4. Protection to other services

4.1 Protection for Landmobile Services

In some areas, landmobile services that have not relocated by 1 June 2010 will limit the use of some LPFM frequencies. Those frequencies and the areas where use is limited are identified in Attachment A - Available LPFM Frequencies during the transition period. The attachment is based on current landmobile transition plans and may change prior to 1 June 2010.

MED will provide an update to the schedule as part of the report on submissions to this consultation paper and maintain an up-to-date copy on the RSM web site prior to 1 June 2010. MED will also use its regular Business Update publication to advertise web updates on progress. LPFM operators wishing to subscribe to that service can do so via the RSM website⁴.

LPFM operators who rely on listening on a frequency to identify current users prior to broadcasting may not identify those users either because the licensed services have not started broadcasting, or, as in the case of landmobile services where users operate on a ‘push to talk’ basis, the service may be temporarily unused.
 

4.1.1 LPFM Distance from Landmobile Services

The protection of licensed services from LPFM broadcasts requires operators to identify the minimum distance between their intended broadcast area and those of the licensed services. That distance is significantly dependent upon the locations of the LPFM antenna and any terrain or other physical radio path obstructions between the antenna and a licensed service coverage area.

Given the difficulty in determining the distance between all possible LPFM, VHF-FM, landmobile and aeronautical transmitter and receiver, MED proposes a minimum separation distance of 50 Km in the GURL and GUSL.

Question 3

During the LPFM transition period, should MED allow use of the 87.5 - 88.0 MHz band for LPFM services but prohibit transmissions on specific frequencies in specific geographic areas?

Question 4

Is the proposed 50 km transmitter minimum distance separation appropriate? If not, is there a better means of minimising the potential for LPFM to cause interference and what are its benefits?

4.2 Protection for Broadcast Services

The introduction of a temporary GUSL for LPFM services to operate 88.5 MHz, 88.6 MHz and 88.6 MHz, and the proposed GURL that will permit LPFM services at the edge of the VHF-FM band, may increase the potential for harmful interference to VHF-FM broadcast services.

Similar conditions have existed in the past without significant issues being identified. Nevertheless, MED proposes that the GURL and GUSL could include a requirement for either:

1. The 50 km LPFM – VHF-FM transmitter separation requirement noted above; or
2. Where LPFM broadcasters operate within the 50 km radius, a frequency spacing of 400 kHz.
    

Question 5

Do you consider the 50 km and 400 kHz separation criteria appropriate for protection of broadcast services? What other criteria is appropriate? Please identify the specific concern and the additional safeguard sought?

If you consider the protection provided is excessive, please identify and explain the level of protection that you consider would be appropriate?

4.3 Protection for Aeronautical Services

Protection for aeronautical radionavigation and COM (voice) services requires that:

1. Aeronautical receivers do not receive signals at power levels and frequency combinations that can cause receiver intermodulation (IM); and
2. Navigational and COM receivers do not receive LPFM transmitter generated IM interference.
    

4.3.1 Aeronautical Receiver Intermodulation Issues

A project to identify the potential for interference into aeronautical radionavigation receivers operating in the 108 - 117.95 MHz band from high power VHF-FM and LPFM broadcasters was recently completed by MED. Discussion on the requirements for analysis and those processes is available in the Technical Monographs, TM2001/003 Review of FMBC and aeronautical coordination issues, and TM2007/001 Aero-FMBC Coordination processes. These are available on the RSM website⁵.

Aero-FMBC coordination calculations are currently based on the use of the older Montreal aeronautical receiver. CAA recently reviewed this requirement and plans to mandate use of the newer ICAO 1998 receiver by all aircraft equipped for flying under instrument flight rules later in 2010. The mandate will take effect 12 months from the date of publication.

The ICAO 1998 receiver has greater immunity to receiver IM and calculations based on its use have indicated that restrictions currently required on VHF-FM service power levels will be significantly reduced. MED therefore proposes that the LPFM GURL and GUSL be reviewed in late 2011 to take into account the ICAO 1998 receiver performance.

