[Milsurplus] AN-104

[jcoward5452 at aol.com]

Mike
  You are delving deeper than I on this.I only remember a cross section 
that showed a modified coax inside the wooden mast and a metal 
sheath.Memory is having selective fading these days.Now if I can only 
dig up the printed page...
 Jay

-----Original Message-----
From: kk5f at earthlink.net
To: milsurplus at mailman.qth.net
Sent: Thu, 11 Jan 2007 2:16 PM
Subject: Re: [Milsurplus] AN-104

>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





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