[AMRadio] The 'Sweeper' and NDB's

CL in NC mjcal77 at yahoo.com
Thu Jan 21 15:39:54 EST 2010

Our local net that met on 3810 and now 3716 on Sunday and Tuesday, has been plagued with the sweeper for years. The one that bothers us, will sweep and lock on to a particular carrier, some it will pass by.  The stronger the carrier, the greater the chance of a 'lock on'.  Here in western NC and Upstate SC, we have at various times DF'd this pest southward, perhaps Ft. Jackson in Columbia.  One of our group is real buddy buddy with Riley, and when he was with the FCC, the complaints direct to him caused him  to poke around a bit looking for answers, and the sweeper seemed to go away, for a few months anyway.  Probably coincidence.

NDB's and LOM's have nothing to do with this.  At one time, NDB's and LOM's had weather information applied to it, and the voice modulation was set to 80% and the ID 20%.  When the FAA removed the voice signal to these units, the ID modulation was set to 90%, accounting many times for now hearing these stations.  Most now only radiate their 2 or 3 letter ID's, and are only monitored through the air, no phone lines or special comm circuit used to deal with it.  The Nautel NDB's I maintained at one time, ran 400 watts of carrier, and when the ID hit them, they peaked at 1200 watts output. Funny that the FAA still autorizes installation of these things, they are just the next step up from the bon fires they used to light on mountaintops for navigation.  Even in this GPS world, a failed NDB will generate almost an immediate inquiry from a pilot.  Perhaps the CW ID is a form of comfort.

NDB's are alway asscociated with an airport approach, and are the cheapest form of bearing establishment around.  I guess there is nothing as reassuring as seeing a simple indicator swing and point and say 'fly this way'.  LOM's are the same equipment most times as an NDB, but are located at a 75Mhz. Outer Marker site,  approximately 5 miles from the end of the runway.  LOM's have only a 2 letter ID.  Powers run from 25 watts to 400, with a few Ikw units on the fringes like Alaska. Antennas run from a simple top loaded 50ft wire, 100+ feet of Rohn 25, 200-300 ft 4 leg steel towers, to custom designed fiberglass encased  LF antennas with a huge wagon wheel top hat.

The FAA has started a program of decommsioning the government owned transmitters, but many are still in use at small town and county airports.

Charlie, W4MEC in NC


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