National SW-3 Plug In Coils
Unlike more modern radio's which allow you to change bands at the turn of a single bandswitch, the National SW-3 changes bands by plugging in a pair of coils. The operator turns off the radio, lifts the hinged top lid, and replaces the coils with the desired frequency set.  There are two kinds of coils, a "general coverage" coil which provides a full dial over a wide frequency range, and the "bandspread" coil set which provides a full dial over a narrow frequency range for fine tuning the ham bands.
Click on the thumbnail photo to bring up a large picture. The photo above shows the bandspread coil set with the rf coil on the right and the detector coil on the left.
Click on the thumbnail photo to bring up a large picture. The above photo shows the general coverage coil set. The main physical difference between the general coverage and band spread coils is that the general coverage coil set does not have a capacitor and no wire and plate cap attached to it.
Click on the thumbnail photo to bring up a large picture. The photo above shows a close up of the unique 6 pin coil bottom. Also shown is the stamped identification number.
SW-3 coils are identified by several possible ways. Earlier coils had a colored dot applied to the bottom of the coil form, in-between the pins. It may or may not have the coil number stamped on the bottom. Along the top edge of the coil form, there is an indentation that may also be colored in.  Later coils had a number stamped on the bottom and may or may not have a colored bottom dot and the top edge colored in. See the table below for coil frequency identification.
Range (Mhz) Range (Meters) 10-20 Series 60-70 Series 30-40 Series
General Coverage 1930 ~1933 ~1940
0.09-0.16 2000-3000 22 72 42
0.15-0.22 1500-2000 21 71 41
0.19-0.28 1200-1500 20 70 4
0.25-0.39 850-1200 19 69 39
0.32-0.65 500-850 18 (purple) 68 38
0.50-0.90 350-550 17 (yellow) 67 37
0.69-1.50 200-360 16 (orange) 66 36
1.5-2.7 115-200 15 (blue) 65 35
2.5-4.5 65-115 14 (green) 64 34
4.2-8.0 40-70 13 (white) 63 33
7.0-12.0 23-41 12 (red) 62 32
12.0-21.0 13.5-25 11 (black) 61 31
19.0-35.0 9-15 10 (brown) 60 30
         
BandSpread        
1.8 160 meters 15A (blue) 65A 35A
3.5 80 meters 14A (green) 64A 34A
7.0 40 meters 13A (white) 63A 33A
14.0 20 meters 11A (black) 61A 31A
28.0 10 meters 10A (brown) 60A 30A
Coil information from an article by William Fizette K3ZJW, titled "Those National 6-Prong Plug-in Coils"
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Last Update: 01/25/04
Web Author: Scott Freeberg WA9WFA
Copyright 2004 by Scott Freeberg WA9WFA - ALL RIGHTS RESERVED