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Author Topic: How close to the ocean ?  (Read 1059 times)
G7SQW
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« on: December 01, 2008, 12:01:40 PM »

I have been operating mobile HF now for the last couple of years and have recently been operating about 20 miles away when I have chance near the North sea coast.

My question to you all is as follows,is there a certain distance from the Sea in wavelengths or just is it a case of being as close as possible to the sea?
I nearly always operate on 17 and 20m with the occassional visit to 15m when it is open.

I am pleased how things have been going recently and I have worked about 190 DXCC over the last couple of years which I think is OK for this stage of the cycle, using just a mk1 ic706, 100w and a outbacker outreach 500 antenna.






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K0BG
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« Reply #1 on: December 01, 2008, 12:41:41 PM »

No one can effectively give you an answer, as there are just too many variables. Here's something to consider, albeit you didn't ask.

The outbacker antenna is about 15% efficient on 20 meters, and a near dummy load on 80 meters. If you were to switch over to a decent screwdriver, or bug catcher, you could effectively increase your radiated power by at least 20 dB, or twice what you'd garner from an amplifier.

Alan, KØBG
www.k0bg.com
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WW5AA
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« Reply #2 on: December 02, 2008, 09:06:17 AM »

I agree with Alan. A better antenna would improve your signal more than being near salt water. Higher is also better than being near salt water. Salt water is over rated!

Go to: www.k0bg.com.
Do not pass go.
Save $200.

73 de Lindy
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WB2WIK
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« Reply #3 on: December 03, 2008, 09:03:56 AM »

There are two distinct advantages to being "near the sea," I have found.  

One is, in many parts of the world, when you're very near the sea, you're also very much "in the clear" of ground clutter and have a very clear radio horizon for a full circle because there are no mountains or other obstructions close by.  This same advantage might be possible in the middle of flatlands, or a big corn field, but it's still an advantage.  Unfortunately, there are other parts of the world where mountains rise right next to the sea, as they do here in California, so here when we are "at the sea," we are only in the clear for about 180 degrees -- the other half the horizon is badly blocked by steep mountains.  

The other is the particular advantage of having an excellent ground plane under your antenna, providing a low-loss ground reflection to improve realized antenna gain.  To take advantage of this one, you have to be *really* close -- 20 miles isn't nearly close enough.  Based on my own experiences operating from near my old home at the New Jersey (USA) seashore, I didn't get the "salt water" advantage until I was right at the "beach," close enough to get my feet wet.  I lived only one mile inland from the ocean, and although I could take advantage of the "no obstructions" issues raised above, I could not take advantage of the low loss ground plane offered by the sea.  A mile seemed to be a bit too far away for that.

WB2WIK/6
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G7SQW
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« Reply #4 on: December 03, 2008, 01:32:20 PM »

Hello Alan, Lindy and Steven,

First of all thank you for taking the trouble to to reply to my question and your comments you have put forward.

I will have to take a closer look at getting myself a screwdriver antenna in the future, but at the moment I will stick with what I have as I do not feel at the moment I can go down the route of a permanant fixed antenna on the family car.

Steven, I appreciate your comments but I think I may not have worded my first paragraph correctly, as I actually live about 20 miles from the sea and drive there when I have free time and operate within 20 metres of the sea.

Over thew last few weeks I have worked VK on several occassions,ZL, JA, VK9 ( Willis island ) JT,V8, YB and Many more DX stations from that location with the above set up, so I am sure it does help a bit.

I also agree fully that the antenna is the most important item in Amatuer instalation and will look to improve it later on, certainly before a amp is purchased.

Thanks again

Andy





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NH7L
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« Reply #5 on: January 24, 2010, 12:58:55 PM »

Out here, a lot of us are forced by housing restrictions to operate portable. We're on islands, so mostly we go to the shore.

Being on seawater-saturated ground helps, even if you're mobile. This can mean a waterfront parking lot built on fill on formerly marshy ground.

