Some astute listeners may notice that when driving down Rt.18 from I-195 to downtown New Bedford, that when you get just past the US-6 turnoff to Pope’s Island…and lasting until roughly the Cannon Street Power Plant…there’s a strange “weakness” to our 89.3FM signal. As in, a lot of bips of static tend to happen.

It’s very sporadic, and doesn’t happen for everyone. And it can come and go by just driving a block or two, even just a dozen feet. And it’s loosely centered on the State Pier Maritime Terminal.

Turns out, there’s two reasons for that:

1. The hill that most of New Bedford is on causes a “shadow” of the 89.3 signal there, making it weaker than usual right at the bottom of the hill where Rt.18 (and the State Pier) is located.
2. There’s a nasty intermodulation product (aka “intermod”) between the two local FM stations in New Bedford: WCTK 98.1 and WFHN 107.1, and it appears on 89.1 MHz…right next to 89.3FM.

The latter is the real problem, but the former certainly exacerbates it.

Intermod is a mathematical combination of two local signals creating duplicates of the signals at other frequencies. It can get really really complicated when you have a whole bunch of FM signals all broadcasting from the same location, but in this case it’s pretty simple: there’s WCTK 98.1 MHz from that giant self-supporting tower near the hurricane barrier, and then there’s WFHN 107.1 MHz from a smaller tower on Pope’s Island.  

WCTK is station A, WFHN is station B. The formula is 2A – B = X. Therefore…

• 2 * 98.1 – 107.1 =
• 196.2 – 107.1 =
• 89.1 MHz…right next to our 89.3 signal!

And indeed, if you drive around the State Pier with your radio tuned to 89.1, you’ll hear this bizarre mashup of WCTK Country music and WFHN Top 40 music at the same time, in a signal that comes and goes from nonexistent to very strong within a few feet.  

The real rub is that I’ve run several tests with the cooperation of the engineers at both stations, and near as we can tell, the intermod isn’t coming from either of their towers. It’s something else in the vicinity of the Pier, re-radiating both signals. And it could be anything. It doesn’t have to be an FM radio or transmitter. It could be a marine radio or antenna. It could be a shore-to-ship (or ship-to-shore) radio. It could be a TV reception antenna. Or a GPS antenna. Or…and I swear I am not making this up…it could just be a loose metal bolt. Yes, I’ve heard a true story of a loose bolt on a 950MHz STL dish on one tower out west that was re-radiating an intermod product. That sucker took years to find.

Considering the apparent strength of the intermod signal near the Pier, and the presence of dozens of commercial boats and the accompanying communications infrastructure for those on land to talk to those boats, I’d guess it’s probably a marine radio with insufficient grounding and/or signal filtering. But there’s no guarantee of that, and it could take months of concerted effort to isolate the offending problem.

Never mind work with the owner of the unintentional radiator to resolve the issue. After all, if you own a commercial fishing boat that needs to makes profit, and you’ve got a $5000 radio? Do you want to admit that your radio needs to be replaced with a $10,000 model when – from your perspective – it works just fine? I can certainly sympathize.

If I had to guess, I’d say this probably wasn’t a problem until WCTK had to build that new tower to accommodate the Mass CEC using the parcel for offshore wind farm equipment. Most likely that subtly shifted various “hot spots” from WCTK’s signal, and now it’s strong enough in just the wrong location to create the intermod product. Also when we moved 89.3 westward from the UMass Dartmouth campus to the Tiverton tower in 2018, it made for a much better signal overall but it wasn’t as super-strong in New Bedford thanks to the physically-more-distant tower location. That super-strong 89.3FM signal was enough to mask any problems from the intermod on 89.1FM.

Unfortunately there isn’t a terribly good solution for this at the moment. Someday when I’ve got more time and/or more engineers we’ll go on a hunt to find and fix the unintentional radiator putting out that intermod product. For now? Just know that, no, you’re not going crazy. 🙂 There’s just a small three-or-four block area where our signal is a little weak.

Ed.note: the intermod product is no longer very detectable ever since WBUH 89.1 moved to their new, taller tower in Brewster MA in early 2020. It’s not clear whether or not the presence of WBUH has changed the nature of the undesired signal such that most radios no longer “perceive” the “interference” to WNPN 89.3 anymore. Your Intrepid Engineer’s car radio does seem to hear less interference to 89.3 in the area by the State Pier, but 89.3 is still kinda “weak” due to terrain shadowing from the hill New Bedford is built on.