"J. B. Wood" wrote in message
...
In article , Richard Clark
wrote:

On Wed, 08 Mar 2006 19:14:02 GMT, "NA"
wrote:

Can anyone tell me about "fan antennas" like those used on the large
navy
ships.

Hi Jim,

It is an array of co-planar, parallel monopoles driven at the same,
common point. The size of each in the array is different spanning
from short to long such that at least one is resonant within a wide
band of frequencies.

snip

Hello, and the only fan antenna that I have any experience with is the
venerable 2-6 MHz twin fan transmitting antenna used on USN combat ships.
This antenna consists of two sets (port, starbaord) of three sloping wires
strung from a yardarm on the stack to the feedpoint on the deck. A VSWR
of 3 or less is typically achieved using a fixed passive LC network. By
design, most of the radiation actually comes from RF currents induced on
the stack by the fan wires. Sincerely,

John Wood

Supplementing, if I may, many fan-wire antennas are fed from the mast. (I've
climbed most of the ships in the Pacific Fleet at one time or another.)
Coax into the matching network box and port/starboard feedwires out.

The traditional design used the fan from 2-6, another antenna, maybe a cage
for 4-12 and a third, maybe a discone or trussed whip, from 10-30. These
are fed by four- or eight-channel multicouplers in Radio.

A newer design, called HFRG, for HF Radio Group, uses a 2-9 fan and maybe
twin whips from 8- 30. HFRG tolerates a 4:1 SWR ... I don't think you can
get a match of 3:1 over a wide range like 2-9 or 8-30. (In a lab, maybe.)
I Googled for AN/URC-131 and got:
http://www.rfcomm.harris.com/product...s/anurc-131v.p
df It's mostly a Harris puff piece, but there's a little techie stuff.

I have to say "maybe" a lot because of differences between classes and even
between different levels of modernization within a single ship class.

John
KD6VKW
US Navy since Jan, 1962, in many capacities.