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Author Topic: Underwater radials- Fresh water  (Read 905 times)
W8JH
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Posts: 6




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« on: April 07, 2008, 08:12:09 AM »

I have a vertical ant. at the waters edge (Lake Erie) and have been wondering about adding radials underwater.  These would be under 5-6 feet of freshwater max.  Any guesses what effect this would have?  Is it worth it???

73,

Joe W8JH
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K7KBN
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Posts: 3693




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« Reply #1 on: April 07, 2008, 08:24:49 AM »

Lake Erie is "fresh" water.  With pollution, it could be more conductive than sea water!

As for the radials, just experiment and see what happens.  It's impossible to predict the results of something like this over the internet.  
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73
Pat K7KBN
CWO4 USNR Ret.
WT0A
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« Reply #2 on: April 07, 2008, 08:27:10 AM »

More radials can't hurt....
Glen
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K6AER
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« Reply #3 on: April 07, 2008, 08:27:46 AM »

Putting radials under ground or in the water will detune the radials. Efficiency will be less for the conducting properties of water are much less than copper on a ground surface. The capacitance coupling to the vertical radiator will be effected by the dielectric loss of water.

Although water may conduct high voltage the resistance of water is very high when compared to metal wire. I know there are plenty of hams who have antidotal stories of working DX with the vertical near water but water is not a substitute for a good radial system. If it was, broadcast stations would not need to put radials for their broadcast station antennas.

Some hams will say then why to broadcast stations always put the antennas in low lying swamp areas, simple, the land is cheep.
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WA7NCL
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« Reply #4 on: April 07, 2008, 08:44:56 AM »

Pure water is a dielectric.

Lake Erie is probably not "pure".  So you have a lossy dielectric.

Soil is well known to be a lossy dielectric as well.  If it is wet, it would have a higher conductivity.

I would treat the dirty water like wet soil.  It would seem that radials would improve things.

The radials would definately have a different electrical length when emersed in a dielectric like dirty water.

You probably could model the situation using NEC.  I seem to recall that it can have several ground media as part of the analysis.  If you are really curious about modeling it, an email to Mr. Cebik might be productive.

Otherwise, I would try it experimentally.  You might find a need to adjust the radials to decouple the coax from the antenna or you might need a line isolator.  Also the tuning for resonance might be affected.  Sounds like a fun experiment........have at it!
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N3OX
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« Reply #5 on: April 07, 2008, 08:58:09 AM »

"Lake Erie is "fresh" water. With pollution, it could be more conductive than sea water! "

No way, not even close.  Amusing use of quotation marks but salt water is pretty darn conductive ;-)

Freshwater and poor, wet soil should have very similar characteristics, I think.

- - - - - - -

Putting radials under fresh water probably isn't going to help much.  I agree with K6AER, the losses from setting up fields in a somewhat lossy dielectric could make those radials less effective than the rest of the set.  If you put a LOT of radials in 5 feet of water it might be good for 160m where it doesn't matter as much that they're buried 5 feet under a lossy dielectric.

I can imagine putting a bunch 160m  radials in shallow water like a swamp for exactly the reason K6AER says: you got the land, you want to use it and the radials happen to land under a foot of water.... but the water doesn't help much beyond keeping the dirt wet and the ions mobile.  

But I doubt it would be worth the trouble to try for a HF vertical.  Too hard to add enough radials to matter on the bottom of the Lake.

Then again, if you are trying to do 160m work, where everyone's a little crazy and deep buried radials aren't such a big problem, you might want to try to run out a bunch of 120 footers into the lake ;-)  Seems like something about 160m DXing leads people to throw practical considerations out the window.

They'll work better if you put little floaters on them to buoy them to the surface.. and then you'll need something to keep them pulled out straight.  Possibly trained ducks tied to to the radials with 6 feet of nonconductive tether?

Dan


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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WA3SKN
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« Reply #6 on: April 07, 2008, 08:59:05 AM »

We have a saying.... "anything conducts better than dirt!"
This might not be so, but it comes close.  Go ahead and try it.  Whatever conductivity there is will help, not hinder the signals.
73s.

