[Milsurplus] AN-104

[Mike Morrow]

>The radiating element in these antennas is just a piece of stripped 
>coax stuffed in a wooden mast.The metal sheath is there for "armor" 
>only.

The outer steel sheath is *THE* radiating element of the AN-74 and AN-104 type of antennas.  The instructions found in various manuals (SCR-522, AN/ARC-1, AN/ARC-3, AN/ARC-4, AN/ARC-5, etc.) for AN-104 installation require the bottom edge of the sheath to be separated from the surface of the airframe by at least 3/16 inch.  The sheath has a hole 2.5 inches from the top tip, and instructions say that any metallic wire attached at this point for the support of a HF antenna insulator must be no longer than two inches.  Clearly these requirements must be observed because the sheath is the radiating surface.   

It is the large effective width of this radiating element at VHF wavelengths that is responsible for the impressive low-VSWR bandwidth of the AN-104.

I strapped an AN-104-B to the side of a large metal filing cabinet and then measured the VSWR with an antenna analyzer.  The VSWR remained below 2.5 from 100 to 160 MHz, but climbed rapidly above and below that range.  Typical VSWR within the range was around 1.5.  A simple vertical rod or wire antenna can't physically realize that sort of bandwidth.

I don't know what other component elements are in the AN-104.  There is a direct DC current path between the center and outer conductors of the SO-239, so something internal is creating a low resistance path.  Perhaps there is an RF choke to minimize the static voltage difference across the antenna connection.  I may remove some paint from the radiating portion of the AN-104 and measure for a direct DC current path between the center of the SO-239 and the radiating surface.

Mike / KK5F