The advantages of a Delta Loop antenna

[Cpt Pat]

Discussions of antennas often stirs some controversy, so I will preface this with the statement that ... this works for me. If you don't like it, don't use it.

I have been employed for 40 years as a communications tech and antenna design engineer. When I bought my diminutive 26 foot sloop, I wanted HF coverage from 160 meters on up with no gaps, covering the entire HF spectrum of both Ham and Maritime bands. Conventional wisdom in the sailing community is to use an insulated backstay antenna; but I vowed to avoid chopping up my rigging with insulators, dealing with the lightning suppression issues of an insulated stay, and juggling the antenna length to avoid exact 1/2 wave multiples at any operating frequency -- which creates untuneable gaps. So I chose instead a Delta Loop antenna.

Insulated backstay antennas 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. Electrically bonding everything on the vessel is the most effective lightning countermeasure - and an insulated backstay is unbonded from the rest of the boat by its very nature of being insulated. 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 (you may want to use flat stainless steel braid instead) 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). The ground to seawater is only needed for lightning suppression - it is completely unnecessary for RF propagation. Caution: don't use any metal but zinc - anything else will cause rapid corrosion of an aluminum propeller. I used bronze at first, and practically dissolved my prop after 6 weeks of immersion.

I've had some great "QSOs" (conversations) with other vessels in New Zealand, all over Europe, Australia, Japan, and Brasil, and solid email coverage with Sailmail stations in Hawaii and Panama from California with 50 watts. Conversations with other distant Hams are often met with incredulous comments when I tell them I'm not using an enormous directional antenna with a high power amplifier.

73 N8QH

[Badsanta]

Thank for the update. Good to know.

[Sharki127]

Hello Pat,

I'll preface this by saying that we are not keen ssb radio users. Really we only use ssb on passages to collect weather data and talk on nets. We don't have a Pactor modem.
On my previous boat, a Pearson Vanguard, I installed a Delta Loop antenna together with an old Yaesu ham radio and a manual tuner. The set up worked great. Sitting in Majuro I could talk to Brisbane and Miami as well as nearby stations. On my current boat, an Amel Sharki, I installed a Delta Loop antenna together with an Icom 802 radio and AT 140 tuner. This set up also works great.
I got the idea for the Delta Loop from reading a SGC Tuner Manual. The Loop is elegant in its simplicity.

[Horizon Watcher]

Excellent detailed post about a subject of great interest to me. My antenna installation is the insulated back stay, and I get good performance on most frequencies, but on occasion, some frequencies do not couple with the tuner. I will be watching this thread closely in hopes of more interesting antenna information for my antenna of the future.

My current equipment is Icom M802 with At140 tuner, and 60' Insulated back stay with copper strap to external ground plate. Also having good luck with Signalink USB soundcard adapter for free email and WX maps. But very slow data compared to Pactor. I am looking for a used Pactor if anyone here has one, PM me with the info.

I would hope that others that have experimented with the Delta Loop on a boat would also ad comments of the experience.

[amytom]

How would you adapt this for catamaran use? No back stay and two bilges. Also more likely to have crew on the foredeck.
Roller furling on forestay and all connected physically to a cross beam with a bow light on it.

[Cpt Pat]

Originally Posted by amytom

How would you adapt this for catamaran use? No back stay and two bilges. Also more likely to have crew on the foredeck.
Roller furling on forestay and all connected physically to a cross beam with a bow light on it.

The back stay isn't necessary and I only ran the counterpoise (ground) line through the bilge because it was a convenient return path. Under the sole of the cabin would work as well. So, presuming you have RF continuity from the stem to the masthead through the forestay, I'd connect the feedpoint at the stem. The bottom of the mast needs to be connected to the "ground" return back to the antenna tuner -- and you have a delta loop.

I have all sorts of electrical stuff at my masthead: wind sensor, two VHF antennas (comms and AIS), steaming (masthead) light, and even red-over-green (navigation rule 25c) lights. All are RF-isolated at the mast base with snap on ferrite RF suppressors. Just don't put a ferrite on the return ground wire from the mast base.

So far as crew safety from RF burns is concerned - you'll have to make sure no one is hanging onto the forestay, the mast, or anything else that's long and conductive whenever you transmit. Otherwise, you'll hear loud screams... But the same holds true for an insulated backstay antenna -- it'll induce enough RF in all of the rigging to risk RF burns whenever you transmit.

[amytom]

So the aluminum cross beam being perpendicular to the forestay and being the stem won't be an issue?

[Cpt Pat]

Originally Posted by amytom

So the aluminum cross beam being perpendicular to the forestay and being the stem won't be an issue?

It might be. Without seeing the configuration, I can't say. I have to admit a lack of familiarity with catamaran rigging.

[Target9000]

I'm totally green when it comes to radio technology so forgive my very ignorant questions but reading this sparked a few that I thought I would ask.

1) The grounding is somewhat confusing to me, if I understand it right... You just connected a wire to the mast and sent one end back to the tuner and one end to the backstay. Why is part of the grounding wire braid and the other part isn't?

2) I know the antenna ground (counterpoise) connects to the lightning ground via the rear chainplate, but does the antenna ground ever connect to anything else such as the primary electrical ground?

