Radar Arch Grounding

[svcattales]

We installed an aluminum arch on our catamaran to mount a bunch of stuff. It supports a radar scanner, an Air X wind generator, a solar panel and a 24' SSB whip antenna. Instructions with the radar specify connecting the shielded cable to the RF ground. The wind generator is supposed to be grounded to sea water and suggest connecting to engine (mine don't ground to sea water since sail drives are isolated to prevent galvanic action). Not sure what I should do about grounding the arch, if anything. If I connect arch to RF ground would that cause interference with SSB transmissions since whip antenna is mounted on the arch? Or would the grounding the arch to RF ground help increase the ground plane? Any advice will be appreciated.

[exposure]

Does the arch connect electrically to the whip portion of the antenna? I would hope not since that would make it an active portion of your antenna and you could get shocked/burned if you touched it while transmitting.

If it were me I would connect the arch to my RF ground. It should add to the counterpoise and improve performance somewhat. This may mean connecting the arch to copper strips by running a strip up to a bolt holding the arch in place. If you can't get a strip all the way to a bolt, then get one as close as you can and use some braided copper to finish the connection. You can get a short length of braided copper by removing it from a piece of coax.

If you did that you could ground the radar shield to the arch. If it is not part of the RF ground then I would only ground the shield to your RF ground at the display end. That is how mine is done, grounded only at the display end. What kind of RF ground do you have?

[Rick]

The base of a whip is the maximum current, minimum voltage antenna drive point. The other "half" of the antenna design is an rf ground ideally immediately beneath the whip base. Therefore, if the whip is mounted on a radar arch the arch should be part of the rf ground. If it is NOT then the metal will "rob" you of effective radiated output power and degrade the standing wave ratio of the transmitter drive.

If you are using an automatic antenna tuner to drive the whip at frequencies not exactly at the whip's quarter wave frequency of resonance then the tuner should be mounted IMMEDIATELY below the base of the whip with the tuner's rf ground connected to the arch. Continuing downward towards the sea you should have each mounting tube of the arch at its base connected to rf grounding elements carrying into the seawater contact. On a catamaran, for example, you ought to have a bronze fitting (even if not for water) in the hull for reliable sea water contact. The surface area of the fitting does not have to be large, as most people believe. Inside the hulls the bronze fitting gets wired with multiples of parallel insulated wires as directly in line as possible to the bases of the arch mounts. The number of wires in parallel should be equal to the length of the wire distance run, in feet, divided by two. For example, if the wire run that conveniently makes the contact between the bronze fitting and the arch base tube flange (or whatever is the metal) is eight feet then use 4 parallel conductors like two 14-2 marine grade wire tie wrapped intimately together. This makes a very low inductance phantom ground appear at the base of the whip and antenna tuner.

After this is done if you merely provide a convient rf ground from the radar transceier/antenna unit to the arch you will comply with lightning saftey in a reasonable fashion. A "shield" will not be necessary or perhaps even desirable.

After doing the above you should be able to verify that you can transmit with good SWR at frequencies near 10 MHz yet need a tuner to make it work on an SSB freq of 8 MHz or the HAM SSB freq of 7 MHz (40 meter band). What most quarter-wave whip users don't realize is that when using them at twice the frequency you don't have a quarter-wave antenna any more you have a half-wave one which is much more "fussy" when it comes to tuning and rf grounds. So, when using 16 MHz SSB freqs (or 14 MHz SSB HAM freqs) you may have a more difficult time radiating a high effective output power into the atsmosphere.

Some of this info flys in the face of some myths which run especially rampant with cruisers regarding radio installation. Even many marine radio so-called "experts" don't operate outside of grunt rules of thumb and this info seems, therefore, foreign to them.

