WJIB-AM 740 translator on 101.3 FM

WJIB is beloved by Boston/Cambridge listeners for its one-of-a-kind playlist and commentary from Bob Bittner, who runs companion AM/FM stations in coastal Maine. WJIB-AM 740 is also on FM translator W267CE 101.3 FM from the same tower as WJIB (Note, the radio-locator website uses the wrong callsign). Much like the Boston Bloomberg Radio 106.1 FM translator, WJIB will displace pirate broadcasters notoriously causing trouble after repeated FCC citations.

FM Translator co-channel interference

The WMRC 101.3 FM translator in Milford, MA is only 34 km away and may limit coverage to the western Boston suburbs. However, this is the nature of modern FM broadcast, and the new approach of the FCC to squeeze in more stations is to be lauded. Modern FM receivers digitally narrow their bandwidth to accommodate strong adjacent signals, and the low capture ratio of wideband FM ensures a relatively pleasant transition between stations at the edge of the coverage area. Roughly 10 dB of desired/undesired (D/U) signal ratio is required on WBFM (broadcast FM 88-108 MHz) for a listenable > 40 dB audio SNR output.

The WMRC 101.3 W267CD translator has mild directivity to the north, but spills signal excessively into the Boston area. This hurts WMRC as well since they are wasting power toward areas where WJIB interferes heavily. WMRC should consider reaiming the W267CD antenna to improve WMRC and WJIB coverage. I would be glad to advise WMRC/WJIB in this effort.

FM translator interference map with interference in red and coverage in green.
WMRC interferes with WJIB on 101.3 FM. Area of significant (< 10 dB D/U) interference in red and coverage (> 10 D/U and > -100 dBm to automotive FM antenna) in green.

Pirate

An illegal broadcaster from Dorchester, MA was previously using 101.3 as well. They caused destruction of nearly 70% of WJIB coverage area. Below is an illustration of just the regions of Boston that were affected–all the southern suburbs and even regions to the east and north of Boston are impacted severely by this pirate. The Fenway area on east was highly negatively impacted by the pirate.

However, the pirate has moved for the time being to 100.3. However there is another legal station coming to 100.3 in the next few months, so the pirate may move again….

pirate FM translator interference map with interference in red and coverage in green Greater Boston elevation map. pirate FM translator interference map with interference in red and coverage in green. pirate FM translator interference map with interference in red and coverage in green.
Pirate interference to WJIB FM translator, summer 2017.

It was futile for the Dorchester pirate to continue operating on 101.3 in any case, since the more powerful WJIB cut pirate coverage to 1/2 - 1 mile radius from the transmitter. The pirate in effect covered about 5 sq. miles, yet interfered with over 350 sq. miles of WJIB coverage. This is why we have an FCC and long distances between co-channel stations, even when one or both are very low power (< 250 watts).

Intermodulation

The Dorchester location has three tall antennas, assuming each dedicated to the illegal transmissions on these frequencies:

  • 87.75 MHz
  • 100.3 MHz
  • 105.3 MHz

these transmissions are clipped/overmodulated, and this causes interference to other Boston area FM stations from splatter. Additionally, intermodulation from the three closely-spaced antennas occurs in the poorly-filtered transmitters. Combinations of multiples of the transmitter frequencies makes out-of-band interference products that bother services such as Logan Airport radio frequencies. Various intermodulation spreadsheet calculators exist. Also a Pascal intermodulation calculation program with source code I modernized to work with current Pascal compilers.

Why AM stations need FM translators

For low power (or no power) nighttime Class D AM stations like WJIB 740 AM, distant AM stations drastically reduce nighttime coverage. WJIB 740 AM single-tower omni-directional coverage uses 250 watts by day, but only 5 watts by night. This means you need to almost see the WJIB tower at night to receive it on 740 AM as illustrated below.

Northeastern US night-time AM skywave overlap 740 kHz
WJIB 740 AM is crushed at night by a Canadian station. Plot courtesy http://www.nf8m.com/patternmaps_night.html.

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