OE-303 1/2 Rhombic Antenna

©Brooke Clarke, N6GCE


Background Information

The AT-984/AG 150 foot wire Antenna may have been a predecessor to the OE-303.  The AT-984 was simply a reel holding 150 feet of wire in a canvas bag.  One end of the wire was attached to the antenna terminal of a PRC-25 VHF Low Band Squad radio in order to greatly extend the useful range.

The antenna pattern of a wire forms a cone that points more and more to the far end as the length gets longer.

The OE-303 is a vertically polarized 1/2 rhombic with an included apex angle of 140 degrees.  The two anchor points are 166 feet apart..  Rated for 30 to 88 MHz and 350 Watts. Vertical radiation angle is 6 to 10 degrees.
Planning distance for two VRC-12 series radios is 36 miles if both are using the OE-303.

This is NOT a normal OE-303, but some variant.  It uses a 400 Watt termination and a BALUN that is labeled 2 to 35 MHz.

Manuals

TM 11-5985-370-12.pdf  -12HR and -20P are all available free on line from ETM.  These are for a different version of the OE-303.

Prototype not production OE-303

400 Ohm Termination, 275 Watt

The version that I have is a special, maybe designed to handle more power.  Instead of using small combined transformers and "dissipation lines" this version uses a termination housed in an Aluminum tube 3" in diameter and 18" long.  The 400 Ohm resistor itself is an inch in diameter and 12 inches long.  This one is broken.  It is marked Carborundum Co., 889 SP4OUT, 400 Ohms P/M 5%.
It is installed in the 3" tube with both ends brought out to U-106/U Silver Push type wire terminals.  The 3" tube is designed to mount on top of a mast, not to be staked into the ground.  This is strange since the termination should be at one end of the antenna, NOT up in the air.  How to mount the termination? If you know email brooke@pacific.net

Photo of broken 275 Watt, 10 kV power resistor - now made by Kanthal Globar -

BALUN 50 Ohms to 400 Ohms

Labeled for 2 to 35 Mhz 400 Watts, but works up to 60 MHz not 79 or 88 MHz.  The four 100 Ohm resistors used to terminate the BALUN only have maybe 4 nH inductance which at 88 MHz is about 2 Ohms. Photo of BALUN s/n X03 = eXperimental 3.  BNC(f) connector for feed coax and two U-106/U Silver Push type wire terminals for ANT and GND.

T Wire Holder

There is a "T" shaped part with a slot on top that looks like it could be put into the top of a mast to hold the upper half of a half Rhombic.

RL-29 Reels

There are two RL-29 reels, but one appears to have much more wire than the other.  Indicating this is a half Rhombic antenna.

Production OE-303 Parts

Links

 

 

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