Thursday, May 19, 2016

Some fan dipole thoughts ...

They found that the wires at the center feed point had to be separated by at least 5 1/2 inches vertically and the ends separated by 38 inches in the 2 to 18 MHz range. As in any fan dipole construction, all of the dipoles are connected in parallel but in the SRI method, the separation between them at the feed point must be maintained.
By this simple change they found that you could accurately cut the antenna element
lengths for given frequencies and eliminate the need for pruning.

In the drawing above, the lowest frequency antenna is on top and is cut 4% short of the standard 1/2 wave length. (Length in feet= 0.96 times 468 divided by the operating frequency in MHz).

The middle frequency antenna (lower in frequency), is cut for an exact 1/2 wave length. (length in feet= 468 divided by the frequency in MHz)

The highest frequency antenna is at the bottom and cut for 1% longer than the 1/2 wavelength (length in feet= 1.01 times 468 divided by the frequency in MHz)

Compared to the construction effort of a standard multi-band dipole the only difference is the fabrication of a feed block or center insulator that is about 12 inches vertically by 3 inches wide, so make sure this is made of a good insulating material such as Lucite, Bakelite, fiberglass, or PVC. 
The end 38 inches of separation can be maintained by separate halyards on each element or a spreader bar with a common halyard.
The bandwidth will be at least plus or minus 2% for a 1.5 to 1 SWR according to Stanford Research Institute.

(Each leg is shown in length so you will need two legs.)

10 METERS = 8'4"
12 METERS = 9'5"
15 METERS = 11'1"
17 METERS = 12'10"
20 METERS = 16'8"
30 METERS = 23'2"
40 METERS = 32'9"
60 METERS = 43'7"
75 METERS = 60'9"
80 METERS = 65'6"
160 METERS = 123'5"

137 KHZ = 1708'1"

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