-Mike.
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N1LO
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« Reply #7 on: April 07, 2008, 09:37:13 AM »

My guess is that it is not worth it.

I was looking at the ground properties drop-down table in EZNEC recently and saw that fresh water was second only to salt water at the high end, above all types of soils.

Mark
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K6AER
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« Reply #8 on: April 07, 2008, 10:37:18 AM »

Just for fun I filled a 3 oz. shot glass with water from my well that ha sa particulate count of 13 parts per million. Pretty pure.

With the Fluke meter I measured a resistance with one inch bare probes in the water set to a distance of one inch and the resistance was 14.5 Mega ohms.

Next we put into the water one teaspoon of salt. Stir and wait for the liquid to go clear. Extremely salty and much higher than any marsh or ocean water. The measures resistance had dropped to 1.21 K resistance at one inch spacing.

In the RF world with a 50 ohm feed impedance this would be a very bad connection. Can you image radials made from 40 feet of 1 K resistors.

The poorest wire made is still magnitudes better than the best salt water marsh as a ground plane.
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W8JI
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« Reply #9 on: April 07, 2008, 04:04:40 PM »

Bad advice.

The conductivity of Lake Erie based on published FCC data is poor. It is typically in the range of 8 ms/m, about the same as typical poor soil. The soil on shore is typically in the 15 ms/m range, although it varies quite a bit locally.

Saltwater is nearly 1000 times better. Seawater is in the 5000 ms/m range, compared to perhaps 8 at best for Lake Erie.

So there is no comparison at all.

Now here is an interesting fact. If we submerge a radial in water the effect is as if we simply cut the wire off at something just beyond the skin depth of the water. Assume I have saltwater with a skin depth of one foot. I toss a 100 feet of wire in the water and let it sink. At radio frequencies only the first few inches of wire has any effect on the signal. Beyond two feet it is as if there was no wire at all attached. I just wasted 98 feet of wire.

Looking at skin depth tables, any conductor running more than a foot or two below the surface in the lake would be pretty useless as a ground above 2 MHz.

You'd actually be much better off with radials laying on the surface of very poor dirt than radials a few feet below lake water in a fresh water lake like Lake Erie.

73 Tom    

 


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W8JH
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« Reply #10 on: April 07, 2008, 06:48:29 PM »

Many thanks to Tom and everyone who replied.
Even though I was first licensed in 1972 I am learning a ton of stuff on the Elmer threads.
For my radials, I will gather them up now and pile them onshore as stretched out as possible.  In the winter I can get away with staking them out- above water.  That seems like the best solution for me.
My big dream is to get on 160m in the dead of winter with a balloon hoisted wire vertical and as much wire as I can lay out on the ice!  Maybe some day.  Thanks again to all who offered info (and did experiments even).
73,
Joe
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NEW2GEN
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Posts: 48




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« Reply #11 on: April 07, 2008, 07:00:55 PM »

"
Some hams will say then why to broadcast stations always put the antennas in low lying swamp areas, simple, the land is cheep."


That is absolutely true. Low cost.
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W9OY
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« Reply #12 on: April 08, 2008, 05:36:59 PM »

W8JI is correct, but you can have some fun experimenting.  When it gets warm enough float a radial or three out in the lake using some milk bottles or something to support the wire above the surface and measure the field strength with the radials connected to the vertical and disconnected.  Then sink the radials and do the same measure.  Calculate the relative efficiencies.  You will learn a lot about radials and vertical antennas both practical and theoretical, if you play with them like this.  A lot gets demystified.    

You will then become the expert on lake Erie radial systems and have some quantitative data to think about.  

73  W9OY
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W8JI
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« Reply #13 on: April 09, 2008, 03:19:06 AM »

Wet soil is pretty conductive. So is salt water. Fresh water by itself, even with all the old tampon applicators floating about and industrial waste, is not.

:-)

73 Tom
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