Radio Interference Along the Beltway

August 24, 2009 9:55 AM

What causes massively scrambled FM radio reception along the Capital Beltway near Falls Church, Virginia?

There is a stretch of the Beltway in Northern Virginia where the FM radio reception is abysmal. If I remember correctly, it is near the Falls Church and Fairview Park exits, but it's definitely south of Tyson's Corner. It was so when I was growing up there 20+ years ago (80s-90s) and friends who live there now say it remains so.

The problem always occurs in the same area and both day and night - you drive into it and no matter where you are tuned on the FM band, the station becomes unintelligible, the reception totally scrambled and overwhelmed by loud static.

The AM band is unaffected - and tuning to AM talk radio was always my father's answer to the problem. After a mile or two, maybe three, everything goes back to normal on the FM dial.

I have looked up the radio towers located in Falls Church, Merrifield, Vienna, and Fairfax.

Those pages list the commercial, private, and broadcast towers in those locations and their assigned frequencies, but I have no expertise that would allow me to judge whether those listed towers are the problem. I have some guesses based on location, but none of the towers I looked at seemed to be broadcasting in the FM band - but then again, I don't know if they'd have to be in order to interfere.

So do you know what is the cause of this Bermuda Triangle of radio, or can you help in interpreting the tower listings to potentially identify the problem? Thanks.

posted by jocelmeow to technology (10 answers total)

FWIW, I live in the center of the City of Falls Church and have never noticed this.
posted by phrontist at 10:37 AM on August 24, 2009

Never noticed it, but isn't that in right in the middle of a polygon connecting the CIA, Dulles, the Pentagon and Reagan National? Insert head in tinfoil hat as you wish...
posted by grateful at 11:03 AM on August 24, 2009

My girlfriend worked down there when we lived in DC, she'd call me on the phone on the way home. Inevitably, the call would drop out right near Falls Church. We always chalked it up to the various intelligence agencies that all have facilities in that area.
posted by Oktober at 11:12 AM on August 24, 2009

Yep, grateful and Oktober, in thinking about this previously it had occurred to me that this "mystery spot" was far enough from all of those "known" locations - Langley being the closest - as not to be thus explained. But I suppose it being metro D.C. means there are always things like those black lines as possibilities...
posted by jocelmeow at 11:15 AM on August 24, 2009

I've never noticed anything, and I drive through that stretch of the Beltway frequently. I typically listen to either WWDC (DC 101), which broadcasts from a tower up in Chevy Chase, or WAMU, which is out of the American University campus in the District. My car's radio, however, is about as sensitive as a rock with a piece of bell wire tied to the end, so if the problem is front-end overload it might not be experiencing it to the same extent that a more sensitive receiver is.

The list of broadcast sites you're looking at is only "land mobile", which is a fraction of the spectrum users that you might encounter. So I'm not sure if that's going to tell you much.

The Beltway and the I-66 corridor in particular is a very RF-dense area, but I've never heard of QRM so bad that it was completely obliterating FM broadcast stations.

isn't that in right in the middle of a polygon connecting the CIA, Dulles, the Pentagon and Reagan National

Not really. The CIA is in Langley, which is on the Potomac near McLean, off of the GW Parkway. Their line-of-sight to the Pentagon wouldn't go anywhere near Falls Church, or even cross the Beltway at all; it would be down through the Great Falls area. (Where, incidentally, Prince Turki Al Faisal has a house/compound, providing much better fodder for conspiracy theories if you wanted to start one. I suspect he just likes the view of the falls, though.) The Dulles/Pentagon line would also be too far north to affect the area in question. (For that theory, you'd want to involve the giant microwave tower in Tysons Corner at Rts. 123 & 7.)

It's likely to be something totally boring like a transformer that's arcing, combined with some specifics of the receiver in question.
posted by Kadin2048 at 11:39 AM on August 24, 2009

That patch of beltway is real, and still exists. I'm not sure exactly why it happens, but most people blame it on the CIA. I have no proof of that, of course.

