CB radio base / mobile antenna

An affordable base station antenna for GMRS, MURS, or CB radio can be made from a mobile antenna at roof or balcony level. Be sure the antenna is well away from power lines for safety and noise. It’s important to get the tip of the antenna higher above ground level vs. antenna length. That is, a shorter antenna up high is often better than a longer antenna at ground level.

A mobile antenna near the roof peak using half a mirror mount can screw into the metal eaves for groundplane. A satellite TV dish mount can also be used, assuming a lightweight CB antenna that doesn’t significantly exceed the weight or wind load of the dish mount.

Groundplane

If the antenna is not mounted to metal, a counterpoise / groundplane can be employed at least by drooping a wire or wires 1/4 wavelength long (27 MHz CB ~ 9 feet) from the ground of the antenna base, connected to the coax cable shield. Consider using a thin vertical wire with a vertical fiberglass or other non-conductive support to keep the center pin-connected wire from drooping and touching the ground or other objects. The groundplane wires should be kept away from the center-pin connected wire element of the antenna to avoid coupling and detuning the antenna. The groundplane wires can be drooped at an angle (ideally about 45 degrees below horizontal) to reduce coupling and detuning.

For higher frequency MURS and GMRS or as a compromise at CB 27 MHz, a magnet mount antenna could be attached to a large dog kennel or magnetic baking sheet. A mobile mag-mount antenna on the roof will perform poorly without a large magnetic surface to attach to. For safety, put weight (say via sandbag) on the groundplane to prevent it from blowing off in the wind.

Specifically for CB radio (27 MHz antennas), a flat metal area is important for the mag-mount antenna to attach to, to establish the proper groundplane coupling. Putting a 27 MHz mag-mount onto a mesh or other non-uniform flat surface is likely to have poor coupling, high SWR, and not great performance. The groundplane can be smaller than you’d expect and still be usable, but with less range than is possible with a larger groundplane. Just because the SWR is low enough (below 2:1) doesn’t mean the antenna is performing well, as the SWR can be low but the antenna may not be radiating efficiently due to poor groundplane coupling.

Maximum communications range

Important factors for maximizing communications range include:

  • antenna tip height above average terrain
  • freedom from interference (noise, undesired signals)
  • antenna efficiency – 1/4 wavelength radiating element is desired – balancing length, weight and aesthetics. The mobile antenna should be of a type at least 3 feet long to be worthwhile as a base antenna for effective communication range.
  • antenna blockage by the roof or adjacent structures
  • ensure the power supply is linear to avoid noise–especially important at 27 MHz CB.

Depending on favorable terrain, base-to-base communication range on CB using omnidirectional antennas with at least 1/4 wavelength antenna with base (bottom) height of the antenna about 20 feet above ground, in clear terrain, and with clear channels can legally reach using various emissions modes:

  • AM / FM: 20+ miles
  • SSB: 30+ miles

In contrast, a base station antenna in an urban area on a first-floor deck (say 5 feet above ground) may have only 0.5 mile to 1.5 mile range on CB, MURS, or GMRS to mobile or portable units. This is similar to simply having a portable unit at the base location. Getting that antenna up on a second-floor balcony or roof (say 15 feet above ground) can significantly increase the range.

Estimate communications range using a voice activated recorder at the base station and drive around, giving location on each transmission. Compare CB range to MURS or GMRS range with multiple radios near the recorder.

FM and CTCSS

FM mode is strongly preferred for family and neighborhood communications. For CB radios capable of FM and CTCSS, the squelch can be set to only open for the group’s chosen CTCSS tone. The choice of FM mode and CTCSS is not about maximum communication range, but about reducing interference from other users on the same channel. If the family members or group turns off the radio because of static and interference, that’s no radio at all. Better to have shorter range village communication than no communication at all. CTCSS on CB radio finally largely levels the playing field with other license-free radio bands like MURS and GMRS that could always use FM and CTCSS.

A CB radio gives a heads up when members of a family or group are coming back home. This gives some peace of mind in case of a breakdown, in that the home base or another base station can be reached on the local channel.