Indoor Antenna Projects 3
Indoor Antenna Projects 3: Attic VHF/UHF Rotables
In many instances, a listener can mount a VHF or UHF beam in the attic, on a rotor. They can achieve good results depending on their location and height off the ground. Antenna size limitations are solely dependent on the available room in the attic that allows rotation of the beam without hindering its rotation.
To check the attic for the maximum size antenna, the hobbyist must first enter the attic and measure the distances in the area planned for the set-up. Trusses, electrical wiring, air conditioning / heating ducts, and the items that are stored up there, can all play a factor in the actual antenna size.
Finding your antenna with a pair of long johns dragging off the front end is not a pretty sight.
After measuring the area in the attic, the antenna size and height off the attic cross members can be accomplished. Remember to allow for a base to accommodate the rotor. This too must be calculated into the system.
Building a sleek system, to rotate two or three small beams, can be dashed, when the system will not turn because of an oversight in the measured values of the attic area in question.
Turning radius is the important factor. Each antenna forms a rectangle ABCD where the maximum distance is AD or BC.
A————————————B | | | | —-|———————————-|—- BOOM | | | | C————————————D
For horizontal antennas, the turning radius is from the mounting point to the tip (either side) of the longest element. Multiple this figure by two for the turning diameter of the antenna.
For vertical antennas, the truss angle places the biggest problem as the height from the boom to the tip of the reflector (B or A). In essence, this will shorten your boom length or require a lower rotor mounting.
Indoor telescoping beams
One problem I have encountered from listeners living in apartments is the space to place a VHF/UHF antenna. This concerns operation of the beam from a bedroom or porch. In these cases, the listener can not keep the antenna up and must remove the antenna after he or she finishes.
Using telescoping elements can solve the problem. Adding a two or more section boom can further the portability for the antenna making it idea for field day, camping, or travel also.
Dimensions for beams have been discussed earlier, so I won’t rehash them here. The best portable and quickest for set-up is the four-element VHF antenna. A two-section quick clamp boom with mounting flange for a camera tripod provides the base. Each element contains two telescoping units and a snap on fitting for attachment to the boom.
The antenna described here can be stored in a briefcase save the camera tripod. A VHF four-element antenna can provide 9dBi of gain for the apartment dweller.