A dipole will exhibit resonance at odd multiples of a half-wave, that is, showing a current node at the feedpoint. This will allow one to feed the dipole with coax and still tune it with the "typical" ATU easily. As the number of half-waves increase, the feedpoint impedance rises, and the SWR is >>NOT<< 1:1, but the increase in SWR won't be enough to drastically increase the losses unless the coax is very lossy to start. This allows one to cut a dipole for 75 Meters, and run it on its 7th harmonic on 10 Meters, or cut a dipole for the low end of 40 and run it on its 3rd harmonic on 15 Meters and the 7th harmonic on 6 Meters. Looking at the chart, you will notice a number of lengths that are useful on more than one band.
|
Length (ft) |
1/2 |
3/2 |
5/2 |
7/2 |
9/2 |
11/2 |
13/2 |
15/2 |
|
50 |
9.36 MHz |
29.04 MHz |
48.72 MHz |
68.4 MHz |
88.08 MHz |
>100 MHz |
>100 MHz |
>100 MHz |
|
52 |
9 |
27.92 |
46.85 |
65.77 |
84.69 |
>100 |
>100 |
>100 |
|
54 |
8.67 |
26.89 |
45.11 |
63.33 |
81.55 |
99.78 |
>100 |
>100 |
|
56 |
8.36 |
25.93 |
43.5 |
61.07 |
78.64 |
96.21 |
>100 |
>100 |
|
58 |
8.07 |
25.03 |
42 |
58.97 |
75.93 |
92.9 |
>100 |
>100 |
|
60 |
7.8 |
24.2 |
40.6 |
57 |
73.4 |
89.8 |
>100 |
>100 |
|
62 |
7.55 |
23.42 |
39.29 |
55.16 |
71.03 |
86.9 |
>100 |
>100 |
|
64 |
7.31 |
22.69 |
38.06 |
53.44 |
68.81 |
84.19 |
99.56 |
>100 |
|
66 |
7.09 |
22 |
36.91 |
51.82 |
66.73 |
81.64 |
96.55 |
>100 |
|
68 |
6.88 |
21.35 |
35.82 |
50.29 |
64.76 |
79.24 |
93.71 |
>100 |
|
70 |
6.69 |
20.74 |
34.8 |
48.86 |
62.91 |
76.97 |
91.03 |
>100 |
|
100 |
4.68 |
14.52 |
24.36 |
34.2 |
44.04 |
53.88 |
63.72 |
73.56 |
|
102 |
4.59 |
14.24 |
23.88 |
33.53 |
43.18 |
52.82 |
62.47 |
72.12 |
|
104 |
4.5 |
13.96 |
23.42 |
32.88 |
42.34 |
51.81 |
61.27 |
70.73 |
|
106 |
4.42 |
13.7 |
22.98 |
32.26 |
41.55 |
50.83 |
60.11 |
69.4 |
|
108 |
4.33 |
13.44 |
22.56 |
31.67 |
40.78 |
49.89 |
59 |
68.11 |
|
110 |
4.25 |
13.2 |
22.15 |
31.09 |
40.04 |
48.98 |
57.93 |
66.87 |
|
112 |
4.18 |
12.96 |
21.75 |
30.54 |
39.32 |
48.11 |
56.89 |
65.68 |
|
114 |
4.11 |
12.74 |
21.37 |
30 |
38.63 |
47.26 |
55.89 |
64.52 |
|
116 |
4.03 |
12.52 |
21 |
29.48 |
37.97 |
46.44 |
54.93 |
63.41 |
|
118 |
3.97 |
12.31 |
20.64 |
28.98 |
37.32 |
45.66 |
54 |
62.34 |
|
120 |
3.9 |
12.1 |
20.3 |
28.5 |
36.7 |
44.9 |
53.1 |
61.3 |
|
122 |
3.84 |
11.9 |
19.97 |
28.03 |
36.1 |
44.16 |
52.23 |
60.3 |
|
124 |
3.77 |
11.71 |
19.65 |
27.58 |
35.52 |
43.45 |
51.39 |
59.32 |
|
126 |
3.71 |
11.52 |
19.33 |
27.14 |