[svcattales]

Thanks for the great advice Rick & Exposure. The arch is not electrically connected to the SSB whip so I will ground it to RF ground. Rick, I will follow your advice and put the tuner right under the arch and run the antenna wire through the deck to the whip rather than up thru the arch leg. I don't have a seawater connection yet and would appreciate some more advice. All my thru-hulls are nylon so I will have to install something when we haul out. One alternative is a dyna plate, but I understand that they really don't increase surface area. Another alternative is to replace one thru-hull with a bronze one. Any other suggestions? Also, I have a huge stainless steel tank (120 Gal) that runs across bridge deck. Should I tie this into the ground plane? If I understand your formula for ground plane radials, if I have a 12' run, for example, I would use 6 insulated wires connecting the tuner to the bronze sea water ground. I don't need the 100 sq/ft of copper strap? Sounds much easier to install.

[exposure]

I have a dynaplate. It came on my boat when I bought it. It is installed on the hull that houses the navstation. It is one of the bigger plates and is actually cut in two with each half mounted on one side of the hull. They are about two feet apart. The tuner is mounted in a locker at the aft end of the hull. It takes about two feet of 3" copper ribbon to reach from the tuner to the dynaplate. The feed is GTO wire lead to an isolated backstay antenna. The ground ribbon continues from the dynaplate another 20 feet to attach to the rear of the SSB, which is an Icom 706 ham rig in my case.

All of this seems to work quite well based on the various signal reports I have gotten and some limited use of winlink. BTW, the dynaplate has been in the water for 1-1/2 years and I am sure is well fouled, but still seems to perform well enough.

[Rick]

Just to make sure that there is no miscommunication I mean that the tuner should be mounted IMMEDIATELY beneath the whip mount which would put the tuner on the radar arch itself. That way there will be only about 6 inches or so of output lead from the tuner to the whip element itself. The tuner rf ground gets connected directly to the radar arch and the arch mounts at the deck level (beneath the deck if you can get to the below deck fasteners) will have the multiple wire attachments which then lead to through-hull bronze fittings. It is not necessary that the fittings be as large as a whole Dynaplate. In fact you could have them fabricated to only be about 3-4 square inches each.

If the arch has sufficient geometry (especially including tubing diameter) then the phantom ground will work so that it appears to be at the level of the whip base. Now when I describe "geometry" in this particular case I refer to geometry which presents a significantly low inductance at the frequencies of operation from the tuner rf "ground" terminal all the way down to the sea water, not necessarily the number of square feet of arch occupation in any plane.

The first thing to do (after mounting and wiring the tuner) is to provide the multiple wires to the through hull "plates or rounds" and check to see what is the SWR of the whip with the tuner in the straight-through mode and at a quarter-wave frequency for the 24 ft element...if your transmitter is not capable of outputing that theoretical frequency then there are other methods...this is one of the easiest. If the SWR is 2 to 1 or better then the phantom ground is working. If not then you add multiple wires all the way up the radar arch (inside a tube if possible for appearances sake yet make the test externally first to verify operation).

If you do not place the tuner immediately beneath the whip then you will not have an ideal feed to the whip and the arch will absorb a significant amount of rf energy, grounded or not.

Every time you observe the usual antenna tuner/backstay installation with the "high voltage" GTO cable leading from somewhere below deck and married up the stay to some lower insulator you KNOW that it is a lousy installation because that whole run of cable is losing energy during transmit into the uninsulated portion of the stay, pulpit and chainplate.

[GordMay]

Rick:
Thanks for the interesting & informative tutorial on R.F. Grounding (etc).

Could you please provide a little additional detail & background on some of the concepts?

R.F. Grounding:
1. Why do parallel ground conductors provide a lower inductance R.F. ground, than would a larger single-conductor ?
2. What is the derivation of the (specific) ½ distance in feet = number of conductors recommendation ?

Lightning Safety:
An arch (/w antennae etc) is likely to become a secondary Lightning Terminal (attachment). You suggest that the described R.F. ground will provide adequate “Lightning Safety”.
3. Would smaller conductors (ie: 2 x 2#14) provide a robust lightning downconductor ?
and
4. Would a bronze thru-hull (small fitting) provide an adequate ground electrode for lightning protection ?

