# Colinear Vhf antenna

Textbook colinear vertical antenna designs show coax cable with alternating braid to centre configuration. Commercial whips seem to utilise a single wire as the radiating element with .66 velocity factor phasing shields along the radiator with no other connections.

How does this work? I can copy the design but am intrigued as to the theory behind it.

Also how do you feed the antenna? A 1/4 wave stub like a j-pole (or something else)? This has to be up in the weather, so simple and robust is better!

I'd explain this better with diagrams but I don't know how to put up a picture?

• To add a picture, click on the sixth icon in the toolbar above the text entry area - the square icon that looks like mountains and sun. Click the "paste" link to insert a picture from the OS clipboard. – Brian K1LI Jun 18 at 8:45
• I'm confused by the comparison between a coaxial collinear and a "commercial whip." Please point us to examples of each for reference. – Brian K1LI Jun 18 at 17:15
• I mean the fibreglass pole type as seen for mobile and base station use where the radiating elements are enclosed, rather than the steel whip types where loading coils etc an be seen. Ie: 1/4 wave a loading coil then 1/2 wave on top. The enclosed fibreglass poles seem to use one continuous conductor up the centre, threaded through tubing of 1/2 wave x the velocity factor as phasing elements. Searching for ideas on line I see numerous references to lengths of coax cable with the inner and outer conductors reversed every 1/2 wavelength. How did the first example evolve from the second? – R Johnson Jun 19 at 9:01
• coax cable with alternating braid to centre [conductor]? It sounds like you are referring to a coaxial Franklin antenna. – Mike Waters Jun 20 at 18:37

## 1 Answer

The ideal dipole antenna is roughly a half wave long. A monopole is half of that, with the other half in the conductive surface below the monopole. The colinear antenna is a series of stacked and phased half wave dipoles with a quarter wave section at the top and bottom.

If your commercial whip is only a half wavelength long or less, then it is just a monopole whip, and not a colinear.

As to how the colinear works... consider it as an antenna array rather than a single antenna. Each antenna is separately fed, and it is phased so that the radiation emitted from the antenna is in phase with the radiation from the other antennas, causing constructive interference in the desired radiation direction.

There are several styles of colinear antennas. A common commercial version uses folded dipoles (that look like flat/oval loops) mounted along a vertical rod. The version you describe combines the phasing harness (the coax) with the antenna elements. The shield and center conductor are swapped at half wavelength intervals to create the correct phase between each section.

Either way, the effect of this is to take the toroidal radiation pattern of the dipole and flatten it, to give decreased gain at higher elevation angles and increased gain horizontally. Sometimes (typically for colinears on towers) the length of the phasing is adjusted to cause the flattened toroid to "droop" to make it better fit the curvature of the Earth.

One of the advantages of using an array of antennas is that the radiation pattern can be "steered" by adjusting the phase between elements which shifts where the constructive and destructive interference occurs.