Re: RF grounding methods for sailboats: A Summary
- From: bob <bob@xxxxxxxxx>
- Date: Thu, 01 Jun 2006 20:50:41 +1000
chuck wrote:
Richard Clark wrote:I would also add that sea waters conductivity is not a consistent as some seem to think. The salinity of salt water varies with temperature. NASA has surveyed most of the world oceans using microwave radar and salinity if far from consistent. This fact combined with tidal flows, man made pollution from storm water drains and other factors makes "connecting" to sea water seem dubious.On Tue, 30 May 2006 09:21:56 -0400, chuck <nospam@xxxxxxxxxx> wrote:
If anyone spots any errors of fact or significant omissions, I'd welcome appropriate "recalibration". Thanks in advance.
1) Grounding plates
Will not work if submerged as much as four feet
Hi Chuck,
Hello Richard,
Perhaps the context for the summary was unclear. My objective was simply to identify several existing recommendations for obtaining RF grounds on plastic and wood vessels over saltwater. Regrettably, I lack knowledge of any published theoretical or empirical comparison of these proposals that provides an objective, quantifiable measure of performance.
Don't know how you got this miss-impression.
That it will not work if submerged as much as four feet? From Roy's report on his NEC-4 modeling. It is presumably based on the known skin depth of RF at 14 MHz.
2) Wire in water
A one-foot length of wire immersed near water surface is sufficient for
near-perfect results based on W7EL's NEC-4 model results. Assumed
performance is similar to grounding plate.
This conclusion is conflict with the first, making it a mystery how
you came to either in summary. The focus on "water surface" is as
though you are trying to force it work like a pool of mercury. Water
is NOT a ground plane in the sense of conductivity. Water is a
terrible conductor. It is only its huge mismatch with air that gives
it such superb propagation, not match, characteristics. Distinguish
between the two.
Well, some of what you are saying is pretty much what I had thought. If you read my posts on the other thread, you'll see where I was heading.
We agree that seawater's conductivity is lower than copper's but greater than earth's. We agree that a single ground rod driven into the earth will perform poorly as the only return path for a vertical antenna over land, but at least in some cases will perform better than if it were not there. Will a wire dipped into the sea perform better than the ground rod driven into the earth? I doubt there is much disagreement on that, even though water might be characterized in relative terms as a "terrible conductor". The only relevant question, then, is "how much better?" and so far, the only numerical answer that I am aware of has come from NEC modeling.
The characterization I reported of a wire dipped into the sea is based on Roy's modeling, assuming I have not misunderstood of course. Others have made the same point, but now we have model results to support it.
BTW, I have just discovered a response from Roy to one of my posts on the other thread that is available on Google, but has never shown up on either of the two newsgroup subscriptions I have. Roy addressed some of my concerns in that post so if you have not seen it you might do a search for it on Google.
3) Radials
Even shortened (loaded) radials elevated over seawater work as near-perfect based on N6LF's NEC-4 modeling. Objections to radials are
The objections are they are wholly unnecessary when ground is so
easily achieved by conventional means. You would need 120 radials to
shield against the loss you perceive, and that loss doesn't matter
What loss is it that I perceive? N6LF's results show near lossless results with only four shortened radials over seawater.
when you stand to gain so much in propagation. You couldn't even
field a tenth of these radials. At HF, and maintaining their tune
and symmetry, you would be lucky to fit in 2.
Other studies have shown a single elevated radial over land to lose less than one dB over a perfect ground plane.
At that stage of thegame, there is absolutely no match advantage over conventional
techniques aboard a small craft (and at HF you don't qualify for any
thing other).
4) Counterpoise (i.e., mast, forestay, shrouds, lifelines, engine, metal
tanks, 100 square feet of copper, keel, rudder, etc. bonded together)
This type of
counterpoise is also the approach recommended by both Icom and SGC.
Only because it is already available and doesn't ask you to go any
further for no obvious advantage.
Well, what makes life interesting is that to advocates of the other approaches, there are obvious advantages.
5) OCF dipole w/horizontal component along deck
Not commonly used,
Who would choose a complicated design over so many simple ones?
Multiple resonant radials that cover the popular marine and ham bands on a small boat are not seen by all as simple. One might ask the same question of those who advocate the counterpoise approach, since the wire in the water is simpler. I think choosing the best system (broadly defined to also consider operation over fresh water and near-vertical radiation when important) will be easy if we can only get some objective, reproducible data and/or analysis.
Is that where it stands, folks?
If you want a dipole, make a VERTICAL dipole, even a lousy one.
Certainly worthy of consideration. Many backstay antennas are probably operated as half-wave vertical dipoles (end-fed, of course) above 10 MHz or so.
Finally, and to repeat, learn the distinction between matching and
propagation. Your focus on matching issues is like seeing your glass
3/4ths empty. Looking at the propagation advantages in comparison is
like seeing a pitcher of water nearby that will fill that glass a
dozen times.
I didn't address any matching issues at all that I can see, Richard. Sorry if I misled you.
All of the alternatives utilize the same seawater for propagation and the same vertical radiator. They differ in whether there is any high-angle radiation from a horizontal radiator, and possibly in the magnitude of their "ground return losses."
Appreciate your comments.
73,
Chuck
73's
Richard Clark, KB7QHC
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If we assume you can connect to sea water, do you want to place most of your current in such a small area? I doubt someone could keep this connection clean enough in seawater to be efficient. This fact combined with electrolysis will lay waste to any connection very quickly. Maybe a gold plated conductor might be able to survive in a salt water environment.
The way most people install radials in yacht would make them elevated, rather than buried below skin depth. N6LF has indicated in his modeling that short radials as long as .1 wavelength work just as effectively as 0.5 wavelength radials over sea water. Since you not trying to combat ground loss whats the point of trying to build a radial field when 2 or 4 .1 wavelength radials will do the job? Besides building a elevated radial system will have some capacitance to ground. A single radial should over sea water capture most of not all of the antenna return currents
Besides from all the posts i have read burying something like a Dynaplate below skin depths is akin to burying your rf current below a copper plate. I also believe there is a rule of thumb that 5 skins depths is the maximum return on investment in using the available conductive depth of any material. Its probably that its only ever the piece of wire to the dynaplate thats working as a short radial. I dont see how anyone can say a dynaplate could ever work when below skin depth?
One thing i am curious about is that if you suspend a radiator or conductor well below skin depth in air whats the radiation efficiency like of that conductor? To me on a yacht it looks like a conductor suspended in a U channel suspended in air with the top open and exposed. Maybe it will operate like a waveguide with some cut off frequency, this is a wild guess.Maybe someone who can model conductors below ground can model this. Most yachts have at least 3ft of freeboard above water to play with.
But who knows there seems to conflicting advice on several points. Maybe Roy can clarify all these issues with his models.
Bob
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