My dad commutes on that part of the beltway, and frequently calls me on his way home from work. Thats why most conversations with my dad include a very frustrating bit of "What? I can't hear you, what?" to go along with the "what are you doing with your life?" and "you should go to law school" or "Get a MBA." Therefore, I can only fake cell static or the call being dropped with passing convincingness. Every once and a while he's actually near that part of the beltway and I luck out. However most times he tries to wrap up the conversation before that stretch, knowing it's coming up.
posted by fontophilic at 11:47 AM on August 24, 2009

Yeah, radio interference is "old school" - stuff that I think of near old embassies as recently as the late 90's... (think Russian above Georgetown... used to be ridiculous interference through the power lines that served the area... had to buy expensive filtering interconnects for stereo)...

I'd imagine lower end dampening fields - lower budget stuff that is less discriminant - around private homes/ businesses/ foreign nationals.
posted by Jiff_and_theChoosyMuthers at 12:33 PM on August 24, 2009

After looking at a topo map, I suspect that the explanation involves that being the place where the Beltway crosses Holmes Run, and is probably a bit lower than the surrounding terrain. Being in defilade there might explain the poor reception, especially on FM but not AM.

Because it's such a built-up area, the USGS maps don't have a lot of contour lines, but I actually happen to have a GPS track, taken a couple of years ago when I drove a route that happened to go from 236W around the Beltway to 66W with my Garmin sitting in the cup holder. (If anyone is particularly interested in this, let me know and I can send you the .gpx or .kmz file. GIS is not really my thing; I'm just looking at it in Google Earth.) It has waypoints taken every few seconds which include an altitude measurement. They're probably only accurate to a few meters at best, but it does confirm that there's a dip in there.

Near the 495/66 interchange, the altitude reports as 114m ASL. This drops down to 110 or 111m near the Rt. 29 underpass. (Google Earth's own elevation measurements show it dipping in there, to around 103m or so. I suspect the difference in absolute measurement is due to map datum issues.) By the Route 50 cloverleaf, it's back up to 117m ASL, and Google's is up to 106m. South of 50 (near the hospital and the Fairview Marriott) it dips again, and then comes back up a small hill near Gallows Road. South of Gallows Road, it slopes back down and there's a significant downhill as you come into Annandale.

My suspicion is that if you had a good topo map that showed you all the surrounding terrain, you'd find out that the little (12' or so) dip south of I-66 is creating a "shadow" that, in combination with other hills and manmade obstacles located between there and various transmitter sites, is hurting radio reception. Maybe if you had a GIS package, you could even draw lines from various known transmitter sites and look at the altitude over them, and see what's going on.

Given that FM is practically VHF, a 12' difference in antenna positioning could very well cause a significant difference in reception depending on the environment and the sensitivity of the receiver in use. Like I said in the other comment though, I've never actually experienced this problem myself, and I've driven all through that area while listening to FM and operating 2m amateur VHF. It's not a total "dead zone" for everyone and everything.
posted by Kadin2048 at 12:35 PM on August 24, 2009 [1 favorite]

Actually, FM is VHF. It's in between channels 6 and 7.
posted by gjc at 4:11 PM on August 24, 2009

Thank you, everyone, for your contributions.

I was hoping that there would be an easy, obvious answer on the source, but that doesn't seem to be the case. Kadin2048, yours sounds most plausible, but I have to say, the interference doesn't sound like the typical hill-and-hollow interference I've encountered. It sounds more like what happens when a signal's being overwhelmed by a stronger one nearby on the band. I'm a total non-expert, though, obviously.

gjc, your thought on VHF and FM's proximity made me wonder if WJLA, the D.C. area ABC affiliate on Channel 7, had a broadcast tower nearby. I looked it up, though, and their transmitter tower seems to be in Tenleytown in the District.

I also posted this question to my facebook profile - several people chimed in to say they remembered the issue back in the 90s and that it was the same today. So though not everyone is encountering it, there are enough who immediately knew exactly what I was referring to that I don't think it is a fluke of one or two flaky car radios.

Anyhow, thanks again for your help - I guess I'm going to have to chalk this one up to "life's little mysteries" or the D.C. metro area's "if you needed to know, you'd know."
posted by jocelmeow at 10:52 AM on September 5, 2009