The KL3JM MODIFIED "SRI" MULTIBAND FAN DIPOLE
FOR 80 - 40 - 20 Meters
I built my SRI dipole for three bands, 80,
40 and 20 and designed it to be as light as possible since there would be
no center support, just hanging between two trees.
In case you don't know what an "SRI" dipole is, read the article above in
the link. The antenna in this project is a modification of the
techniques used to design a multiband fan type dipole with little or no
Since I only had 105 feet between my trees I had to use a loaded wire for the 80 meter band at 90 feet long.
K7MEM has a good web site at www.k7mem.com that I used to get the specs for this short 80 meter section loading coil.
It was made with a 51uh coil set between a 9 foot and 36 foot wire for a total of 45 feet on each side. This 90 foot length set the design for the rest of the assembly.
The center connector/insulator was made from a 14 inch length of 1 1/4 inch PVC. See photo (1) below.
The 1 1/4 inch PVC is not big enough to get your hand in but much lighter than 3 or 4 inch PVC.
While a bit like building a ship in a bottle, it wasn't too bad to get it together.
I used 6 stainless #10 eye bolts as wire anchors and 6 stainless #10 machine screws for the terminal connectors, 3 per side.
The terminals were spaced 6 inches apart, a bit more than the 5 3/4 inch spacing suggested in the original SRI design. (It is important to remember that all 3 center insulator terminals are wired together on each side of the center insulator making each half of the dipole parallel with the other band dipole legs on the same side. Each half of the dipole is connected to the SO-239 connector. One side of ALL of the dipoles is connected to center pin on the S0-239 connector and the side to the shield side of the connector.)
The three terminals for each side were connected with 12 gauge wire with ring terminals. The nuts and washers for the middle terminals and eye bolts were held in place by putting them on the end of a long screw driver with a bit of axel grease to hold them on the tip.
Note that the spacing between each
dipole on the end spacer is 19.5
I built the antenna on the ground and
tuned all three bands with my MFJ analyzer.
I found this to be a simple and inexpensive multi band antenna to construct and I am very happy with the results.
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