INTRODUCTION

The coordination of the Brookdale ATV Repeater System in Lincroft, New Jersey has been delayed many years due to internal conflicts within TSARC and the lack of an established 70-cm bandplan. While frequency coordination on a band filled with a multitude of emissions is not a trivial matter, it is important to realize that in the eyes of the Federal Communications Commission, every communications mode available to Amateurs has a right to exist, and no frequency coordination council, including TSARC, can exclude any mode of operation in favor of another. If TSARC is to operate in the interest of all the Amateurs in the area they claim to represent, then the FCC requires the coordination council be sensitive to the concerns of all Amateurs under their jurisdiction.

The 426.250 MHz in / 439.250 MHz out frequency pair proposed for the Brookdale Amateur Television Repeater System was recommended by past TSARC President Mr. Charles Motes. The discussion that follows provides justification for adoption of this frequency pair in the TSARC 70-cm bandplan, and tackles interference issues of concern to other users of the band and offers possible solutions.

FREQUENCY CONSIDERATIONS FOR COORDINATION OF THE
BROOKDALE AMATEUR TELEVISION REPEATER SYSTEM

The designers of the Brookdale ATV Repeater system wish to make their system available to the maximum number of users possible to encourage the educational and public service aspects of Amateur Radio, as well as foster further experimentation and research in the field of telecommunications as provided through the Amateur Radio Service and encouraged by the Federal Communications Commission.

The Brookdale ATV Repeater system will use a horizontally polarized antenna system and receive amplitude modulated NTSC Amateur Television transmissions having a video carrier frequency of 426.250 MHz. The use of 426.250 MHz will keep all high frequency video sideband components as well as the FM audio subcarrier of repeater users within the 70-cm Amateur band and 70-cm repeater subband, while at the same time avoiding interference both to and from horizontally polarized weak signal operations centered around 432 MHz. 426.250 MHz is a well recognized ATV video carrier frequency for both repeater and simplex communications. In fact, half of all ATV transmitters sold to date by PC Electronics have had this frequency requested. A video carrier frequency lower than this will cause the lower sideband of the AM video transmission to fall below the 70-cm band edge at 420 MHz which is a violation of FCC rules. A higher carrier frequency will cause upper sideband energy to fall within the 431 MHz to 433 MHz subband where weak signal operations currently take place and repeater communications are forbidden by the FCC.

Considering the current state of the art, it is unrealistic to expect that ATV users whether communicating on a simplex channel or through an ATV repeater system will generate vestigial sideband (VSB) ATV emissions. VSB is impractical for variable frequency Amateur TV operations. The only commercial manufacturer of VSB equipment, Advanced Electronic Applications, discontinued production of its 1 watt VSB-70 ATV transceiver. When amplified using practical amplifying devices and circuits available to Amateurs, vestigial sideband emissions become distorted due to non-linearities and intermodulation distortion products generated by the amplifier through the amplification process. The result is a regeneration of the lower video sideband, lower color subcarrier, and lower FM audio subcarrier that was so carefully removed by the VSB filters in the exciter. An external combline or interdigital bandpass filter could be used in-line between the transmitter output and the antenna feedline to reduce the amplitude of the lower video sideband and produce a VSB emission. However these filters are expensive and their fixed frequency nature makes them unattractive to Amateurs and their bulk makes them impractical for portable transmitters.

In contrast, the Brookdale ATV repeater system, being a fixed frequency Amateur station, will use an interdigital bandpass filter and VSB duplexer between the output of its transmitter and the antenna feedline and will transmit using a vestigial sideband emission. Not only is VSB a TSARC requirement for ATV repeaters, but the use of transmit and receive bandpass filters is a necessity when operating a repeater station such as the Brookdale ATV repeater system in a high RF environment as it reduces the possibility of transmitter cross modulation and its resulting spurious emissions. The Brookdale ATV repeater will be sharing tower facilities with a large and growing number of commercial radio services as well as a 11 kilowatt ERP FM broadcast station, WBJB-FM.

