Wednesday, March 9, 2011

Loop Skywire antenna



Are you looking for a multiband HF antenna that is easy to construct, costs nearly nothing and yet works well?

You might want to try this one. The Loop Skywire antenna is a full-sized horizontal loop. Early proponents suggested that the antenna could be fed with coaxial cable with little concern for losses, but later analysis proved that this was a bit of wishful thinkingthe relatively low values for SWR across multiple bands indicate that cable losses were part and parcel performance. The best way to feed this versatile antenna is with open-wire ladder line, with an antenna tuner in the shack to present the transmitter with a low value of SWR.

The antenna has one wavelength of wire in its perimeter at the design or fundamental frequency.

If you choose to calculate Ltotal in feet, the following equation should be used:

Total L = 1005/F

Where F equals the frequency in MHz.

Given any length of wire, the maximum possible area the antenna can enclose is with the wire in the shape of a circle. Since it takes an infinite number of supports to hang a circular loop, the square loop (four supports) is the most practical. Further reducing the area enclosed by the wire loop (fewer supports) brings the antenna closer to the properties of the folded dipole, and both harmonic-impedance and feedline voltage problems can result. Loop geometries other than a square are thus possible, but remember the two fundamental requirements for the Loop Skywire—its horizontal position and maximum enclosed area.

There is another great advantage to this antenna system. It can be operated as a vertical antenna with top-hat loading on other bands as well. This is accomplished by simply keeping the feed line run from the antenna to the shack as vertical as possible and clear of objects. Both feed-line conductors are then tied together, and the antenna is fed against a good ground.

Antenna construction is simple. Although the loop can be made for any band or frequency of operation, the following two Loop Skywires are good performers. The 10- MHz band can also be operated on both.

3.5-MHz Loop Skywire

(3.5-28 MHz loop and 1.8-MHz vertical)

Total loop perimeter: 272 feet

Square side length: 68 feet

7-MHz Loop Skywire

(7-28 MHz loop and 3.5-MHz vertical)

Total loop perimeter: 142 feet

Square side length: 35.5 feet

The actual total length can vary from the above by a few feet, as the length is not at all critical. Do not worry about tuning and pruning the loop to resonance.

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