Shorty 40 analysis

A Shorty 40 is a shortened 40 meter dipole which uses a matching transformer/loading coil consisting of a 30 turn coil of AWG 12 wire on a 2.5" diameter form 6" long. The elements are typically 18'6" long (on either side of the center) for a total length of about 11.3 meters.

Using the program ACMI from Paul Nicholson (http://www.abelian.demon.co.uk/acmi/) , we can calculate self and mutual impedances for the coils. Of course, that program does NOT calculate the stray capacitances, which are significant. The coils were modeled as 3 windings:

	tapped at 2 turns	tapped at 3 turns
feed L (uH)	0.46	0.90
feed R	0	0
sec 1 L (uH)	7.95	7.13
sec 1 R	0.01	0.01
sec 2 L (uH)	9.61	9.61
sec 2 R	0.02	0.02
M Feed:sec1 (uH)	0.57	0.75
k	.2981	.2970
M feed: sec 2 (uH)	0.59	0.79
k	.2788	.2685
M sec 1: sec 2 (uH)	1.10	0.90
k	.1255	.1082

Input file for ACMI

; Shorty 40 matching coil
; 6 tpi, 30 turns on 5" long 2.5" diam form
; 
; A closely coupled helical impedance matching transformer

feed {

   radius .03175

   height1  0.0
   height2  3/6*0.0254

   conductor  12 awg  ; wire radius
   turns 3
}

sec1 {

   radius 0.03175		; 2.5 inch diam

   height1 3/6*0.0254
   height2 0.0635

   conductor 12 awg
   turns 12
}

sec2 {

   radius 0.03175		; 2.5 inch diam

   height1 -0.0635 
   height2 0.0

   conductor 12 awg
   turns 15
}

ACMI output

2 turns
      FD.L|   FD.R|     SC1.L|  SC1.R|     SC2.L|  SC2.R|   FD-SC1.M|   FD-SC1.K
|   FD-SC2.M|   FD-SC2.K|  SC1-SC2.M|  SC1-SC2.K
   0.46 uH|   0.00|   7.95 uH|   0.01|   9.61 uH|   0.02|    0.57 uH|     0.2981
|    0.59 uH|     0.2788|    1.10 uH|     0.1255
3 turns
      FD.L|   FD.R|     SC1.L|  SC1.R|     SC2.L|  SC2.R|   FD-SC1.M|   FD-SC1.K
|   FD-SC2.M|   FD-SC2.K|  SC1-SC2.M|  SC1-SC2.K
   0.90 uH|   0.00|   7.13 uH|   0.01|   9.61 uH|   0.02|    0.75 uH|     0.2970
|    0.79 uH|     0.2685|    0.90 uH|     0.1082

Here's a NEC analysis of the antenna itself, with the feed point impedances..
CE
GW 1 41 0 -5.716 0 0 5.716 0 .001
GE 0
EX 0 1 21 0 1 0
FR 0 1 0 0 7.2 1

F	R	X
6.90	15.08	-837.11
6.95	15.33	-826.76
7.00	15.58	-816.51
7.05	15.83	-806.37
7.10	16.09	-796.33
7.15	16.35	-786.40
7.20	16.61	-776.57
7.25	16.87	-766.83
7.30	17.14	-757.19
7.35	17.41	-747.65
7.40	17.68	-738.19
7.45	17.96	-728.83
7.50	18.24	-719.55

Some other links:

An inductively loaded 80m Yagi: http://www.qsl.net/ve6wz/intro.htm

radio/antenna/s40.htm - 30 Jan 2003 - Jim Lux
(antenna home page) (radio home page) (Jim's home page)