Thanks,
Gord

[Ryan]

There's so much mysticism in HF setups these days that until I see an explanation based in physics it's hard to put much credibility into some claims.

I recently heard a "guru" claim that GTO was crap and the best antenna feed wire is vinyl coated SS lifeline wire. Some installers swear by backstay standoffs; the theory being that the high current in the antenna feed wire can magnetically induce a current in the backstay, robbing the signal of some strength. Since the magnetic field decreases exponentialy with distance, even a small separation helps. Others say this is BS.

Another thing I never hear mentioned is the conductivity of the wire used as the antenna. Would copper wire behave differently than SS?

Every time you observe the usual antenna tuner/backstay installation with the "high voltage" GTO cable leading from somewhere below deck and married up the stay to some lower insulator you KNOW that it is a lousy installation because that whole run of cable is losing energy during transmit into the uninsulated portion of the stay, pulpit and chainplate.

So if the tuner has to be below decks, and the backstay is used as the antenna, and the lower insulator has to be above where it can be touched by unsuspecting crew, how would you do it?

I'm no guru, just a curious former EE

Thanks,

-Ryan

[GordMay]

Stainless-Steel (Rigging Wire) is less than 1/3 as “Conductive” as Copper Wire.

Copper is the standard by which electrical materials are rated and conductivity ratings are expressed as a relative measurement to copper. These ratings will frequently be expressed as "28 IACS". IACS is the abbreviation for International Annealed Copper Standard and the number preceding "IACS" is the percentage of conductivity a material has relative to copper, which is considered to be 100% conductive. This does not mean that copper has no resistance (is 100% conductive in an absolute sense), but rather that it is the standard by which other materials are measured. The higher the % IACS, the more conductive the material is. This standard refers to a pure, "standard" copper having a resistivity of 1.7241 microhm-cm at 20̊C or 68̊F (equivalent to a resistivity of 1/58 ohm per meter for a wire one square millimeter in cross section).

Approximate IACS conductivity values of some common materials.
Silver 105%
Copper 100%
Gold 70%
Aluminum 61%
Brass 28%
Zinc 27%
Nickle 22%
Iron 17%
Tin 15%
Phosphor Bromze 15%
Lead 7%
Nickle Aluminum Bronze 7%
Steel(s) 5 - 25%

[Rick]

Steel, iron (and stainless) is ABOUT 50 times more resistive than is copper (one reason that I always advise using discrete battery negative return cables from high current alternators rather than relying on the engine block to carry the current).

Any wire that is not properly stranded, tinned, and strain relieved may be considered "crap". I suppose that the tech Ryan quoted may have only encountered non-tinned GTO cable and, therefore, found a lot of premeture corrosion because antenna tuners are usually mounted in very moist environments.

The usual formula used to calculate the equivalent inductance of parallel wires assumes infinite separation....it makes the formula easier to calculate. With infinite separation the equivalent inductance is decreased multiplying by the reciprocal of the number of wires (assuming equal length and equivalent geometry of all wires). If the wires are in proximity to each other then the inductance is FURTHER reduced by the mutual inductance between each wire to all other wires. You can see that this produces a geometric reduction so that, for a given frequency, it is easy to see that a practical number of wires quickly reduces the inductance to a negligible value. The value of "m", the mutual inductance INCREASES with a DECREASE in separation between teh wires. The more intimate the wires are the lower the inductance. That is why I say to tie wrap them all tightly together.

My formula was heuristically determined when I made inductance measurements in the HF band and does not apply to much lower frequencies. This technique of using the added mutual inductance effect to gain low inductance is not new. Litz wire was used in early high-frequency transformer winding for switch-mode power converters but is very expensive. It looks like many fine wires of a normal cable yet each wire is insulated with a clear laquer and, therfore, terminating the ends of the cable requires careful chemical removal of the insulation only at the ends. In addition, printed circuit board layouts using high speed logic or switching waveforms often benefit with the use of parallel closely spaced conductors to avoid signal degradation.