4.3.1.1 Previous Aero-FMBC Calculations

MED’s recent Aero-FMBC coordination calculations, which were based on the current LPFM (maximum e.i.r.p. of 0.5 Watts) and VHF-FM broadcasting installations combined with Montreal receiver parameters, identified that:

• There are no current coordination (harmful interference) issues between aeronautical and LPFM of VHF-FM broadcasting services due to aeronautical receiver IM or overloading (desensing);
• There are some locations where marginal coordination situations exist that could, if the aeronautical and broadcasting services were operating outside their normal operating or licensed parameters, result in harmful interference to aeronautical services; and
• LPFM lower band frequencies (below 88 MHz) will not normally contribute to aeronautical-FMBC coordination issues.

4.3.1.2 Updated Calculations for Expected Services (1 June 2010 on)

More recent Aero-FMBC coordination calculations have been completed based on:

• Use of both Montreal and ICAO 1998 receiver parameters;
• Both 0.5 Watt and 1 Watt LPFM services;
• Inclusion of existing, planned and likely future VHF-FM broadcasting services (likely future services were considered only for major VHF-FM sites adjacent to major airport landing approaches); and
• Inclusion of an allowance for normal equipment operational variations, e.g. aging components.

These calculations using the Montreal receiver parameters identified some locations where the aeronautical receiver safety margins could be exceeded. In such areas, there may be a need to prohibit transmissions or require reduced transmitter output of one or more LPFM or VHF-FM broadcast services. This would ensure that IM components do not exceed appropriate safety margins.

Calculations completed using the ICAO 1998 receiver parameters indicated no locations where the aeronautical receiver safety margins could be exceeded.

4.3.1.3 Protection from Aeronautical Receiver intermodulation

MED considers that the protection of aeronautical services from aeronautical receiver IM requires that one or more of the signals contributing to that IM are reduced in level or removed entirely.

Where a contributing signal is radiated by a high power VHF-FM signal, its power level is determined using Aero-FMBC calculation processes prior to the service being licensed.

Where the contributing signal is radiated by an LPFM service, reduction of the signal level is difficult to monitor and prohibition of use of that signal is necessary in the area where the IM would otherwise occur.

Areas where LPFM services on particular frequencies are not permitted are identified for the purposes of this document and the proposed GURL as ‘LPFM exclusion zones’. It is proposed that where such zones are required, they will be included in an LPFM GURL and maps of the zones will be published on the RSM web site.

Table One (below) identifies the LPFM exclusion zones and prohibited LPFM frequencies for 0.5 Watt and 1 Watt LPFM transmit powers and aeronautical receive types.

Table One: LPFM Exclusion Zones required for Protection of Aeronautical Services

The size of the exclusion zones does increase when LPFM power is increased from 0.5 Watt to 1 Watt. However, because of the effect of antenna vertical radiation patterns, the variability of ground heights and margins allowed for aircraft lateral position when landing, the increase is insignificant for the first three zones in the table.

Having regard to the limited number of exclusion zones and the shortness of the period they are required (i.e. less than 18 months), MED considers the following questions are appropriate.
 

Question 6

Do you agree with the proposed LPFM exclusion zones to protect aeronautical navigation services? If not, what specific measures do you consider appropriate?

4.3.2 LPFM Transmitter Generated Intermodulation Issues

Transmitter generated IM, a component of transmitter unwanted emissions, can occur anywhere where there is an LPFM transmitter operating in close proximity to other transmitters, including high power VHF-FM, landmobile and other LPFM transmitters.
MED investigation following reported incidents of LPFM service interference to aeronautical COM (voice) services noted that:

• Transmitter IM can be generated as a result of signals received via the transmitter antenna mixing together and with the wanted transmitter signal to produce new signals that are then radiated via the transmitter antenna;
• Harmful interference occurred to COM services used by low flying aircraft that was caused by co-sited LPFM services in pilot controlled airspace, adjacent to helipads and airport zones; and
• One incident involved two co-sited LPFM transmitters, one fitted with an antenna circulator and the other without. In this instance the IM was found to be from the transmitter without the circulator.
   

4.3.2.1 LPFM Transmitter Susceptibility to Transmitter Intermodulation

MED understands that most LPFM transmitters are generally low cost devices using Class C amplifiers. These tend to generate and transmit IM in the presence of multiple signals (particularly when operated close to and at transmit power levels above their design limits).

Unlike high-powered broadcasting transmitters, LPFM transmitters do not have equipment between the antenna and transmitter to prevent the reception of high level signals.

