Citizens Band or CB radio reached its heyday in the 1970s, leaving an abundance of surplus 11-meter CB equipment available for recycling. Of particular interest to amateur radio operators are 27-megahertz yagi beam CB antennas, which can easily be converted for the 28-megahertz (10-meter) ham radio band. Convert an old CB antenna for amateur radio use and install it on your existing amateur radio antenna tower to save yourself hundreds of dollars while recycling some useless aluminium into a useful and highly directional 10-meter antenna fully capable of worldwide wireless radio communications.
- Skill level:
- Moderately Easy
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Things you need
- 11-meter yagi beam CB antenna
- Mounting hardware
- Amateur radio antenna tower
- Television antenna rotator
- Rotator control box and wire
- Coaxial cable
- Antenna analyzer
- Penetrating oil spray
- Medium- or fine-grit sandpaper
Obtain an 11-meter (27 megahertz) CB antenna and any mounting hardware, such as U-bolts and antenna mast sections, that come with it. Give all surfaces a thorough cleaning and a light sanding to remove any oxidation. Spray hardware, screws and bolts with penetrating oil to free them for adjustments.
Install the antenna on a mast section about 10 feet above the ground for testing and adjusting. Attach the coaxial cable transmission line to the antenna and to your antenna analyzer. Set the band switch of your analyzer to the setting for 25 to 30 megahertz. Dial the tuning knob to determine the existing resonant frequency of your antenna. The Standing Wave Ratio (SWR) should be about 1:1 near 27 megahertz.
Loosen the screws and slide the sliding adjustable clamp toward the boom in increments and check for the resonant frequency on your antenna analyzer if your antenna has a gamma match section. If the resonant frequency goes up, continue adjusting the clamp until you reach your desired centre frequency in the 10-meter (28 megahertz) amateur radio band.
Shorten the antenna's elements in 1-inch increments with a hacksaw until resonance is achieved if your antenna does not have a gamma match section or if you cannot attain the desired resonant frequency. The centre-driven element for a 10-meter yagi should be about 15 1/2 to 16 1/2 feet long depending on other variables such as desired centre frequency, tube diameter and element spacing.
Shorten the parasitic elements. Shorten the director element (shortest element at front end of antenna) by cutting it symmetrically so that it is about 95 per cent of the length of the centre-driven element. Shorten the reflector element (longest element at back end of antenna) so that it is about 110 per cent of the length of the driven element. Readjust the gamma match for resonance on your desired centre operating frequency.
Install your newly converted 10-meter antenna on its mast and antenna tower using the appropriate hardware and a rotator motor. A 10-meter antenna is small enough and light enough that an inexpensive television rotator and control box will work. Make sure all hardware is secure and make a final test for resonance over the 10-meter band. Attach the coaxial cable to your radio transmitter or transceiver.
Tips and warnings
- Adjusting yagi antennas can be tricky because each variable affects several other variables as they are changed.
- Make your changes and especially cuts in small increments and check continuously for resonance and lowest SWR with your antenna analyzer.
- Use an online antenna modelling program to enter your antennas physical parameters to see how your antenna should perform under ideal circumstances and use the information to make modifications and to design new antennas to specifically meet your needs.
- Keep antennas and people away from electrical power lines.
- Disconnect and ground antennas during lightning storms and when not in use.
- Use a safety belt and harness when climbing radio towers.
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- SignalEngineering.com; Directional Antennas: The Yagi Beam
- D.J. Healy; W3PG: An Examination of the Gamma Match
- YagiAntenna.net; The Yagi Antenna
- Al Alvareztorres; AA1DO: Two Element 10-Meter Beam: Two On 10
- American Radio Relay League; ARRL Antenna Book
- Firestik Antenna Company; Measuring SWR and Things Every CB'er Should Know