As with our input frequency, the use of 439.250 MHz as a repeater output frequency is a compromise between conflicting factors that constitute the reality of today's Amateur Radio Service. A VSB-AM Amateur Television emission having a video carrier frequency of 439.250 MHz will occupy a 6 MHz bandwidth between 438 MHz and 444 MHz. The use of 439.250 MHz effectively keeps the lower vestigial sideband out of the 435 MHz to 438 MHz International Amateur Satellite subband, while maintaining a substantial (20 dB minimum) degree of isolation to FM voice repeaters and packet radio operations above 440 MHz by using a horizontally polarized antenna system. The interference threat to satellite communications is very real if DSB-AM were used on 439.250 MHz as would be the case if this frequency were used as an ATV input frequency. This would cause the lower video sideband to extend as low as 434.750 MHz, blanketing the entire OSCAR satellite subband. In addition, satellite signals are weak in nature and satellite communication links utilize circularly polarized antennas that offer no isolation to horizontally polarized ATV signals.


70-cm band


While it may appear that an interference potential exists to FM users up to 444 MHz, this fear is unfounded. The spectral power density of a VSB-AM video transmission varies with picture content, but typically 90 percent of the video sideband power is contained within 1 MHz of the video carrier, and those sidebands are typically 40 dB below the video carrier amplitude. An additional 20 dB or greater cross polarization isolation with FM users is provided since the ATV repeater is horizontally polarized, and the FM and packet users are vertically polarized, making the interference risk from the ATV repeater to FM users at least 60 dB below the ATV video carrier. This equates to a signal level of 60 microwatts, hardly a cause for alarm. In addition, FM is a strong signal mode and exhibits a capture effect that offers protection against such weak interference.

The interference situation is not mutual, however. Wideband AM television communications requires a carrier-to-noise ratio in excess of 40 dB and a dynamic range at least as high for satisfactory image quality. This means an FM carrier 40 dB below the video carrier amplitude of a television signal will cause considerable interference to the television signal if it is placed anywhere within the 6 MHz television bandwidth. Wideband AM television is very susceptible to interference as anyone who has struggled with TVI will attest. Conversely, a video sideband product 40 dB below the amplitude of an ATV video carrier will, in all probability, cause no harmful interference to a narrowband FM radio link. With this in mind, it is only logical that 439.250 MHz be used as an ATV repeater output frequency rather than an input frequency as this will eliminate the possibility of the ATV repeater transmitting a video signal corrupted by packet bursts and continuous FM repeater interference received on its input.

In addition to providing greater repeater communications reliability, a 439.250 MHz carrier output frequency also makes readily available the reception of repeater transmissions using varactor tuned handheld television receivers such as those marketed by Radio Shack, Casio, and other manufacturers. Reception of 439.250 MHz with these receivers is possible simply by tuning below UHF channel 14.

The ease of reception these portable receivers provide for ATV signals along with the elimination of a downconverter requirement is a valuable asset in providing Amateur Television communications especially during periods when the repeater is used for relaying emergency or public safety communications, such as during walk-a-thons and parades. It should be realized that the true value of a repeater system is in its ability to make portable operations practical, and making portable operations practical is one of the reasons behind having an ATV repeater.

The use of 439.250 MHz as an output frequency also makes possible the reception of Amateur television signals using consumer-grade television receivers or VCRs with "cable ready" tuners by attaching a UHF antenna and tuning to cable channel 60. This ability will not only help introduce newcomers to the fascinating world of Amateur Radio, but will also accelerate the involvement of local schools in ATV related events. The Brookdale ATV Repeater System will have the ability to re-transmit live NASA Select Television programming during US Space Shuttle missions providing local schools the ability to view and participate in classroom in space activities without the need for a TVRO groundstation.

In further support of the ATV repeater frequencies outlined here, it is important to realize that FM interference to ATV repeaters using high in / low out frequency pairs is a matter of record in some areas of the United States with the rapid proliferation of FM repeaters and packet radio stations on the 70-cm Amateur band. In some cases this interference has rendered previously existing ATV repeaters useless. Moreover, such reverse splits would not be in the interest of good engineering practice since they would place users at risk of causing interference to weak satellite communications and would be a clear violation of FCC rules and regulations.