34.95 |
42.76 |
50.57 |
58.38 |
|
128 |
3.66 |
11.34 |
19.03 |
26.72 |
34.41 |
42.09 |
49.78 |
57.47 |
|
130 |
3.6 |
11.17 |
18.74 |
26.31 |
33.88 |
41.45 |
49.02 |
56.58 |
|
132 |
3.55 |
11 |
18.45 |
25.91 |
33.36 |
40.82 |
48.27 |
55.73 |
|
134 |
3.49 |
10.84 |
18.18 |
25.52 |
32.87 |
40.21 |
47.55 |
54.9 |
|
136 |
3.44 |
10.68 |
17.91 |
25.15 |
32.38 |
39.62 |
46.85 |
54.09 |
|
138 |
3.39 |
10.52 |
17.65 |
24.78 |
31.91 |
39.04 |
46.17 |
53.3 |
|
140 |
3.34 |
10.37 |
17.4 |
24.43 |
31.46 |
38.49 |
45.51 |
52.54 |
|
200 |
2.34 |
7.26 |
12.18 |
17.1 |
22.02 |
26.94 |
31.86 |
36.78 |
|
205 |
2.28 |
7.08 |
11.88 |
16.68 |
21.48 |
26.28 |
31.08 |
35.88 |
|
210 |
2.23 |
6.91 |
11.6 |
16.29 |
20.97 |
25.66 |
30.34 |
35.03 |
|
215 |
2.18 |
6.75 |
11.33 |
15.91 |
20.48 |
25.06 |
29.64 |
34.21 |
|
220 |
2.13 |
6.6 |
11.07 |
15.55 |
20.02 |
24.49 |
28.96 |
33.44 |
|
225 |
2.08 |
6.45 |
10.83 |
15.2 |
19.57 |
23.95 |
28.32 |
32.69 |
|
230 |
2.3 |
6.31 |
10.59 |
14.87 |
19.15 |
23.43 |
27.7 |
31.98 |
|
235 |
1.99 |
6.18 |
10.37 |
14.55 |
18.74 |
22.93 |
27.11 |
31.3 |
|
240 |
1.95 |
6.05 |
10.15 |
14.25 |
18.35 |
22.45 |
26.55 |
30.65 |
|
245 |
1.91 |
5.93 |
9.94 |
13.96 |
17.98 |
21.99 |
26.01 |
30.02 |
|
250 |
1.87 |
5.81 |
9.74 |
13.68 |
17.62 |
21.55 |
25.49 |
29.42 |
|
255 |
1.84 |
5.69 |
9.55 |
13.41 |
17.27 |
21.13 |
24.99 |
28.85 |
|
260 |
1.8 |
5.58 |
9.37 |
13.15 |
16.94 |
20.72 |
24.51 |
28.29 |
|
265 |
1.77 |
5.48 |
9.19 |
12.91 |
16.62 |
20.33 |
24.05 |
27.76 |
|
270 |
1.73 |
5.38 |
9.02 |
12.67 |
16.31 |
19.96 |
23.6 |
27.24 |
For the calculations, I used the long-wire formula from the ARRL "Antenna Book" which allows for the end effects in a long wire. The formula is:
L(ft)= (492(n-.05))/F(MHz), in which n is equal to the number of half-waves in the wire.
If you are cutting a long wire for, say a Vee Beam or a Rhombic, the formula, using wavelengths is:
L(ft)= (984(n-.025))/F(MHz), where n is equal to the number of wavelengths in the wire.
I have used a dipole cut for 3.83 MHz on 80, 10 and 6 meters with good results for years. By adding a 40 Meter dipole, cut for the low end of 40 (7.02 MHz) I get good results on 40 and 15 also. This gives me 5 bands on a single feedline.
If you can latch onto a copy of Ed Noll's book, 73 Dipole and Long Wire Antennas you will see other combinations that will work for you, and not just dipoles.
The snow is flying, so the >>REAL<< antenna season is here. What's a little frostbite between friends, anyway?