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Thread: Delta Loop HF Antenna for the Ariel

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  1. #1
    Join Date
    Nov 2010
    Location
    Sunnyvale, CA
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    104

    Delta Loop HF Antenna for the Ariel

    The long time standard HF/MF antenna for sailing vessels is the insulated backstay antenna. These work very well on vessels that are large enough to offer a ground plane that is a substantial portion of a quarter wave length at the lowest expected operating frequency. The ground plane is usually accomplished either by connecting to a conductive hull - or in the case of fiberglass hulls - with a wire mesh laid inside the hull below the waterline that establishes capacitive coupling through the fiberglass to the sea.

    But with smaller vessels, as is the case with my 26 foot Pearson Ariel, the length of the vessel is insufficient to provide a good counterpoise at longer wavelengths - since the Ariel is only 18.5 feet long at the water line – just slightly longer than a quarter wave at 20 meters. To operate efficiently at 30, 40, 80 and 160 meters, I needed another solution.

    There are also some serious disadvantages to the conventional insulated backstay antenna: 1) it requires the installation of insulators in the backstay, which operates at high mechanical tension. If an insulator fails, you can lose the mast (a very inconvenient experience at sea). Using "Johnny Ball" insulators (the type you see on utility pole guy wires) will prevent complete loss of the backstay (it'll slack the backstay instead) because they interlock the lines, but a failed Johnny Ball insulator still results in a short-circuited and therefore failed antenna. Insulators require at least four (usually swaged) mechanical connections, which are also vulnerable to failure. 2) The insulated backstay has no DC continuity to the rest of the vessel, making anything connected to the antenna vulnerable to static buildup and lightning damage. Connecting lightning suppressors at the output of an antenna tuner is a tricky compromise since the legitimate RF voltages can be over 1,000 volts when the antenna length approaches a half wave or multiples. 3) The antenna “tuner” (correctly called a coupler) may not be able to tune the antenna when its length approximates a half wave, or multiples of half waves, because an end-fed half wave antenna presents infinite impedance at its feed point. If the objective is to operate on ALL of the Ham bands, and on ALL of the maritime bands (which are interspersed between the Ham bands), then choosing the correct length of an insulated backstay antenna presents a daunting problem of avoiding half wavelengths. A loop antenna, on the other hand, presents much more moderate excursions in its impedance up and down the bands.

    I’ve been operating a Delta Loop antenna as illustrated (see attachment) for over two years with very good success. It is tunable from 1.7 to 30 MHz, with no gaps. There is complete DC continuity between all of the antenna elements, negating the need for lightning suppression (I trail a zinc plate at the backstay chain plate during thunderstorm activity and whenever at the dock). And the biggest advantage, in my estimation, is the fact that no modifications whatsoever need to be made to the sailing rig. I chose a feedpoint at the forestay because when the forestay is energized with RF, it's less of a hazard of giving the crew (me) an RF burn (I've had a few - they're nasty and take forever to heal). The feed connection is made on the inside at one of the three large through-hull bolts attaching the stem. The ground line is 2 inch tin-plated flat copper braid that runs through the bilge and connects at the backstay chainplate with one of the three chainplate bolts. The antenna "tuner" (coupler) is located in the space previously used by the water tank. I used an SGC model SG-230 tuner, but others should work as well. I used inexpensive snap-on ferrite RF suppressors on all of the wires that exit the mast base, installed at the mast base. Note that the ground wire also connects to the mast base (don't use an RF suppressor on that wire) and is bonded to the wire that runs through the bilge. That wire is very important: without it, the boom is "hot" when transmitting (burn risk). The shroud wire chainplates are not connected to anything, and the shrouds act as resonators at shorter wavelengths. And the shroud lines attached to the mast smooth out impedance excursions.

    The lightning discharge path is a straight shot down the backstay to the submerged zinc plate, providing a "zone of protection" under the backstay (protecting the cockpit). Caution: don't use any metal but zinc - anything else will cause rapid corrosion of an aluminum outboard motor shaft and propeller. I used bronze at first, and practically dissolved my prop after 6 weeks of immersion. (I leave my outboard in the water - a new outboard is cheaper than back surgery.)

    You can see my position report beacons which are transmitted hourly on 30 meters (10.148 MHz) , listed here (see "Stations which heard N8QH-8 directly on radio"). My range record for the antenna so far is 8,000 miles -- using 20 watts of power. I've had some great "QSOs" (conversations) with other vessels in Australia, Japan, and Brasil, and solid email coverage with Sailmail stations in Hawaii and Panama with 50 watts. Who needs a sat phone?

    73 N8QH
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    Last edited by pbryant; 02-13-2013 at 11:16 AM.

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