Insulated standoffs separating GTO cable from the backstay BEGIN to help avoid the parasitic loss into the rigging but my measurements taken on real boats has shown that in order to effectively gain back the loss the GTO would have to be over 3 feet away....not very practical. I recommend NOT installing a lower insulator at all. If you have one jumper around it with a piece of S/S rigging wire the same alloy as your rigging. Jumper around the toggles as well of the lower chainplate and turnbuckle. Drive the chainplate from INSIDE the hull, avoiding having to drill a hole for the GTO completely. If you have one, fill it in.

Back to the physics. With a GOOD phantom ground in place (this infers that there will be no significant voltage gradient between the sea and the ground lug of the tuner) AND with a judiciously chosen backstay length for your frequency band(s) of operation you should not have a too-horrible an SWR on the backstay itself. Note that the function of the tuner is NOT to lower the SWR on the antenna element (it CANNOT) it is to match the impedance between the tuner elements and the transmission line from the transmitter. AS A RESULT the transmitter will be able to put out a maximum rated power WITHOUT its internal protection circuitry causing automatic power reduction.

All of the automatic HF tuners today can literrally "tune" to a garbage can. In fact, if you take away the can it will STILL achieve a tune (try it with no output wire). SO, merely getting an indication of a "tune" is insufficient to assume that you are generating any effective radiated output power. SIMILARLY by merely "talking" to another location is no objective observation to conclude that your installation is good as many installation techs do. Why is this? Because even with 200 Watts from the transmitter it can all be lost within the elements of the autotuner without a real radiater to generate effective output power. Some radiation loss occurs within a tuner and only ONE MILLIWATT may be sufficent to "talk" to someone quite some distance away on good conditions. With bad conditions you will NOT be able to talk.

Here's one statistical manner in which to determine good lightning protection wire size. Approximately 90 % of all lightning strikes are less than 10,000 -20,000 Amps. A number 8 AWG wire will fuse open at about that level. If you "ground" items which you wish to "protect" with wire having a total amount of cross-sectional area as that of #8 wire then you are probably doing a good job without dipping into the law of diminishing returns. Keep in mind that a good rf ground is a good lightning ground and vice-versa. The only proviso is whether or not the rf ground carries sufficient copper to sustain a sufficient passage of coulombs before destruction. Two 14-2 conductors together will rate a current capacity close to that of a #8 AWG wire (slightly more).

[Rick]

In answer to Gord, yes, bronze throug-hull fittings around 2 1/2 inches in diameter, or so, are sufficiently large to work as rf grounds in sea water. Keep in mind that the inside cylinder leading to the outside adds surface area to the sea. The ones with a grating over the cylinder work fine as well.

In answer to Ryan, the physics of a quarter-wave radiator shows that at the lower end (where you might grab the backstay) the voltage is MINIMUM and the current is MAXIMUM. This phenomenon helps to keep you "safe" from an rf burn. Now the real issue here is that if you touch the backstay whilst someone is transmitting with, say 200W out of the transmitter, will you ge a shock? NO! You don't get "shocked" with rf you get burned on the skin! The energy does not go into your heart, it cannot. I'm not telling you to do this yet if you were to grab hold very hard onto any part of the backstay (the higher up you go the more difficult it will be to stay also in contact with a "ground", of course) you will likely feel nothing. If you slightly BRUSH up against it you might get burned by the rf high-frequency arc-over to your skin. I have had many rf burns working in a high-power HF transmitter site with 65 transmitters having between 500 W and 100kW as receiving them was unavoidable while working on down transmitters in the proximity of all of the open transmission lines. The burns are an annoyance yet not life threatening except for the possible jerk-away reaction one might have when perceiving a burn and falling or hitting something in the process.

Does this help?
Regards,
Rick

[exposure]

I recommend NOT installing a lower insulator at all. If you have one jumper around it with a piece of S/S rigging wire the same alloy as your rigging. Jumper around the toggles as well of the lower chainplate and turnbuckle. Drive the chainplate from INSIDE the hull, avoiding having to drill a hole for the GTO completely. If you have one, fill it in.

