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.

It was based on an article on Hamuniverse.com that can be seen here. 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 tuning involved.

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.

Photo 1. Finished center insulator with SO-239 connector on end.
Dipole terminals spaced 6 inches apart.

Finished Dipole Spacer

Note that the spacing between each dipole on the end spacer is 19.5
 inches between each wire as suggested by the army's total length of 39 inches
 in the original SRI article. Although not mentioned in the article, small diameter PVC tubing can be used for the spacers (sometimes called spreaders) between dipoles or any non-conductive material.


Yellow drawn in lines represent separate dipoles with spacers.
 80 meter coil is in upper right of picture.

I built the antenna on the ground and tuned all three bands with my MFJ analyzer.
The 80 meter wire started at 45 feet per side, the 40 meter wire at 32 1/2 feet per side and the 20 meter wire at 17 feet per side.

Starting with the analyzer on the top wire, each band needed to be shortened a bit. After about 5 adjustments all bands were resonant in the middle of the band with an SWR of 1.3 or less.

After raising the antenna up 64 feet to its final position and putting the analyzer back on, there was no need to lower it for more tuning. The same resonant points stayed as they were with SWR at 1.3 for the 20 and 40 bands and 1.8 for the 80 meter band. I have made a number of good contacts between Fairbanks and Miami with signal reports of S-6 to S-9 on all 3 bands.

I found this to be a simple and inexpensive multi band antenna to construct and I am very happy with the results.

73

Scott KL3JM
Don't forget to refer to the original SRI article on Hamuniverse.com