Tuesday, September 05, 2006

Longwave radio beacons

I recently got a nice digital shortwave receiver, the Degen 1103. It's pretty nice, but this isn't a place for a review. In addition to shortwave, it receives the FM and AM broadcast bands, plus a frequency region below the AM broadcast band known as low frequency (LF) or longwave.

This band is defined (PDF) as 300kHz down to 30kHz. From 525kHz (the bottom of the AM dial) to 300kHz is technically still medium-wave but feels miles away from the AM band. It is mostly used for radionavigation for aircraft and ships, plus there are small allocations for the WWVB time broadcast at 60kHz and the 500kHz maritime distress frequency.

It's been neat to scan this new piece of spectrum I've never listened to before. Once you get past all the horrific noise (from switching power supplies, powerlines, etc. you start to hear the morse code IDs of all the radionavigation beacons around the airports near you.

In Chicago, I've heard five stations and if you record the morse code IDs, you can find where they broadcast from.

frequency(kHz) / ID / bearing(degree) / power(watts) / location / airport

There is also a slow "T" morse code beacon at 317kHz-318kHz, repeating precisely every 23 seconds, that I've been unable to identify. It's faint enough that I can't find the direction because I only hear if I null out all noise, which has it NE/SW.

DXinfocentre has a NDB list that is handy for quick ID. You can get a lot (by a lot I mean a lot) of more info if you enter the morse ID into Airnav's system here

Take the coordinates from either site and paste them into Google Maps to quickly locate the beacon.

Most of the beacons line up with runway approaches at the various airports, although RZL's is just at the airport itself. In the case of "ME", you can see it's an active approach here. and here. and here.

What's neat about the bearings (which came from the Degen's internal loopstick AM antenna), is that you can figure out where you are in relation to each beacon. Two beacons, roughly 90 degrees apart at your location, can give you a good fix about where you are. Any radio with an internal antenna like the Degen can be used like this for direction finding: the antenna is most sensitive to the front and back of the radio (as the loop inside is usually mounted horizontally lengthwise in the radio).

The Degen 1103 suffers from overloading from strong AM radio stations, and it exhibits a lot of fake radio stations (called images) in the longwave band that are just nearby transmitters.

There are other beacons and radio stations out there in this band (Europe has a whole set of actual commercial radio stations down there), but I haven't heard them with all the noise in the neighborhood. It looks like I'll need to make a better antenna to catch anything else than these stations.


Milligan said...

Sounds like fun. I had a grand old time doing similar stuff back in high school after hours in an electrical engineering department I had access to. Although instead of a real shortwave radio, I was using a spectrum analyzer that had an AM demodulator. Being able to actually see the spectrum made pinpointing stations dramatically easier.

Incidentally, something that piqued my interest in this post -- how does noise nulling work in this context?

Dean W. Armstrong said...

Loop antennas are sensitive to the magnetic component of the e&m wave, or rather to changes of the magnetic flux. For a wave coming down the axis of the loop there is no change in magnetic flux and hence that's the least sensitive direction. With a multi-turn loop, you can have very sharp nulls that drop signals right along the axis down by a lot--I think I've read 80db. For my apartment, pretty much the loudest noise is the computer/monitor/DSL/network, which is roughly all in the same place.

Steven said...

How do you know the "T" morse code beacon is actually a morse code beacon and not just a tone of fixed duration repeated every 23 seconds?

Dean W. Armstrong said...

That's a good question. The length of the tone is nearly the same as the dashes in the aeronavigation beacons. I should check, but I thought the offset of the tone from where the carrier would be was the same as the beacons as well.

Anonymous said...

An ignorant question, but there is an interesting antenna setup at the following location: I'm not sure if this is a longwave antenna, but it is northwest of Rockford, IL and has been around since at last the 1970's (when I first discovered it) when the family drove out Latham Road near Meridian Road. Maybe this 4 antena array is one of your stations? Or, maybe you know for what it is used? http://terraserver.microsoft.com/image.aspx?T=1&S=10&Z=16&X=1616&Y=23463&W=2&qs=%7crockford%7cil%7c

Although Google Maps shows a slightly closer view of this image, I was unable to bookmark it to link in this post... The driveway also has a "US Government Property: No Tresspassing" (or something similar to that) on a sign at the beginning of the driveway.

Dean W. Armstrong said...

This one?: Google Maps

Entering the coordinates into the FCC database (which I used for a radar question here) I found this one: http://wireless2.fcc.gov/UlsApp/AsrSearch/asrRegistration.jsp?regKey=2642708

Operated by U.S. Cellular.

Milt Poulos said...

Hey Dean,

I'm a amateur radio operator in Western Michigan and logged a few beacons tonight with my Kenwood TS60S transceiver and 40 meter dipole antenna which is 30 ft elevation. I heard CLB on 220 KHZ, GLS on 210, GR on 260, DDP on 390, YPO on 402. See if you can hear these. The antenna makes the difference. Milt

Dean W. Armstrong said...

Thanks Milt. I have been thinking about a fixed loop for VLF work, and I realize it would be good for longwave, albeit nonsteerable. I will have to make a trap for all the AM broadcast stations though--I get over 100mV/m according to the http://www.v-soft.com/ZipSignal/zip_answer.asp database. This overloads the Degen 1103 for longwave. I've had better results hearing many stations up at Yerkes Observatory in Wisconsin--much less noise and overload.

Stephen said...

Hi Dean,

I am a hobby DXer, not a very serious one (actually just rediscovering the hobby.) I live in Rockford, so I will try to check some of those frequencies. I only have a Sony ICF-2010, stupidly took down my antenna a few years ago.

Anyways I will see what I can get. So far only one thing jumped out at me and it sounds like static.

Dean W. Armstrong said...

Hi Stephen, try Gilmy NDB
Identifier: RF (.-. ..-.)
Frequency: 275 KHz

and De Kalb
209 DKB

Stephen said...

I picked up both of those! Pretty cool. I am on the far North East side of Rockford.