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).

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).

UPDATE: The application was approved by the CRTC on Nov. 24, 2016. On Oct. 25, 2018, the station was given a two-year extension to implement the change, explaining that it had undertaken the process, including buying the antenna, doing an engineering study of the tower, and seeking submissions for the actual antenna installation.