Antenna Theory

[VK4DCM]

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.??        

[VK4ADC]

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 !

[VK6ZFG]

Russ

The coax matching length is usually a quarter of a wavelength though sometimes they use odd multiples. Single quarter wave give wide bandwidth but multiples results is narrow bandwidth. I had a rather fat SMD made for 450 MHz that looked promising for 70cm but it was useless on anything other than the specific frequency it was made for. On dismantling it I found it was using a coax length something like seven quarter waves. This made manufacture easy as no inline coax joints were required. Reducing the matching length to a quarter wave made it extremely broad band.

I was told by a ham that worked at an major commercial antenna manufacturer that with a certain brand of RSG213 coax (not all work) it was possible to join a quarter wave section of desired coax impedance (125 for SMDs (RG63), 75 ohm for beams (RG11)) and pull this into the RG213 sheath and braid so as to take place of the existing RG213 inner. The result is that the coax still looks like RG213 but for a section of it is actually 100 or 75 ohm. You can only tell by carefully feeling down the coax until you find the connection between the two cable inners. You can be easily led to believe into thinking it is just RG213.

The coax inside the dipole acts as a balun.

73 Igor