Practical_Antenna_Handbook_0071639586

C h a p t e r 2 2 : R a d i o A s t r o n o m y A n t e n n a s 501
Figure 22.2B Ring radiator: balanced.
Either version of the loop or ring is mounted about 7 or 8 in above a ground plane
made of chicken wire, metal window screen, copper sheeting, or copper foil. (Copper
sheeting or foil is best but costs a lot of money and turns ugly green after a couple
weeks in the elements.) If you use screening, make sure that it is metallic. Some window
and porch screening material is made of synthetic materials that are insulators.
Figure 22.3A is a mechanical side view of the ring radiator antenna, while Fig. 22.3B
is a top view. The antenna is mounted above the screen with insulators. These can be
made of wood, plastic, or any other insulating material. The frame holding the groundplane
screen (Fig. 22.3B) can be made from 1- × 2-in lumber. Note that the frame is
strengthened by interior crosspieces that also support the antenna. The larger outer
perimeter is needed because the screen ground plane should extend beyond the diameter
of the radiator element by at least 10 to 15 percent of that dimension.