This would work great on my boat. However, is it advisable from a safety perspective? Could harmful voltages be present at the lower backstay? My backstay is in a handy spot to steady yourself when on the aft deck. I also have young kids who are a little to forgetful of warnings, as most kids are.

BTW, what is the best way to electrically attach GTO to the backstay above the insulator?

[GordMay]

Rick:
I remain confused.
Thanks for bearing with us on this - it’s not only interesting, but important that we understand.

You answered: “...yes, bronze through-hull fittings ... are sufficiently large to work as rf grounds in sea water... “
but
I asked: Would a bronze thru-hull (small fitting) provide an adequate ground electrode for lightning protection ?

And advising that a good rf gnd is a good lightning gnd, answered:
”... Two 14-2 conductors together will rate a current capacity close to that of a #8 AWG wire (slightly more) ...”

But:
#14 AWG is rated 20 Amps x 2 conductors = 40 A (NEC)
40 A derated 80% (per NEC & CEC Codes) = 32 Amps equivalent
#8 AWG is rated 70 Amps, more than twice as high as 2-#14's.
ABYC specifies (E-4.5) a Primary Lightning path with a conductivity at least equivalent to #4 AWG Cu. - #4 AWG, rated 135 Amps. (more than 4 times 2-#14's)
Secondary Lightning Bonding conductors are specified (E.4.5.2) as #6 AWG minimum, which is rated 100 Amps, triple that of 2-#14's.
and
Thru-Hulls & Seacocks shall NOT be connected to the main lightning down conductor, but may be connected to the external Ground Plate or internal Equalization Bus (E-4.9).

You state "... Approximately 90 % of all lightning strikes are less than 10,000 -20,000 Amps..."

I think that these figure represent an arbitrarily low guestimate. Others have indicated average currents of around 20 to 30 kA. However you cut it, lightning is a very high energy phenomenon.

According to Martin Uman*, most lightning current measurements have been in the range 5,000 to 20,000 amps, though a famous strike, just before the Apollo 15 launch in 197, was measured at 100,000 amperes, by magnetic links attached to the umbilical tower. Currents over 200,000 amps have been reported.
Most commonly, the lightning current ceases in about a millisecond for a given stroke, but sometimes there is a continuing current on the order of 100 amps following one or more of the strokes. This is called "hot lightning" and it is the cause of lightning fires according to Uman. The temperatures of lightning are 15,000-60,000̊F for both "cold" and "hot" lightning - it is the continuing current that starts some 10,000 fires per year in the U.S. in the estimation of Uman.
* Uman, M - "Lightning", Dover, 1984 -&- "All About Lightning", Dover, 1986.

According to the National Severe Storms Laboratory, based in Norman, Oklahoma, “A typical lightning bolt contains 1 billion volts and contains between 10,000 to 200,000 amperes of current.”

According to the National Lightning Safety Institute (NLSI), “the average lightning strike contains 20,000 amps”.

According to NOAA’s, Lightning FAQ’s (‘Jetstream - Online Weather School) “the average lightning bolt carries about 30,000 amps of charge, has 100 million volts of electric potential, and is about 50,000̊F.”

[Rick]

Gord, Notice that your sources are in apparent conflict with each other and that the first one that you site agrees with my original statement. You may also have noticed that reporting of various studies often misquotes the exact figures in terms of "average, peak, etc." For example it is erroneous to state that ."a typical lightning bolt contains 1 billion Volts". This statement is ridiculous and you can discern that merely by looking at the wording which is not used by scientists. That is layman wording. Scientists do not refer to strikes as "bolts" and lightning does not "contain" any voltage it exhibits voltage which is determined by measurement. In additon, when a large amount of charge in the vicinity of any means to discharge will do so WAY before any billion Volts can be developed. In fact, the scientific wording is voltage gradient in terms of the number of Volts per millimeter reached which may cause imment discharge My original statement came from the results by the Atomic Energy Commission studies (at that time this commission controlled all of the nuclear research in this country) which HAD to be realistic in order to establish safety procedures during the above ground detonation of nuclear weapons....who wanted one of those beasts going off unexpectedly from a lightning strike? Now there has been an "upper end" measurement of over 100,000 Amps and, no matter where you desire to draw the line, mother nature can always generate one bigger.

