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CB Antennas

By Tore Aasli, LA1SSA

Citizen band transceivers operate on 27 MHz, and home brewed antennas are easily made.

Most of these antennas are 1/4 λ with radials (ground plane). Usually 3 or 4 radials give an acceptable ground plane solution for home use. The more shortened the antenna is, the worse it will perform (efficiency). These shortened are antennas with coils or capacitance hats.

The antenna below has two radials. The effect of this is shown in the Total Field where the antenna radiates more in the direction of the radials than in the opposite direction.

Explanation

This is a simple antenna model. It has one radiator of 5.36 meters, and to similar length radials - one in the Y direction and the other in the X direction. These two radials work as an artificial earth, and they are tilted lightly downwards

Total field 1

Explanation

The elevation plot shows that this model antenna will have a "gain" in one direction (in the vector sum of the two radials), since the radial are in angeled 90 degrees as the figure above shows.

The outer ring "0 dB reference" is showing 1.52 dBi, which is equal to -0.63 dBd.

This figure also shows that this simple CB antenna has a very low take-off angle which is good for local work, but also for DX-ing when conditions are favorable.

The Straight Dipole CB Antenna

 

 

Explanation

This is also a home-made CB antenna, a regular dipole. It's hung up in a pole 10 meters above ground. The antenna overall length is 5.36 meters divided into two electrical poles (hence the name di-pole). The feed point is exactly in the middle, 2.68 meters from each end. Therefore, this antenna is symmetrical.

Explanation

The field plot of this simple yet quite powerful home made antenna shows much of the same characteristics of the antenna in the example above: Low take-off angel and also quite good for DX-ing.

If you have a pole or a tree to hang it from, why spend a small fortune buying an expensive ready-made antenna? This antenne can be made of an ordinary plastic insulated lead, just connect the center lead of the coax to the top element, and the coax shield to the lower element are weather-proof it with electrical tape or something similar. A 1:1 (current-) balun is preferable, though.

Explanation

A 3-D plot of a vertical dipole, the antenna above.

 

 

 

VSWR of this latter dipole antenna shows a nice 1.1:1 SWR at 27 MHz.

The antenna impedance is approx. 73 Ω and therefore fed with a 75 Ω coax. A 50 Ω might also do the job, but SWR will probably rise to about 1.5:1 and the antenna should be a little more broad-banded.

 

 

 

18.02.2012