WWII Aircraft Antenna Insulators

IN-81 and IN-82 Standoff Insulators

The insulators below were mounted on bulkheads, struts, and other surfaces to provide support for the antenna lead-in between the fuselage feed-through insulator and the transmitter or antenna relay. The IN-81 is 1.25" in diameter and 1.5" long, while the longer IN-82 is 1.25" in diameter and 3" long. The hole through the insulator is not used for the wire itself, but for lacing cord to hold the wire in the saddle on the end. Each type is held onto its mounting surface by a single #10-24 screw.

View of standoff insulators IN-81 and IN-82


IN-83 Insulating Beads

Because of the potential for RF burns to operating personnel, some method had to be devised to insulate the antenna lead for higher power liason transmitters that might operate in the interior of the aircraft. That problem was solved in two ways - first by using spark plug wire (the Navy was partial to this approach), but more often through the use of ceramic beads strung on solid wire, shown below. IN-83 is a Signal Corps designation, and is 5/16" in diameter, 48 beads per foot, which is about 1/4" long nested in the next bead. The hole through the center accommodates up to #8 AWG wire, though it was normally used with smaller diameter wire.

The practice of using the beads was only for safety reasons, so although it was common practice in the 1930s with lower power transmitters like the SCR-183 series, it was discontinued fairly early in the war by both Services for anything other than the higher power liaison transmitters. The ARC-5 manual specifically mentions no need to use them. Still, they look neat and do no harm unless they get damp with salt air or dirty enough to arc over.

The Navy had their own version of the antenna beads that had the number NAF 212989-3 (the suffix denotes the size). They were the same size as the IN-83. Both Services actually had a variety of sizes for different diameter wire, but these are the most common in the manuals.

Package of IN-83 beads, sometimes called fish spine insulators because of the way they fit together


Tension units

Part and parcel of the design of a long wire aircraft antenna is the need to have a tension unit of some sort. The tension in a standard 18 guage Copperweld antenna is nsormally set to 15 pounds in the middle of its range. Below is an example of one I made for the Enola Gay, based on original Boeing drawings, which can be viewed here.

Overall assembly of a B-29 antenna tensioning unit for the command antenna.

Exploded view of the tensioner.

Adjusting the tension to 15 pounds. The scale is located at the other end of the antenna.

The Navy took a different approach to their tension units, using rubber to avoid salt spray binding problems. An example of these tensioners is shown below.

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