CB radio base with 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. Whether mobile or base, it’s important to get the tip of the antenna higher above ground level vs. antenna length. That is, an antenna up high and clear of obstructions within about 1/2 wavelength 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. A 3 ft long loaded mobile antenna (like Wilson Little Wil) may perform about 13 dB worse than an antenna like Antron 99 mounted at the same height and location.

At 27 MHz CB, the communication range is significantly affected by antenna efficiency, which included antenna loading and groundplane coupling. A 3 ft long loaded mobile antenna may have significantly less range than a 9 ft long 1/4 wavelength antenna, even if the SWR is low enough to be acceptable. An Antron 99 (USA Patent 4,360,814) is a popular base station antenna introduced circa 1984, but as with most CB end-fed monopole vertical base station antennas, it’s about 18 feet (1/2 wavelength) long and may be too long for some locations.

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 (e.g., a sandbag) on the groundplane to avoid it from blowing off the structure from high winds.

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 degraded performance. The groundplane can be smaller than expected 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: 15 miles
  • SSB: 25 miles

Whereas a less ideal situation about 10 feet above ground with some obstructions about 10 feet away may have maximum base-to-base range like:

  • AM / FM: 5 miles
  • SSB: 10 miles

Or base at 10 feet above ground to mobile may be have maximum range like:

  • AM / FM: 3 miles
  • SSB: 5 miles

A base station mobile antenna in an urban area on a first-floor deck (say 5 feet above ground) may have only:

  • AM / FM: 0.5 mile to 1.5 miles
  • SSB: 1 mile to 3 miles

This is similar to simply having a portable unit at the base location. Getting the antenna up with a reasonable groundplane on a second-floor balcony or roof (say 15 feet above ground) can significantly increase the range, say 3 miles to 5 miles to other base stations, and perhaps 1.5 miles to 2.5 miles to mobile units. A primary limit on range for antennas below the average roofline in the neighborhood is the blockage of the roof and adjacent structures, which can significantly reduce the effective range. Consider an antenna on a pole 20+ feet away from taller structures to reduce blockage and increase range - experiment first being making permanent installations, to find the best location for the antenna. Use conversations with other distant radio users, and measure the background noise level at various locations. Compare background noise levels at various locations; a change of just 3 meters can make a significant difference in noise and interference levels, and thus effective receiving range.

In any RF commmunications or broadcast system, range is not fully predictable without experimentation. use a handheld or mobile radio temporarily to test a proposed location before making significant purchases.

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. CB radio now largely matches other license-free radio bands like MURS and GMRS that have always supported FM and CTCSS.

A CB radio provides advance notice 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.