Parabolic Dish with BiQuad feeder - Page 4
Saturday, 12 September 2009 22:20


the element soldered onto the copper pipe

The element must now be attached to the reflector. Note that only the two "ends" of the copper wire are to be attached to the copper pipe - the centre of the copper wire must not touch the copper pipe (hence the notch which was cut into the end of the copper pipe. The copper wire element should be approximately 15mm away from the reflector surface.

strip the outer sheath Strip approx 30mm of the outer sheath from the end of some coax cable.
fold the braid back, trim the centre conductor Fold the braid back over the outer sheath, and trim the centre conductor till about 4mm is still protruding.
solder the centre conductor to the element
another view

Insert the braid into the copper pipe, so that the end of the centre conductor lines up with the extreme end of the copper pipe, and solder the centre of the element to it, ensuring the centre of the element is not in contact with the copper pipe. Click on images for a detailed view.

Note that the feed between the rear reflector and the biquad element needs to be shielded. Using coax to feed the biquad element directly, and positioning the coax inside the copper tube achieves this. Use of bare conductors as a feed between the reflector and biquad element results in a radiating feed, which will have a detrimental effect on performance.

the copper pipe crimped onto the coax the completed biquadI used a coax crimper to crimp the end of the copper pipe onto the coax. This ensures that the coax would not move inside the copper pipe.

Now terminate the other end of the coax with an N connector.

the completed biquad

Weather proofing

If you intend to mount the biquad outside, it's best to place it into a weather-proof enclosure, to prevent corrosion, and to prevent water ingress into the coax. Numerous people have used small tuppaware containers successfully. This can be achieved by drilling a hole in one side of the container, and pass the coax tail through the hole, leaving the biquad itself inside the container. Seal up the hole for the coax with some silicone, and your biquad should be protected.

Note: Test the plastic box or container first by nuking it (2-3 minutes) in the microwave with a cup of water on the other side. If the water's hot (boiling)... plastic cool to the touch, then plastic has no microwave absorption and is good to use.