Indoor Antenna Projects 2

Indoor Antenna Projects 2: VHF/UHF Discones

The discone antenna is a rather unique antenna for VHF and UHF. The discone has no gain to speak of, yet can provide the user with a range of ten times the design frequency for reception and transmissions. This means that a discone designed at 140 MHz will work fine up to 1.4 Ghz.

Hiding the discone outside may prove to be quite a feat. Unlike most antennas, the discone has a large skirt and is tall to boot. Its shape can draw attention. If you can place the antenna outside, it will give you excellent coverage over its range. Inside, the discone works well minus the attenuation caused by the building it’s in.

There are many Discones available on the market if you choose not to build one. Building one can be fun if you take the time and lay everything out ahead of time. There are a couple items I would like to point out about discones.

1) The gap between the top-hat and the skirt is critical.

2) The area under the skirt is a null to the antenna.

Design parameters are easy.

** The top-hat diameter is:

(0.1778 * (984/f MHz))*12

Example: at 140 MHz –> (0.1778 * (984/140))*12) = 15-in

** The diameter of the skirt, at the base, equals the length of the skirt elements. This gives the user the closest impedance 50-ohms. Thus a skirt element length is:

(0.2675 * (984/f MHz))*12)

Example: at 140 MHz –> (0.2675 * (984/140))*12) = 22.56-in

** The gap is:

(0.007114 * (984/f MHz))*12)

Example: at 140 MHz –> (0.007114 * (984/140))*12) = 0.6-in

A tin funnel works well as a starting base, to which you can solder brass skirt elements. A SO-239 chassis connector can be fit into the cut-off funnel end and the top-hat soldered to the center pin of the SO-239. Insulating spacers can be used to strengthen the gap. I’ve used a brass screw that was soldered between the center pin and the top-hat, but you can use anything that you can solder.

Wire beams indoor

For many of us, the antenna is the biggest problem. Radio fans have to contend with neighbors and the XYL in order to pursue their hobby. An interesting HF and VHF idea I’ve seen used is the wire beam.

The wire beam is unrolled and suspended in the direction required for operation. After the operation is complete, the wire beam is rolled up and stored for the next time. This type of beam is excellent for 10m and up to 1.25 m. You may be able to set up a wire beam for frequencies below 10m if you have the space to do so. This antenna is also excellent for suspension in an attic.

A simple model is shown below:

R DE D ———–||–|————-|———-|–||———– || | | | || || | | | || || | | | || ———–||–|————-|———-|–||———– || | | | || || | | | || || | | | || ———–||–|————-|———-|–||———–

Aside from the elements, the rest of the configuration is non-conductive. Wooden dowel supports are outside the ends of the antenna and string/rope can be used to support the elements. The wooden dowels are not required if suspension is taught.

You can see that variations of this set-up can be incorporated to accommodate most any frequency from 10 to 1.25 meters.

 

INDOOR ANTENNA ARTICLES

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