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Friday, September 12, 2008

Centre Fed Dipole

This would have to be the simplest Centre Fed Antenna to contruct and is the basis for many broadside and yagi arrays. Quite simply the antenna is a half wave of the operating frequency and fed in the middle where current is of the maximum value and voltage is of the lowest value.Thus the feed point impedance is low in comparison to an end fed half wave because the voltage is at maximum at the ends of a half wave radiator.The characteristic impedance of a centre fed half wave dipole is a little over 70 Ohms. The same length antenna fed from one end would have an impedance of 4000 Ohms.So centre feeding a half wave definately has its pro's as 75 Ohm coaxial cable will provide a very close match.The length of a half wave antenna for a particular frequency can be found by:468/Frequency(Mhz)= length in feet.It is a good idea to cut the antenna on the long side and using a tuning device trim the antenna to resonance.Take care as to cut equal amounts from both ends as this will cause the feed point to shift to a diferent impedance point and also cause a feed point imbalance encouraging feed line radiation.These type of antenna can be built for almost any band and out of a mutitude of materials.Diploes of this kind built for 40m and 80m and sometimes the higher bands are usually made from wire.On the higher bands they can be made from aluminium tubing and are often not directly fed but are coupled to the antenna through simple matching networks.

3 comments:

Safal Trading said...

nice antenna blog


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Santosh Singh said...

nice informative post,thanks for sharing..


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Steve Berke said...

I enjoyed reading your article :) PLease continue publishing helpful topics like this. Regards, from Let's get gadget, a company whose into selling
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