Quadrifilar Helix Antennas
Construction notes and dimensions

The finished model should look like the one labelled "Tall narrow QHA".

Element Dimensions

Mast - 1.5m of 32mm PVC waste pipe.
Elements - 8mm mini-bore soft copper tube - 8 copper elbows for the corners.
2 - 190mm lengths - bottom horizontal tubes
2 - 903mm lengths - short helix elements
2 - 1002mm lengths - long helix elements
4 - 90mm lengths - top horizontal elements

Note - these are cutting dimensions and assume that 90 degree elbows NOT bends are used - The dimensions on the drawing are from centre to centre of the repective elements - you may have to adjust your cutting sizes accordingly.

4 self-tapping screws for feed.
Suitable length of RG58 or UR43 for balun and feed.
32mm cap to plug top end of mast.

Drill 4 - 8mm holes at 90 degrees to each other 25mm from the end of the mast - make sure the holes are Square and in the same plane!

Mark and drill the bottom holes remembering they are in opposing pairs spaced 100mm apart - you're advised to check the measurements at least twice before drilling!!!

Drill a 7mm hole near the top of the mast for the cable entry. 

Drill pilot holes in the 4 top elements for self-tapping screws and assemble top tubes, push coax through hole and make top connections with screws. Wrap the coax 4 times around the mast and tape/glue in position (balun).

Push the elbows onto the top tubes and measure from the centre of each leg - it should be 200mm, NB  -you may have to cut more off if you used swept bends rather than tight 90 degree elbows.

Assemble bottom tubes, make sure they are central and square to top tubes.

Bend helixes to suit - tip - try and find a former of some type; a suitable log or large pipe makes the bends nice and neat.

When your happy with the shape solder up the elbows. It should appear circular when viewed from the end.

Check the connections and cap the top end of the pipe.

The copper tubes can be fixed to the mast using glue/silicon sealer and/or tape - make sure you seal the coax entry hole.

Push a suitable piece of wood up the bottom end to avoid crushing the PVC tube too much when clamping to the mast.

I hope you'll give it a try - it really is worth the effort.


Steve Blackmore
steve@sabcom.u-net.com

Copyright 1997 John Boyer & Steve Blackmore