3) We have a SS bowsprit and bowpulpit that are bolted to the chainplates. Would this make these parts dangerous when transmitting?

[deckofficer]

On a loop antenna, there is no counterpoise. Each end of the loop in this case goes to the hot and ground terminal of the tuner. Very simple, so that a small sailboat that isn't large enough to have an effective counterpoise can run a delta loop.

If you feed the loop at the center of the base leg it will be horizontally polarized and a cloud burner, good for short propagation distances, not so good for long haul. Feed it at one of the base corners it takes on a vertical polarization and will have a lower angle of radiation, thus a better signal at longer distances.

[Cpt Pat]

Originally Posted by Target9000

I'm totally green when it comes to radio technology so forgive my very ignorant questions but reading this sparked a few that I thought I would ask.

1) The grounding is somewhat confusing to me, if I understand it right... You just connected a wire to the mast and sent one end back to the tuner and one end to the backstay. Why is part of the grounding wire braid and the other part isn't?

2) I know the antenna ground (counterpoise) connects to the lightning ground via the rear chainplate, but does the antenna ground ever connect to anything else such as the primary electrical ground?

3) We have a SS bowsprit and bowpulpit that are bolted to the chainplates. Would this make these parts dangerous when transmitting?

You ask really good questions for someone who's green!

1) The antenna is actually two loops. One smaller loop that takes the path: tuner -> stem -> forestay -> mast -> tuner. The larger loop is: tuner -> stem -> forestay > mast -> backstay -> backstay chainplate -> tuner. The nice thing about RF is that it will radiate from whichever element is near resonance, and the rest of the rig acts as an open transmission line. The elements can be the "small loop", the "large loop" or for the higher frequency bands: the shroud lines. Technically, this is called a "periodic" antenna.

There are many antenna designs that utilize this effect to gain efficiency over a broad bandwidth without the need to switch antennas between bands, the most common is the broadband multi-element beam antenna. The ordinary TV antenna mounted on the roofs of homes is one common example - it needs to function over a very broad bandwidth that's wider than the entire HF band. All the elements of the antenna are connected, but only the ones nearest resonance are active.

Ideally, every conductor that carries RF would be a flat strap. The reason for this is an effect called "skin effect." At RF frequencies, all of the RF current is carried on the outer surface of a conductor. At HF frequencies, just the outer tenth of a millimeter. The rest of the conductor is wasted. In fact, even at a frequency as low the 60 hertz we use for AC power, there is no point in using a conductor thicker than about 2/3rds of an inch, because with anything thicker, none of the current is travelling down the middle. Coax cable designed for high RF power will have a hollow center conductor because it saves weight and materials by omitting the useless center of the conductor.

Skin effect and geometry make a flat strap a batter shape than a round wire for carrying RF. A flat conductor with the same amount of material has more than three times the outer surface area of a round conductor, and so RF is conducted through that larger surface area with less resistive loss. A round conductor also has more effective series inductance, which will cause additional loss.

But since we can't change the shape of our stays - it's round stainless - we just have to accept those losses.

Provided you don't break the loop, you can connect the ground strap to the primary electrical ground, but I recommend choosing one "common ground" point on your boat to which all of the grounds from all devices (usually including the negative terminal of the batteries) are connected. This is especially important on a fiberglass boat where you don't have a metal hull to use as a really low resistance common conductor throughout the boat. If you don't pick a common ground, you can get "ground loops" that will cause voltage drops between devices, and worse, in the case of RF, stray RF voltages that will do annoying and unpredictable things to your other electronics like reboot computers, lock up chart plotters, freak out your autopilot, fry your stereo, or cause the navigation lights to flash with every word spoken on SSB. Even if you do everything just right, you may still have to attach ferrite suppressors to some RF-sensitive devices.

3) Yes, the SS bowsprit and bowpulpit should be treated as "hot" whenever you transmit, regardless of whether you are using a delta loop or an insulated backstay antenna. You may find that it's "hot" at some frequencies and not at others, depending on wavelength. But it's prudent to stay away from any long conductive object while transmitting - especially if it's connected to the standing rigging. RF burns are nasty.

[denverd0n]

Originally Posted by Horizon Watcher

Also having good luck with Signalink USB soundcard adapter for free email and WX maps.

I'm curious what mode and software you use for your e-mail.

[thannever]

Originally Posted by Cpt Pat

It might be. Without seeing the configuration, I can't say. I have to admit a lack of familiarity with catamaran rigging.

I am also green but getting an amazing education in a hurry here. I have a 37' wood tri with synthetic rigging so the forestay and mast are my only long metal pieces. Does this mean I can do a Delta loop but some frequencies will not be as effectively tuned? thanks to all contributors.

[Cpt Pat]

Originally Posted by thannever

I am also green but getting an amazing education in a hurry here. I have a 37' wood tri with synthetic rigging so the forestay and mast are my only long metal pieces. Does this mean I can do a Delta loop but some frequencies will not be as effectively tuned? thanks to all contributors.

With such a large foretriangle, I suspect you'll be able to tune the entire HF band with a good tuner.

[belizesailor]

Re cat rigging. No backstay, and forestay hard to integrate into this configuration, but you could do a similar loop incorporating both side stays and mast or a single side stay and the mast. Should work the same as a small boat rig if the stay/wave lengths work out...right?