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UHF Antenna Gain


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When purchasing an antenna, you will hear terms of 3db, 6db, 4.15dbi, 2.15 dbi, 9db, 8.1 etc.

Some manufacturers use db other use dbi (because it looks better)

Db - refers to the gain compared to a ¼ wave roof mounted antenna.

Dbi – refers to the theriacal gain of an antenna in a free space vacuum. To compare antennas, take 2.15 of a dbi Antenna. E.g. – an 8.1dbi antenna = 6 db gain antenna.

What does gain do?

An antenna with a gain of 3 dB gain, means that the Transmitted (radiated) power is 3 dB higher (twice as much) – gain is logarithmic.

E.g.

  • A uhf CB is transmitting 5 watts into a 3 db gain antenna; therefore the radiated power is 10 watts
  • A uhf CB is transmitting 5 watts into a 6 db gain antenna; therefore the radiated power is 20 watts

Impact of having higher gain antennas

I always use a balloon to explain the impact of gain on the radiated energy, with someone stepping on it and gradually flattering the balloon.

  • 0 db – the balloon is round and provides the best all round coverage – ideal for mountains
  • 3 db – the balloon is ¾ round and provides the good all round coverage – ideal for mountains
  • 6 db – the balloon is ½ round and provides the best compromise for mountain driving and driving on the flat
  • 9 db – the balloon is ¼ round and provides the best for driving on the flat - dessert driving)

The higher the gain the bigger the antenna.

For further reading look here http://www.lakecomm.com.au/Antenna-Prop.html (im not asscoiated with this site)

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Reformatted to take up less space
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Sorrry I posted this late last night.

Yes take 2.15 OFF a dbi antenna.

Extending cables using connectors.

You can extend the antenna cable by using the correct 50 ohm connectors that have been properly terminated. But, you have

insertion loss" . Insertion loss is the loss of power across the connector typicaly 1 db.

Therefore it is always better to replace the antenna cable if possible and adviod using connectors.

Always use good quality connectors - i dont like the cheap screw together connectors that dick smith sell. I use crimp connectors with the correct crimping tool ( we but most off our connectors from Turnbulls)

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Antenna placement

Try and mount the antenna as high as possible on the vehilce.

If you mount the antenna on a bull bar, you will have some loss / reduction behind the vehilce. ( I have my antenna mounted on the fron of the vehicle)

Dont have any other antennas within 500 mm of your antenna.

Dont mount the antenna next to metal, ie at the base of the bull bar

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  • 3 weeks later...

So what is a good brand antenna as the fiberglass one I have receives well but is very poor on transmitting had a lot of trouble up the cape last year with 4 trucks

Sounds like have more than likely a fault, could be a shorted coax, and open circuit coax or a blown P.A in the radio antennas them selfs r reasonable reliable, is the antenna u have designed for UHF?

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