home-built antenna experiments on CB Radio

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:

  1. cut an 18.5 foot piece of wire (deliberately long to account for effects of environment)
  2. cut again in the middle yielding two 9.25 foot wires
  3. soldered each piece to the center and shield respectively of RG-58 coax
  4. 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:

  1. possibly severe pattern losses off the dipole ends
  2. cross-polarization losses (also possibly severe)
  3. 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.