These tests used AM-only Johnson Messenger 123A.
CB radio antenna dipole antenna
My dad had an old Shakespeare CB antenna but it’s made for a drill-in mount. From reading old ARRL Antenna Handbook at the library, I see that:
468/27 ~ 17.3 feet
So to construct my CB Radio dipole antenna I:
- cut an 18.5 foot piece of wire (deliberately long to account for effects of environment)
- cut again in the middle yielding two 9.25 foot wires
- soldered each piece to the center and shield respectively of RG-58 coax
- hung dipole up in the attic using a nail at each end into the wood.
Tune CB Radio dipole VSWR
Instead of cutting the dipole wire for best VSWR, I folded it over at each end until the VSWR was minimum at channel 20. I got about a 1.8:1 VSWR, perhaps due to 120Vac power wires a few feet away.
Due to physical space constraints, I ran the dipole diagonally across the wires instead of perpendicular to them as would have been ideal for minimum interaction.
Performance test: Horizontal attic dipole for CB Radio
I waited for a semi-truck to drive by, and asked on channel 19 if they could hear me using the company name on their door to help ensure a response. To my delight the first truck did have a CB on 19 and replied booming in! The old Johnson Messenger 123A AGC apparently was set to a soft background noise, or perhaps due to aging had reduced RF sensitivity. The net effect was a gentle swoosh of static to blasting loud voice 10 times louder!
I only got about 2 miles range, but he was mostly off the end of the dipole. So despite being about 25 feet above ground level, I only got 2 miles range. I think I lost him before he lost me–only moderate receiver sensitivity perhaps. The S-meter barely deflects with no one talking, so I don’t think it’s man-made interference as the range-limiting problem.
Vertical dipole to monopole antenna for CB radio
After a few more tries with similar results, I decided the issue was I needed a vertical antenna, to avoid:
- possibly severe pattern losses off the dipole ends
- cross-polarization losses (also possibly severe)
- coupling with household wiring beneath dipole (raising peak elevation angle of radiation pattern)
I could not simply find 18 feet of vertical space. Was not going to drill a hole in the ceiling or roof. I cracked open the antenna book for the next iteration: vertical monopole.
The coax center wire raises vertically. I found bending a few inches of the top perpendicular like a capacity hat gave better VSWR. I soldered two additional wires to the shield and spread them out approximately evenly with 90 degree azimuth separation. The VSWR was now 1.4:1, excellent!
Even better, base to mobile range is about 10 miles. Base-to-base, I can hear about 30 miles and I say that because of my low ambient noise level, I can hear base stations in nearby cities that can’t hear me due to their local noise level.
A better CB base-station antenna design
I think I have tapped out the best I can do indoors, short of making a yagi antenna in the attic. I think I might get a better range improvement by having an antenna on the roof outside, but due to the difficulty of accessing the roof, I want to do this with a commercial antenna. I’ll keep my eyes out and ask around if anyone has an old base-station antenna they don’t want anymore.