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CB Radio

CB Radio Antennas

  CB radio antennas are usually designed using either 1/4 wavelength or 1/2 wavelength principles.  A radio wave in free space will have a wavelength equal to 984 / MHz = feet.  However that same wavelength when traveling through wire or other metal conductors tends to travel a little less than the speed of light, and hence its velocity factor will be different.  Because of this the wavelength for any radio wave through wire will be about 95% of that of a radio wave in free space.  Thus the formula for a full wavelength antenna becomes (984 x 0.95) / MHz = feet.

  A 1/4 wavelength antenna will be (984 x 0.95 x 0.25) / MHz = feet or rather 233.7 / MHz = feet.  Even with the wire precisely measured, factors can effect its performance and so in practice it is the norm to make it about 2 to inches over to trim it down to lower its SWR for resonance into a 50 ohm coax cable at the feed point.  The same is true of a 1/2 wavelength antenna, however the 1/2 wavelength has a different feed point resistance and hence it can be designed for the exact calculated length and its resonance adjusted at the feed point matching network.  If needed it can be adjusted in length hence you can tweak it up, this is true of 1/2 wave vertical foremost.

  The 1/2 wavelength antenna is calculated by (984 x 0.95 x 0.5) / MHz = feet or rather 467.4 / MHz = feet. 

  If you want to experiment with building antennas then the usual place to start is to build a dipole antenna out of wire.  The full length of the dipole end to end is 1/2 wavelength, however each section of wire is 1/4 wavelength long and hence calculated using 233.7 / MHz = feet.  Since the antenna is fed in the middle, at this location the impedance is equal to 50 ohms.  Adjustment to the length is required for the best possible SWR ratio.  In the below graphic you see a dipole antenna illustrated, however this graphic also shows a 1:1 balun (balanced to unbalanced) transformer being used at the feed point.  You do not have to use this transformer since you can connect the shield of the cable to one side of the antenna and the center conductor of the cable to the other side of the antenna.  What the balun transformer does is help to balance the current or voltage (depending upon balun type) along the antenna, and hence helps to match the unbalanced 50 ohm feed line to the balanced dipole antenna.  This also helps to reduce losses to ground, where with the antenna balanced the portion of the signal normally lost to the ground without the balun transformer, gets reflected back up into the sky when the balun is used and hence the antenna achieves its specified gain of 3dB.  Also since it is a horizontally polarized antenna, it receives less vertical polarized noise, and so it is possible to hear signals on a balanced dipole that can not be heard in the noise of a vertical ground plain antenna.  If you want more detailed insight into dipole antennas then check out this resource here "Antenna Here Is A Dipole."  Also use this link to find an only antenna calculator and its downloadable software version "Dipole, Inverted V and Ground Plane Antenna Calculators."

 

 

  For the best possible performance of this antenna and any CB radio antenna of the ground plain kind, is to locate the antenna 1/2 free space wavelength above ground or some multiple of 1/2 waves, so that the radiation angle will result in the lowest angle possible to the horizon.  This is important if you want to talk some DX.  Also if you want to mostly listen in a certain direction and can not afford a beam antenna, then the dipole is the thing to use, however it is bidirectional in that it receives and transmits off of its two broadside plains.  You are facing one of those plains in the photo above, which means it would radiate at you and away from you on the other side as you see the antenna above.  Also the dipole is not much of an antenna for use locally since you have to aim it in the direction of a station, however for long distance way out across the nation, if it has a balun, then you got a good antenna, and without much expense too!  So you might want to try this sometime, merely just to see if you can build an antenna.  It might inspire you later on to build vertical ground plain antennas or who knows, a Sterba Curtain wire array.

  In general it is common to use the antennas most commonly seen on the market, which can consist of both the 1/4 and 1/2 wavelength types.  Not mentioned above are the short height mobile antennas of which one can also use mounted outside their home, although their performance is not like that of a full sized base antenna, as for which mobile antenna works the best on a vehicle or mounted at home the 102 inch whip fits the bill since it is a 1/4 wave radiator.  We will mention that the K 40 mobile antenna is a good buy for the price, since we know personally that you can talk skip barefoot with this antenna mounted on your car when the conditions are right.  The kinds of antennas available are too numerous to list here on this site at the moment, but given some time when we can edit things, we might try to list the most well known antennas here with pictures.