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Thread: Delta Loop HF Antenna for the Ariel

  1. #1
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    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 12:16 PM.

  2. #2
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    Counterpoise

    I'm responding here because nobody else has run with this thread yet
    Am one who needs a tutorial. Can understand this is very important stuff
    but need to be taken by the hand and shown what to do. Even how to think about it?

    I like the idea of an Ariel being able to put out an 8.000 mile GPS signal.
    Seems like it would be useful to set something like this up for cruising.
    Don't know. Don't know options.

    If I needed to speak with authorities, with homebase, loved ones, as they say....
    picking up a satilite phone, if that is correct, is about all I can imagine.
    Can see the internet and some of the toys used for texting might be useful.
    Haven't an idea.

    Can see from the delta diagram that the horizontal leg running thru the interior
    could bring a lightning strike inside............
    Last edited by ebb; 02-15-2013 at 09:06 AM.

  3. #3
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    Thanks Ebb. It's always good to hear from you.

    I'm a Ham, so this all radio stuff seems sexy to me. But my point is: before chopping up the backstay, try a Delta Loop first. I'm able to tune the mast and rigging from 10 to 160 meters without mechanical modification.

    Regarding lightning: as a former broadcast engineer who worked years at AM stations that took continual direct hits with no damage, the three secrets to lightning survival are grounding, BONDING, and surge suppression. EVERYTHING electrical on my boat is bonded to a single point that terminates at the backstay chainplate, which is connected to seawater with a submerged zinc plate. The one wire shown in my diagram running to the tuner is only one of many grounds connecting everything to that bonding point. The discharge path for lightning needs to be as direct and vertical as possible -- without turns or sharp bends. In my case, the path is from the masthead, down the backstay, to the submerged zinc plate. All other connections to the backstay are bonding wires.

    The whole point of bonding is to keep everything conductive at the same potential (voltage) so that, during a lightning hit, there is no difference-of-potential between objects on the boat -- and therefore -- no current flow. It doesn't matter if the entire boat suddenly jumps to 1 million volts, provided there is no difference in the potentials between everything on the boat. Ohms Law still applies to lightning.

    The best guide I've found for lightning protection of sailboats was produced by the University of Florida: http://edis.ifas.ufl.edu/sg071
    Last edited by pbryant; 02-18-2013 at 07:22 PM.

  4. #4
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    Once did extensive (for me) downloads on everything lightning I could find.
    And I remember Prof Thomson's paper is in a thick file in a box here in the kitchen.
    Altho I will have to read it again to remember anything!

    My impression is that there is no route you can direct a strike to. If it wants
    to come down the mast and jump sideways, it will. If lightning chooses any
    or all at once of the stays & shrouds it will....... My conclusion,
    remember, I really know nada about electricity, domestic or heavenly, is to
    attach heavy duty battery cables to the wire at the forestay, backstay, and
    top shrouds and throw the ends with wired-on chunks of zinc into the water.
    Whatzat, baiting the lightning god?

    I will go below if I can. If I can't, I'll pray that the Faraday tent works the way
    you guys say it's supposed to. I will not ground the mast thru the interior
    of the cabin. Last I heard, we ground to scintered bronze plate because it has
    a lot of surface area in its structure. But somebody else says, wrong, it has to be
    of lot of square footage of solid bronze to guanantee enough mass for a ground.
    I have an objection to fastening a great hunk of metal on the boat underwater,
    because I've also heard that a lightning strike can blow out a big hole....
    right where the ground plate was.

    If I remember, I'll wrap any loose electronics in an aluminuzed survival blanket.
    And throw the breakers on the built in stuff. Make a cup of tea.

    Really have misgivings about grounding the boat thru the interior.
    Last edited by ebb; 02-19-2013 at 02:07 AM.

  5. #5
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    > My impression is that there is no route you can direct a strike to.

    Benjamin Franklin answered that question a while back when he invented the lightning rod. Lightning CAN be directed, and it's a good thing too, or there'd be a lot of dead broadcast engineers and power utility workers. Seawater makes a pretty good ground. Fresh water is much more of a problem. If I were stuck in a thunderstorm in fresh water, and had enough all-chain rode to get a decent scope, I'd connect my common-point ground to the anchor chain and drop anchor to wait it out. A jumper cable from the forestay to the rode would do the trick.

