Off Center Fed Antennas

OCF80 with the 5KW Guanella Current Balun

This will give you a general idea of how they work:

If you take an 80-meter 1/2-wave antenna and feed it in the middle, the impedance is around 72 ohms and is a very close match for 50-ohm coaxial cable. This same antenna is resonant on numerous other bands, such as 40, 20, and 10 meters. If you take this same antenna and use it on forty meters and keep the feed point in the center of the antenna, the feed point impedance will be around 3000 ohms. This is not a good match for coaxial cable. This antenna is a full wave antenna on forty meters but has to be fed at a different location if one wants to use 50-ohm coaxial cable. Move the feed point of this antenna so it is positioned one quarter-wave length in from the end of the antenna. The feed point impedance of this antenna will again be about 72 ohms and is a good match for 50-ohm coaxial cable. If you use this antenna on 10 meters, again move the feed point of this antenna so it is one quarter-wave in from the end. Again you will have a good match for 50 ohms coaxial cable and now have the makings of a long wire antenna with gain.

To make this same 80-meter dipole usable on numerous bands without moving the feed point for every band, you have to find a common point where numerous frequencies converge at a given impedance. By feeding this antenna one-third of the way in from the one end, you will find this so-called, "Sweet Spot". The only thing left to resolve is the high impedance of this antenna at that point. The impedances of the numerous bands, on this antenna, will range from as low as 150 ohms up to around 300 ohms at this feed point. By placing a 4:1 Guanella Current Balun at this point you will have an almost perfect transformation of impedance so the 50-ohm coaxial cable can be used. The end result would be an antenna that is usable on seven ham bands.

The overall length of this antenna is 135 feet. One leg of this antenna is 45 feet long and the other leg is 90 feet long. This will place the feed point of the antenna 45 feet in from one end.

160 Meter Off Center Fed Antenna

8 Bands With One Antenna

Radiation Patterns

Our Hy Power 160-meter off center fed antenna is made with the same material as our 80-meter off center fed antenna. The only differences are three:

1. The overall length of the 160-meter off center fed antenna is 268 feet.

2. The antenna is fed about 87 feet in from one end of the antenna.

3. The OCF160 covers the following ham bands. They are 6, 10, 12, 17, 20, 40, 80, and 160 meters.

40 Meter Off Center Fed Antenna

5 Bands With One Antenna

Radiation Patterns

Our Hy Power 40-meter off center fed antenna is made with the same material as our 80-meter off center fed antenna. The only differences are three:

1. The over all length of the 40-meter off center fed antenna is 66 feet.

2. The antenna is fed about 23 feet in from one end of the antenna.

3. The OCF40 covers five of the ham bands. They are 2, 6, 10, 20, and 40 meters.

The OCF40Q Off Center Fed Antenna in action on You Tube: You Tube Video

The OCF40, OCF80 and OCF160 all use our Guanella 4 to 1 Current Balun. This balun uses two special ferrite cores and is rated at 5KW. More details on this balun can be found on our Balun web page.

Prices

4:1 Guanella Current baluns are installed on all OCF antennas

Micro Series OCF40

The “Micro Series” Off Center Fed antenna is made using #18 stranded insulated wire and a BNC for connection your coaxial cable. The GU4-HF-250-M balun, used on this antenna, uses the same cores that are used in the standard QRP OCF baluns. The balun enclosure is much smaller, 3" x 2" x 1". The end insulators are made from 3/8" round fiberglass. This antenna is not meant to be used for a permanent installation. It is designed for hams on the go. Backpackers love this antenna due to its compact size and weight. They can handle up to 250 watts.

For information on the OCF40Q, OCF80Q and the OCF40QM go to: QRP Webpage

Click here for the order form: Order Form

Return to TOP