It should also be pointed out that 439.250 MHz has been a well established ATV video carrier frequency for many years. Amateur Television enthusiasts were in fact the first users of the 70-cm Amateur band. Unfortunately, self-serving interests and poor spectrum management decisions made by frequency coordinating councils in the past have allowed FM voice repeaters and packet radio stations to infiltrate the 438 MHz to 444 MHz subband over the past several years. This has occurred despite the long established recognition of 439.250 MHz as an Amateur Television frequency by the US ATV Society and the American Radio Relay League.

The 420 MHz to 450 MHz 70-cm frequency allocation is a spectrum resource that is shared with government fixed, mobile, and radiolocation services who are primary users of the band. Considering that Amateurs are secondary users, the FCC requires that adequate measures be taken by Amateurs to avoid interference to government services. High power pulsed radiolocation transmissions are easily detected on an ATV receiver, and considerate ATV operators cease operations when radar signals are seen to avoid interference with radiolocation services sharing the 70-cm band. It would be an interesting exercise to determine what mechanisms, if any, FM repeater and packet radio users, who employ hard limiting receivers designed for immunity against pulsed interference, use to identify the presence of radiolocation transmissions, and what measures these users take to avoid interference to primary government radiolocation services on the band.


INTERFERENCE POTENTIAL AND POSSIBLE SOLUTIONS


There has been some concern among 70-cm FM repeater users over a perceived ATV interference potential to their systems. Much of this concern, however, is based on emotions, not technical facts.

A 100-watt PEP ERP vestigial sideband AM video transmitter modulated by NTSC source material on a video carrier frequency of 439.250 MHz occupies a 6 MHz bandwidth between 438 MHz and 444 MHz and has the following attributes:


439.250 MHz Video Carrier at 0 dBc = 60.0 Watts ERP horizontal
442.830 MHz Color Subcarrier at -22 dBc = 0.4 Watts ERP horizontal
443.750 MHz Audio Subcarrier at -15 dBc = 2.0 Watts ERP horizontal


Considering a 20 dB cross-polarization loss, the ERP at these frequencies as seen in the vertical plane is as follows:


439.250 MHz Video Carrier 0.6 Watts ERP vertical (600 mW)
442.830 MHz Color Subcarrier 0.004 Watts ERP vertical (4 mW)
443.750 MHz Audio Subcarrier 0.02 Watts ERP vertical (20 mW)


The most significant component within this bandwidth is the video carrier which occupies only a single frequency. The second most significant component is the audio subcarrier which is typically at an amplitude of 15 dB below the video carrier. This is kept below the level of broadcast standards to minimize the effects of intermodulation distortion in the ATV repeater's linear final amplifier. The audio subcarrier employs frequency modulation and is limited to 25 kHz deviation. Assuming a 3 kHz maximum modulating frequency, the audio subcarrier occupies a 56 kHz bandwidth.

An analysis of VSB-AM video sideband energy shows that sideband products are not homogeneous. Instead, sideband energy "bunches" at the video horizontal repetition rate of 15,734.264 Hz within the entire video sideband. Armed with this knowledge, it is possible to determine on what particular frequencies these "bunches" or "pips" fall. It is also possible to select an optimum video carrier frequency that would avoid video sideband interference with other mode users on the 70-cm band. If a "pip" should cause interference to an FM voice channel, it is a simple matter to offset the ATV video carrier frequency several kilohertz to place the FM voice channel in a sideband hole. The maximum frequency offset required would be half the video horizontal repetition rate, or 7,867.132 Hz.

Commercial television broadcasters employ +/- 10 kHz frequency offsets as a method of reducing co-channel interference. There's no reason why Amateurs should not employ a similar mechanism for achieving a peaceful co-existence between Amateur Television enthusiasts and other mode users in the 70-cm band.

With this in mind, I recommend the adoption of a 426.250 MHz in, 439.250 MHz out Amateur Television repeater pair by the Tri-State Amateur Repeater Council in their 70-cm bandplan. I also encourage the adoption of transmitter frequency offsets as outlined here to be used as needed to reduce or eliminate bona fide cases of interference to other mode users.


John A. Magliacane, KD2BD




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