Bottom of the mast is a 2el 70MHz twin-book Rigid Quad while above are a pair of 110Ohm 2el twin boom rigid quads for the 2m band.

 

This pair are fed with 2 x 1/2 wavelengths (wl) of 75Ohm RG11 and are joined with a 50Ohm 'T' N-type connector where they join to Ecoflex 15 feed line.

 

The 1/2 wl cable lines (rather than odd 1/4 wl multiples) are made possible as a result of the optimisation of the feed point of each antenna to110Ohm (1/4 wl of 75Ohm coax is required to match each antenna to 50Ohm, then an odd 1/4 wl multiple to provide 50Ohm from to two 50Ohm 'virtual' feed points.

 

The 2 x 1/2 wl sections of RG11 (x.66 velocity factor ) provide just enough space to allow for feeding the top antenna with favourable spacing (1.5m centre to centre) to allow the top antenna to be fed at the bottom, and the bottom antenna at the top. Doing this, provides an extremely high Front to Side ratio (F/S), re-establishes pattern symmetry in the elevation plane (which quads tend not to have) and reduces feed line length required to join the two antennas. The final benefit (which is associated with higher feed point impedance) is the wide band capabilities of the antenna. Without any finalisation or adjustment from model, the antenna has a 1:1.2 bandwidth of 4.980MHz between 142.890MHz and 147.870MHz

 

Notice anything else? The single parasitic element between the two quads enhances Front to Back ratio (F/B) and adds just a little to forward gain too. the result? 11.01dBi free space gain over an impressive 72.2 degrees beamwidth.

 

De G0KSC