Dipole Antennas
Do not forget the dipole antenna for a single and multi-band use. It is possible to install a tuner in the attic and bring coaxial cable down from the antenna system to your station for a 1:1 SWR on all Bands.
Dipole Lengths (1/2 wave antenna) = 468 Feet / freq(MHZ)
Band MHz Length
10 28.4 16' 6"
15 21.1 22' 2"
20 14.1 33' 2"
40 7.1 65' 10"
80 3.6 130' 0"
10 28.4 16' 6"
15 21.1 22' 2"
20 14.1 33' 2"
40 7.1 65' 10"
80 3.6 130' 0"
Radiation Elevation Plot for ¼ Vertical vs. Dipoles
The Black line is the dipole and the Red line is the ¼ wave-length vertical
The Black line is the dipole and the Red line is the ¼ wave-length vertical
Percentage of DX Elevation Angles for 2500 Miles
Look 95% of the Contacts are below 20 Degrees and compare the vertical and dipole patterns
Look 95% of the Contacts are below 20 Degrees and compare the vertical and dipole patterns
If the attic area is not large enough to accommodate an almost full-size dipole, simply erect as much of the antenna as possible in a straight line, then bend the ends of the dipole up, down, or sideways from the main portion of the system. You should attempt to maintain symmetry in the system by bending the ends of the antenna in equal amounts. See diagram of 40M OCF along roof ridge line. With the North to South orientation you would expect a stronger East to West pattern but this antenna is omnidirectional due to the low elevation (less than ¼ wave-length above ground). Most afternoon and evening contacts are Canada and South America.
Ridgeline OCF Dipole Layout
Physically shortened dipoles are practical too for an attic antenna. When there is insufficient area to mount a full-size dipole;
Physically shortened dipoles are practical too for an attic antenna. When there is insufficient area to mount a full-size dipole;
- You can a 50% linear loading line in each leg of the dipole without loss.
- You can build a double bazooka with the same results as a linear.
- You can install a loading inductor in each leg of the dipole with some loss.
Seriously consider the OCF dipole to find an impedance on a 68 Foot antenna that can provide a reasonably good match across multiple bands which are even harmonically related, such as, 80, 40, 20, 10 and 6M. The OCF dipole is like a standard horizontal dipole but fed at a position other than the center typically at the 1/3 – 2/3 position. The OCF antenna gives lower SWR with wider bandwidth over more bands than the small length center fed dipole. Remember the OCF dipole has an impedance of 200 to 300 Ohms so you will need a 4:1 or 6:1 BALUN if you do not use a remote auto-tuner. See SWR plot for center vs. OCF feeds. Click here for detail explanation of how the OCF Dipole works.
SWR Plot for Center vs. OCF Feeds