Antenna Theory

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Hi simple dipole i don't see many people using this method of construction but used in commercial applications..
any like to explain how it works.??

Russ

In essence it is just a folded dipole with a 200-300 ohm feed impedance, although mounted vertically in basically all cases.
The coax feeder is composed of two sections, the first runs through the tube - a half wave (or quarter wave ??) long - and is usually an impedance of 100/120 ohms (or so) connecting at the open feed point. The  remainder of the coax that comes out of the mounting tee is 50 ohms.

The  100 ohm (or so) section is a transmission-line impedance matching arrangement to transform the 50 ohms to 200-300 ohms to provide a better match than a direct connection at the dipole's feed point. 

Note that the matching section only provides a 'flat' response over narrow-ish band of frequencies around the 1/2 wave resonance point, too far away in frequency introduces capacitive or inductive reactances, altering the measured SWR value.

It has been a while since I even looked/worked/thought on one of these devices hence my poor recollection of the 1/4 or 1/2 wave matching section length. Thinking about it now (just before posting), it has to be half wave so the transformation process works correctly.

Hope that helps..

73 Doug


PS : From memory, no 'Googling' involved !