have a long way to go to get this through. There’s never a guarantee that the FCC will ever adopt this. They have already rejected it twice under different circumstances, but REC is not giving up on this fight. If your station decides it wants to plan and start modifying infrastructure to prepare for a potential LP-250 service, you do so at your own risk.
Due to an extremely busy regulatory calendar and upcoming filing windows, REC is not doing any “one-on-one” consultations regarding specific station qualifications for LP-250 at this time. Please understand our position on this as our time is limited for other issues that are eminent and time critical. REC does provide information regarding LP-250 availability at the following two locations:
To check for on-channel upgrades at the current LPFM station site:
https://check.lp250.com
To check for other LP-250 qualified channels or to upgrade at a different location:
https://mylpfm.com
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Original version: June 13, 2021
