Châteauguay’s CHAI-FM seeks to replace two transmitters with one

If you look at a list of radio transmitters in the Montreal area, you’ll find a listing at 101.9 MHz for CHAI-FM, a community radio station in Châteauguay. But you’ll also find one called CHAI-FM-1 in Candiac, also at 101.9 MHz.

It’s an unusual solution to a coverage problem to have a repeater on the same frequency, and CHAI is the only one in the area that attempted it. There’s a reason for this: talk to any broadcast engineer and they’ll tell you that while it can be done, it’s very tricky. If the stations aren’t perfectly synchronized, people between the transmitters can hear unpleasant sounds and echoes.

CHAI-FM proposal: A new transmitter (green) replaces the two old ones (red and blue).

CHAI-FM proposal: A new transmitter (green) replaces the two old ones (red and blue).

So CHAI has decided after less than a decade to abandon that plan and instead seek changes to its primary transmitter (an increase in power, change in pattern and shift of location and height) to allow it to cover both the city of Châteauguay and the MRC de Roussillon with one signal. As you can see from the map above, the engineers have done a pretty good job of replicating the two coverage areas with one signal.

The new CHAI-FM would transmit from atop the Châteauguay water tower in the eastern corner of the city, using a directional antenna and a power of 238 watts, up from 100. The height above average terrain would go from 50 to 66.7 metres. (The city approved the installation unanimously in a council meeting on Dec. 1, 2014, setting a $350 a year rent plus taxes and $460 a year for electricity, a deal of five years renewable twice.)

Being so close to Montreal, the signal has to be careful not to interfere with other existing ones. An engineering analysis found potential interference issues with a half dozen stations but managed to minimize them:

  • CBMG-FM Cowansville (101.9): CHAI and this CBC Radio One transmitter would cause interference to each other, but CHAI notes that the area of CBMG’s signal it would interfere with, centred around Iberville, would be covered by Radio One’s main Montreal transmitter at 88.5 FM, which carries identical programming. CBMG could cause interference to CHAI in Candiac, Delson, Lery and the southern West Island.
  • CJSS-FM Cornwall (101.9): Though they operate on the same frequency, the analysis found CHAI would not interfere with CJSS and CJSS’s interference with CHAI would be minimal, confined to a sliver of its pattern southwest of Lery.
  • WCVT-FM Stowe, Vt. (101.7): No potential interference was found here unless WCVT were to increase to its maximum theoretical power, which it couldn’t do anyway because that would interfere with CBMG.
  • CHPR-FM Hawkesbury, Ont. (102.1): The stations are far enough apart in space and frequency that there are no interference issues.
  • CIBL-FM Montreal (101.5): CHAI would cause some interference to reception of the Montreal community station in the area southeast of CHAI’s transmitter, but that interference would be less than is currently caused by both CHAI-FM and its retransmitter. CIBL would not cause interference to CHAI.
  • CINQ-FM Montreal (102.3): The mutual interference situation for CINQ is virtually identical to that of CIBL.

The worst interference issue both ways is with the Cowansville station, and that’s the only one that would actually increase a non-trivial amount under this scenario. CHAI’s proposed parameters go as far as they can without leaving a coverage hole for CBC Radio One (an area that isn’t within either coverage area of CBME-FM 88.5 or CBMG-FM 101.9).

The CRTC is accepting comments on this application until Jan. 18. You can download the application here (.zip) and comment here. Note that all information submitted, including contact information, becomes part of the public record.

4 thoughts on “Châteauguay’s CHAI-FM seeks to replace two transmitters with one

  1. Dilbert

    For me, the case could be made to ask CBC to move or change the frequency of their retransmitter in Cowansville, or perhaps to work to recontour it as well. Clearly it covers more area than it needs to.

    Other than that, this looks like a winning idea – better signal, lower costs, and improved coverage without hurting anyone else.

    Reply
    1. Fagstein Post author

      For me, the case could be made to ask CBC to move or change the frequency of their retransmitter in Cowansville, or perhaps to work to recontour it as well. Clearly it covers more area than it needs to.

      How is that clear? Its signal goes from the Richelieu river to Magog, covering the area between the Montreal and Sherbrooke signals. There’s some overlap, but the alternative would be having more gaps in coverage.

      Reply
      1. Dilbert

        The issue is that the lobe of the pattern towards St Jean is perhaps a little strong and is in some cases an overlapping signal with an equal signal from the Montreal transmitter. snipping a bit off the lobe on that side to avoid interference doesn’t appear to cause much change in service for those people, but would ADD a new service for them to listen to on 101.9. It would seem to be a win for the citizens of the area.

        Reply
  2. Tim

    I confirm that 88.5 CBME-FM is crystal clear in the far end of Iberville with a car radio antenna. And, you know, mine is probably the only antenna tuned to that CBC Radio One when I’m there.

    Reply

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