[ARC5] WWII Aircraft Antenna AN-104-series

[Mike Morrow]

I have a couple of AN-104-B VHF aircraft antenna masts.  The AN-104 is designed for 100 to 156 MHz, and also contains an anchoring hole for an HF wire antenna support.  It thus serves as the VHF antenna and the forward support for an HF antenna.

According to VHF AN/ARC-5 and SCR-522 manuals that I have:

AN-104-A    -  Used by USAAF
AN-104-AX  -  Used by USN

I have no information how these two versions differ from each other.  I also have no details of the improvements for the AN-104-B version.  Apparently the AN-104-series is the most common US VHF aircraft mast of WWII. 

The upper 30 inches or so of the AN-104-B is an oval cross-section *steel* mast, wrapped around a wooden bottom section perhaps about seven inches long that has a SO-239 connector at the bottom.  I'm having a hard time understanding how such a structure was actually anchored on the aircraft.  Apparently the bottom wooden section would fit into some type of matching oval socket.  The bottom of steel mast would have to be maintained somewhat above the skin of the aircraft, otherwise the metal antenna surface would short to the airframe.

I jury-rigged a mast to a metal file cabinet and ran a MFJ antenna analyzer across its design range.  Very surprisingly, SWR from about 105 to 160 MHz was below 2.5 across the range, and typically less than 1.5 from about 110 to 155 MHz.  Above and below these ranges, SWR increased rapidly.  It surprises me greatly that the SWR of such a simple antenna mast would be so low and flat over such a wide range.  Is there something special about the electrical design of this mast?

The SCR-522 manual briefly mentions a proposed variable-length VHF antenna whose length would be preset and selected through the small connector found on the BC-602-A (but not -B) SCR-522 control box.  With the flat SWR of the AN-104, it becomes clear why a variable-length model was never required.

I've been searching without sucess for years for an AT-8/AR late-WWII rod-type VHF antenna, apparently used mostly by the USN.  I can't imagine any way that a simple metal rod could show as flat a SWR over the 100 to 156 MHz range of WWII-era VHF sets.  For that matter, I'm having a hard time imagining how the AN-104 performs as well as it does too.

Mike / KK5F