Application Note #6
Subject: Dipole Antenna with Rainbow Bridge
Although this is more of a note about constructing a center-fed dipole, it also addresses use of the Rainbow Tuner.
A center-fed dipole consists of two quarter wave pieces of wire joined in the center by an insulator and connected to other insulators at the end. It has an impedance that closely matches coaxial cable, so I recommend direct connection of coax (RG-58 is fine) right at the center.
Strictly speaking the antenna is a balanced antenna and coax is unbalanced, but in practice, for non-directional antennas direct feed with coax usually works well. If you are concerned by this, you can put a balun at the feedpoint or coil up about a half-dozen turns of coax cable at the feedpoint in a coil 6 or 8 inches in diameter. I usually don't bother.
Simply connect the coax center conductor to one side of the center insulator and the shield to the other side. You should also provide some sort of strain relief for the coax at the center.
16 ga wire is fine for the dipole legs and as mentioned earlier, the length on each side of center should be a quarter wavelength. This can be easily calcualted kusing the formula
L(FT) = 234 / F (MHZ)
where L is the length in feet, and F, the operating frequency in megahertz. For 10.125 MHz, this work out to about 23 feet, 1 inch either side of center.
It is best to start with the antenna a little longer than calculated (6 inches or so) and measure the SWR, then trim a little at a time until the SWR is lowest at the frequency you want.
The other end of the coax cable should go to the bridge portion of the Rainbow to check SWR. It goes to the OUT and GND terminals and your transmitter goes to the IN and GND terminals. The manual shows a switch to bypass the bridge after adjustment and this is a good idea since the bridge uses up 3/4 of your power while in use.
The dipole center should be as high as you can get it. For 30 meters, at least 20 feet above ground is best. The ends can either be supported at the same height (end-supported dipole) or the center can be supported with the ends drooping (inverted Vee dipole.) For the inverted vee, the ends should be no less than 7 or 8 feet above ground. As with most antennas, the higher you put up the antenna, the better it will work.
Joe E., N2CX
from Southern New Jersey, y'all