4.3.2.2 Protection from Transmitter Intermodulation

Steps that can be appropriate to reduce transmitter IM include the:

• Specification of appropriate minimum distances between LPFM and other transmitters before the use of antenna isolation devices⁶;
• Specification of maximum antenna mounting heights;
• Use of LPFM exclusion zones adjacent to sensitive aeronautical receiver sites and where aircraft are close to the ground; and
• Use of antenna isolation devices between the LPFM antenna and transmitter for LPFM transmitters operating in close proximity to other LPFM, VHF-FM broadcasting, landmobile or aeronautical transmitters⁷.
   

MED acknowledges that the costs of antenna isolation devices (typically $200 to $600) will be of concern to LPFM operators. Nevertheless, MED is concerned at the number of interference incidents that have resulted from transmitter IM and believes that while there are standards in place aimed at the prevention of the radiation of unwanted emissions⁸, further steps could be taken to further protect aeronautical services.

Question 7

Do you agree with MED’s proposal to mandate the use of transmitter–antenna isolation devices when transmitting within the following separation distances:

1. Everywhere; or
2. Within 100m of other LPFM transmitters; and
3. Within 100m of other transmitters operating pursuant to a radio licence in adjacent frequency bands (75 - 87.5 MHz and 108 - 137 MHz); and
4. Within 2000m of any licensed VHF-FM broadcast service?

4.4 General LPFM Transmitter Issues

4.4.1 Other Transmitter Unwanted Emissions

The current LPFM GURL includes in its terms, conditions and restrictions the need for transmitters must conform to the requirements prescribed in notices made under Regulation 32 (1) (b) of the Regulations. Those notices include the Radiocommunications Standards Notice which identifies unwanted transmitter limits in Table 3 – Low Power FM.

It is suggested that this table could be included in the LPFM GURL to ensure LPFM operators are aware of the requirement.

Question 8

Do you agree with the LPFM transmitter unwanted emission limits being repeated in the LPFM GURL?

Question 9

Are there alternative options that will provide equivalent or better protection from the transmission of excessive unwanted emissions and prevent harmful interference to aeronautical COM and other services?

If so, what are those options and how are they better?

4.4.2 Transmitter Frequency Stability

Some LPFM transmitters have been found to drift in frequency across the LPFM and adjacent frequency bands when transmitters fitted with phase lock loop (PLL) technology do not have fail safe control. Such transmitters have been known to cause interference to broadcast, landmobile, aeronautical and other local LPFM services.

In order to comply with the proposed GURL and GUSL, LPFM operators may need to upgrade their equipment to reduce the potential for the radiation of interfering signals.

Question 10

What other changes should be implemented to improve the protection of adjacent band licensed services from transmitter stability?

4.5 Mobile LPFM Transmitter Issues

MED is concerned that there is potential for mobile LPFM units to contribute to aeronautical receiver IM, aeronautical transmitter IM in LPFM exclusion zones. Use of mobile LPFM devices has not been considered as significant potential cause of harmful interference.

MED therefore considers the following LPFM exclusions appropriate:

• Adjacent to airports and helipads;
• Those identified in Table One: LPFM Exclusion Zones required for Protection of Aeronautical Services; and
• Aircraft. This is also intended to limit the potential for interference to broadcasting, landmobile and other LPFM services.

Question 11

Do you agree with the use of LPFM exclusion zones adjacent to airports and helipads for the protection of low flying aircraft, as identified in Attachment B – Proposed LPFM GURL and the Attachment C – Proposed LPFM GUSL?

Is there a better means of achieving the equivalent protection? If so what are they and how do they achieve the protection?
 

Question 12

Do you agree with the proposal to prohibit LPFM

Footnotes

4 Direct link at http://www.rsm.govt.nz/cms/customer-support/business-updates.

5 Direct link at http://www.rsm.govt.nz/cms/engineers-and-examiners/rsm-technical-monographs

6 MED is aware that there can be difficult when the radio path parameters are unknown, e.g., losses due to path obstructions and antenna types, azimuths and mounting heights.

7 Directional couplers, circulators or band pass filters in the transmitter to antenna cable are available at a cost of between $200 and $600.

8 LPFM transmitters are required to conform to the unwanted emission limits contained in the Radiocommunications (Radio Standards) Notice, Table 3 – Low Power FM, which includes a requirement to minimise transmitter generated IM.