A Really BIG Antenna

By Steve Mollman – KD9HL

As a teenager and adult living in the shadow of the US Navy Radio Station NSS-Annapolis, I was able to watch much of the development of this gargantuan antenna system. Such things as 800 and 1200-foot towers with supporting 600-foot towers.  A sight for an amateur operator to behold and drool over!

NSS-Annapolis first came operational in 1918 with two 350 KW arc transmitters.  Annapolis was strictly a transmitting site-the receivers were originally located at the Navy Yard Annex in Washington, DC. Later the receiver site was relocated to Cheltenham, MD, about 30 miles from Annapolis.  Portions of the site, including some of the log periodic antennas mounted on wooden utility poles, were visible to motorists traveling on US 301.

Over the years improvements were made with the spark-gap transmitters being replaced in 1931.  By 1969, the “big” transmitter was a 1000 KW VLF machine using four distilled water cooled 250,000-watt Eimac tubes in a “push-pull” configuration. (Four spare tubes were kept in a “ready” mode for instant replacement should one of the on-line tubes fail).

Three of the Eimac 250KW tubes

The antenna system was a 1200-foot guyed center tower surrounded by nine 600-foot towers (three of which are identical to those erected in 1917). The “Goliath” antenna consisted of the 1200 tower and the “top hat” assembly supported by the 600-foot towers, covering about 200 acres. To make the antenna appear to be the correct length for the transmitter, the huge capacity top hat was employed. A “top-hat was necessary because at the 21.4 KHZ operating frequency, the wavelength is about 8 miles! The actual tower, of course is only 1200 feet.  Ceramic insulators supported the center tower system which was reported to weigh “about as much as a destroyer.” (7000 to 9500 tons).

The Ceramic Tower Base

A Drawing of the Antenna with its “Top-Hat”

 The output from the transmitter coupling coils was fed to the center tower at the 300, 600 and 900-foot levels and these “lead-ins” were part of the antenna system.  One can imagine the size of the wire.

Rendered obsolete by satellites, frequency hopping technology and the end of the cold war, NSS ceased operation in 1999 and all of the antennas and most of the towers were removed. Three of the smaller 600-foot towers were preserved as a historical site and to serve as aids to navigation for boaters on the Chesapeake Bay. The site today is the Naval Academy Golf course. The penalty for a golf ball hitting one of the towers is a stroke and distance.

A couple of side notes:

  1. The NSS transmitter fed one million watts of RF to the antenna, and during idle times, transmitted in Morse code the string “W W W VVV VVV VVV DE NSS NSS NSS”. The power was so high and the frequency so low, one could often hear the signal on many of the cheap AM receivers of the day. Messages were sent in Morse code, but were encrypted.
  2. Because it was somewhat secluded, the site was popular with local teenagers who would park nearby. The Marine Security Patrols took a dim view of these activities.


Reprinted with the author’s permission, via Bill, N4SV.     Thanks, Bill.


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