CQTF

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CQTF er tímarit Félags íslenskra radíóamatöra, ÍRA
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TF3JA

Antenna-lifting kites

By Gudmundur Loeve TF3GL

Purpose

The two primary purposes of using kites to support antennas are in my mind:

• Raising antennas where there are no supports for portable operations
• Raising antennas that are too big for mechanical supports for portable operations

In the former case, a lightweight wire and a simple parafoil-type kite without struts may be perfectly adequate for 20-40 meter operation using an end-fed half-wave. In the latter case, nothing short of a 1 m ² (10 square feet) will do for a stable and reliable flight at an acceptable angle.

Antenna configurations

Obviously, kites lend themselves to supporting vertical antennas of different kinds. It must be pointed out that a vertical doesn't need to be totally vertical; in fact, a 45° sloping wire will have half of its radiation vertical and half horizontal, and in actual practice, the vertical polarization will dominate at low angles over real earth whereas the horizontally polarized energy will primarily radiate upwards. This makes a sloping kite vertical perfectly adequate for both low-angle DX purposes and NVIS communication.

On the topic of verticals: Where better to put them than by the sea? And, incidentally, where can you find a steadier and more reliable breeze than precisely there?!

Typical kite antenna configurations: Left, a vertical half-wave. Right, a vertical quarter-wave with two elevated radials at 2 meters off the ground.

In the below I will be going a step further than the venerable half-wave vertical, investigating the extreme case of a 160 meter vertical Yagi on the seashore; a feat by no means impossible or out of reach of any ham with three kites.

To whet the appetite of potential experimenters, quick – and entirely unoptimized – design in EZNEC shows a whopping 10.5 dBi of gain on 160 meters with a reflector, driven element and director with the lower end 2 meters (7 feet) off the ground, and the closest element 50 meters from the sea (close-spacing (<0.1 wl.) beams were not examined, a certain emphasis was put on achieving a reasonable feedpoint impedance).

Even the most stable kite may fly erratically due to eddy currents in the air and the varying force of the wind. This boils down to varying feedpoint impedance for sensitive designs like the end-fed half wave, whereas a quarter-wave with a ground plane would be much more forgiving (but more lossy, as an elevated ground plane vertical would have about 3 dB more overall system losses than the half-wave, but perhaps as little as 1 dB less gain towards the sea provided it is close enough to "see the sea" equally well as the half-wave).

Half-wave vertical — EZNEC modelling

The base-case setup is a 78 meters long radiator starting at 2 meters above average ground an pointing upwards at a 45° angle pointing away from the sea, true to the typical nature of sea breezes.

Modelling the anticipated performance of this configuration in EZNEC yields some interesting results:

• Low-angle radiation (<10°, suitable for DX) is -1 dBi towards the sea 500 meters away, but 3 dBi 50 meters away from the water. This demonstrates the far-field importance of the reflecting plane of the sea; the antenna needs to "see" it well.

• Increasing the angle from 45°to 60°(a tall order with such a long wire, even for a "pully" kite), yields a 1-3 dB improvement. Regrettably, this improvement is not directly additive to the sea-proximity improvement above.

• Adding a reflector (another kite, antenna wire and 3 meters of isolating cord to tie it to the ground), yields 2-4 dBi 500 meters away from the shore, but 7-8 dBi at 50 meters (no further gain is obtained by getting your feet wet; this antenna is way high enough for 50 m to suffice, and besides, half-waves don't need ground to be efficient).

• Adding a director at 50 meters from the sea with the rest of the elements further back, yields 10.5 dBi in an unoptimized design that I haven't yet tried in practice, but which consists of an 80 meter reflector, 78 m driven element and a 76 m director, all spaced at about a quarter wavelength (closer spacing will provide a bit more gain, but will definintely increase the capacitive or inductive reactance, perhaps to the point of being difficult to match in an end-fed antenna).

For comparison, my 12.8 meter (42') high vertical ground plane with two radials at 2 meters height yields -3 dBi at a 10°take-off angle on 160 meters, which is more than most non-elevated radial antennas manage to do, but still a tantalizing 12.5 dB down from the seaside Kite Yagi – and what's more, the radiation from the seaside Yagi peaks at 4° take-off angle, which leads to about 7 dB lower path loss plus reflection attenuation at 8000 kilometres (night-time conditions), according to a rough calculation.

DX from the seaside Kite Yagi should be a breeze, with up to 20 dB better performance than even an efficient ground plane vertical. I can't wait to try it: It would be like running 10,000 watts instead of my regular 100!

Performance of the Kite Yagi: 10.5 dBi at a DX-friendly 4° take-off angle, and may still be improved upon.

Close-up of the antenna base, fed against my car (no, it's not exploding, this is EZNEC's way of showing currents). Being a half-wave, the antennta needs very little counterpoise.