Let's face it. If we cruisers were like most North Americans we would live behind layers of false security believing that it is for our own good and never venture out into nature without perhaps at least being on a monster cruise line ship. I've worked on many boats which have been hit by lightning and MOST of them do not have burned out #8 AWG bonding wires. ABYC has most recently changed their recommendation from #8 to #6. That does NOT make existing bonding systems using #8 wires unsafe or unacceptable, does it? I've seen evidence of lightning leaving "wounds" on the outside of the bonded bronze through-hull fittings which fared quite well and did not need replacement.

My boat was hit by lightning when in Panama and it literally blew out the outside surface of my depth sounder yet did not cause a leak because the bronze housing was just fine. Love that bonding wire (#8AWG by the way).

My example was for TWO sets of 14-2 wires which make a total of 4 #14 wires. Now that combination will conduct MORE lightning current than a SINGLE wire having the same copper cross section due to the lower parasitic inductance. The faster that you are able to dissipate the energy the less likely that damaging voltage will be developed across the conductors so that arc-over to other objects does not occur.
By the way, one does NOT derate the current carrying capacity of wires when considering lightning protection because one is interested in how much current can be carried before destruction in a very short period of time. NEC deratings are based on current carrying times essentially considered as infinite as to compared to the time of lightning, including "hot" lightning.

Here is yet another example where I differ with the recommendations of ABYC (keep in mind that these ARE merely recommendations and do not carry any legal authority). In not recommending that thu-hulls and seacocks be connected to "the main lightning down conductor" is ignoring the fact that in a particular installation lightning WILL arc over between the two in the absence of a discrete conductor. What lies in between gets damaged. In addition, their statement infers that there is only one down conducton. In cruising sailboats a single downconductor cannot protect the entire vessel. The more down conductors that you have the better. The straighter the better, and the lower the inductance the better. To my visualization that means that any and all thru-hulls need to be considered as entrance/exit points for lightning and my personal observations of lightning damage verifies this. If one visualizes lightning protection wiring as rf "grounding" leads then you will endeavor to make better and more effective installations.

[Rick]

Exposure,

One good manner to attach GTO wire to a backstay is to use a piece of monel or stainless wire to serve the GTO wire to another piece of rigging wire (like used for running rigging so is flexible like you would use for "jumping" around toggles or a lower insulator). Serve or clamp the rigging wire to the backstay (using compatible alloy material). Do NOT cover the backstay attachment. It should remain visible for inspection at all times so as to not allow degradation of the stay to occur for any reason.

The dissimilar metal contact where the GTO is served should be covered with something like 3M Scotchkote part no 054007-43906 else that contact will not last long.

Your question of the possibility of dangerous voltage being present at the backstay in combination with someone grabbing the stay is a good one and conjures up great fear in most people. With an ideal quarter-wave element and a 250 Watt transmitter there is little danger. The time that high peak voltage gets deveolped at the lower end of the element is when there is high SWR (a non-ideal element). One must plan for the worst case, of course. Again, keep in mind that one does NOT get electrocuted with rf at these power levels and frequencies. What happens is that a white burn can occur on the skin at the point of contact either with the stay or "grounded" pulpit, etc.

Contrary to what you might imagine you do not need a 5 kilo-volt rated insulation to protect against such burns. Almost any plastic protection will prevent a burn. Use your imagination. Some people use those plastic sleeves normally seen covering lowers for grabbing onto. There are many plastic cable covers used in dressing various wires in automobiles, RVs, boats, etc. which allow almost instant installation. If you put a section of black PVC over the turnbuckle and toggle when the stay is removed then it can be easily slid up for inspection and turnbuckle adjustment. The chainplate can be covered with 3M mylar tape. That tape is also great for insulating between stainless fittings which get attached to aluminum booms, masts, etc.