    The next time you are out in the country and see those high power transmission lines, look closely at the thin wire that has no insulators running along the top of the towers. That's a "lightning leader" intended to direct lightning strikes away from the three-phase power lines hanging from big insulators below.

    Of course, the "more is better" approach of connecting everything, shrouds and stays, to the water like you describe has merit. But if you want continual protection, you may feel a bit silly dragging all those wires through the water all the time, and leaving them hanging at the slip. And there's also a problem with copper corrosion. Copper will rapidly dissolve in moving seawater because of an effect called: "erosion corrosion" (also known as impingement attack). If you stream copper wires underway, they won't last very long. Soon, you'll pull up stubs cut off at the waterline. But still, if you have no other protection, streaming jumper cables is a good form of episodic protection.

    Bronze is chosen for "Dynaplates" because it doesn't rapidly corrode and doesn't develop an insulating layer on its surface. Cupronickel, a mixture of nickel and copper (10% nickel to 90% copper) which is highly resistant to corrosion from moving seawater, would also be good. Aluminum oxide is a bad conductor - so is copper when it forms compounds with sea salts (the "green stuff") which you will find rapidly develops on the submerged ends of your jumper cables. Pure zinc doesn't develop an insulating layer, and although it does corrode by dissolving, the corrosion is "beneficial" to any more-noble metals nearby. We're all familiar with that affect. If I used a bronze plate, besides being more expensive, it would corrode my aluminium outboard's prop. I already tried that. It's amazing how fast aluminum dissolves when its close to bronze.

    >I have an objection to fastening a great hunk of metal on the boat underwater, because I've also heard that a lightning strike can blow out a big hole....right where the ground plate was.

    Yes it can! It's like setting off a depth-charge right next to your hull! During a strike, a few thousand watts are dissipated into a small volume of water, the surrounding water is turned to very high pressure steam, and if there is a rigid attachment between the plate and the hull - since water is incomprehensible - but the hull is compressible, the vapor pressure blasts a hole in the hull. If it's attachment is weak (and you're very lucky), the plate may blow itself away from the boat - ripping somewhat smaller holes in the hull below the waterline where it was once attached. That's why I float a zinc plate a foot or so away from the hull behind the boat, suspended in the aft end of the engine well by its terminating wires alone. It just streams along behind the boat. I may lose the plate in a strike, but not the boat, and I've got spare plates (that cost $20 apiece). I admit that it would be with considerable trepidation - and many poured libations to Thor, that I would grab hold of the backstay chainplate to install a spare plate while underway after the previous plate just got blasted away. But lightning never strikes the same place twice, right? (Because that place has now been vaporized.)

    I also object to making more holes in my hull below the waterline to mount a Dynaplate.

    Wrapping up your loose electronics in anything conductive (foil, space blanket, etc.) is a good idea. I have a microwave oven on board (I ain't puttin' no propane inside my cabin!) and that's a great place to toss my spare handheld transceiver and GPS. I'm not sure about tripping the breakers, it's a coin-toss whether that would help or hurt by letting the positive side of the power supply float unimpeded rather than keeping everything connected to the low impedance path back to the battery. I'd be inclined to turn everything on instead. That's what we did at the broadcast transmitter: tower lights, building lights, and all the gear got powered up. We let the utility company, or the diesel generator when the power failed, absorb the hits. By the way, on the subject of surge suppression, on offshore oil rigs (talk about an extremely vulnerable spot to be during a thunderstorm!), they suppress lightning-produced voltage spikes on their AC power by connecting a bunch of 12 volt batteries in series. Two sets: each through opposite polarity steering diodes. The batteries had a voltage slightly lower than the peak voltage of the AC power. The AC keeps them partially charged, and when a spike occurs, the batteries absorb the spike as a sudden charge current. Ordinary automotive batteries (and boat batteries) will absorb hundreds of amps for a few tens of milliseconds without damage. Brute force. Works great! Cheap and effective.The folks at Exon-Mobil shared that little secret with me. They said they'd taken countless hits, and never had a battery blow up.

    One thing I've noticed that puzzles me: I've never seen a single bit of marine life clinging to my zinc plate. Why doesn't anyone use zinc in anti-fouling paint - instead of copper? Because it dissolves too quickly?
    Last edited by pbryant; 02-19-2013 at 04:08 PM.

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