Other than that, get as close as you can to the water. For ground-mounted verticals, best results seem to happen when the antenna is no more than a 1/4 of a wavelength from the water. Local conditions won't always let you get that close, but if you can, you may be amazed at the DX you can work.
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K0BG
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« Reply #6 on: January 24, 2010, 01:09:25 PM »

Ernest, please explain why you suggest being with in a 1/4 wave of the ocean for best results? Also, how much better do you think the signal path is (close to water)?

Alan, KØBG
www.k0bg.com
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WB5JEO
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« Reply #7 on: January 27, 2010, 06:33:12 PM »

A DXpedition bunch some time back had a good situation and the inclination to play around with a vertical near the water. They had some sort of landmass a fair distance downrange, out of the near field, where standing on it put them at about a ten degree elevation relative to the beach. So they could watch the change at ten degrees as they moved it around. They found that it didn't make much difference at that elevation angle unless they were within what amounted to 1L/4 on the frequency they were using. I forget, but I think it was 10 or 15 meters, but it might have been 20. The increase at ten degrees was pretty significant when they got within 1L/4. They believe they were seeing a reflection effect, rather than a ground quality thing. They were very impressed but recognized some practical difficulties that close, especially in a highly variable tidal region. I don't find it right now, but it wasn't too long ago, so I know it's an active web site and could probably be found with some searching if no one happens to know which group it was.
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K0BG
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« Reply #8 on: January 28, 2010, 06:38:53 AM »

Here's the issues. As Steve points out, the conductivity tends to be better as you near saltwater. However, the real-world advantage, as Steve also pointed out, is the clear view. And in fact, this is what accounts for most of the signal improvement.

There is a bugaboo in generalizing the statement about being close to seawater; Not all soils exhibit an increase. Shorelines made up of coral are a very good examples. Some types are very porous, so their conductivity is reliant on the water table below them. At high tide they might just make a good spot, but at low tide they're not.

We're also talking about mobile radio, where the antenna is not mounted atop the ground, but atop a vehicle. While conductivity changes in the surface under the vehicle does have some effect, it is mitigated by this mounting fact.
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WX7G
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« Reply #9 on: January 28, 2010, 02:56:53 PM »

It is often said that the ground in the far field of a vertical out to 100 wavelengths matters. For 20 meters that says a distance of more than 2 km will not matter.

And the difference is roughly 10 dB. I suspect further than 250 or 500 meters from the ocean will make a difference of less than a few decibels.

This can be explored using NEC-WIN PLUS where there is a local raised GND modeling mode. The ground in the circle and beyond the circle can be of different types.
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K0BG
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« Reply #10 on: January 28, 2010, 05:04:49 PM »

Precisely David! Rudy's comments (buried in the data here: http://www.antennasbyn6lf.com/), confirm the hypothesis.

I just have to say...

Far too many facts just creep into the amateur lexicon, and shortly become inalterable fact! I don't wish to put words into Tom Rauch's (W8JI) mouth, but I just love his reference to bovine fecal matter science!

And, I have to put credence to my old math teacher's (Stella Grinstead) observation; detritivores feed upon their own!
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VO1GXG
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« Reply #11 on: January 28, 2010, 06:29:45 PM »

I live about 100 meters from the atlantic ocean pointing to the ocean and I am on top of marsh land perfect grounding low noise. All roads in Newfoundland have a view of the ocean some get washed out by the ocean and I've noticed that my reports are always really good in the mobile. So in my opinion as long as you can see a good bit of ocean then its a good thing for mobile work!
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K0BG
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« Reply #12 on: January 28, 2010, 07:07:24 PM »

Matt, this is again an issue with clear view. Wouldn't it be nice if we all could have a clear view.

Steve Katz, WB2WIK/6, did post on the thread, and I'm sure he can tell you what a good clear view can do for you. He is, after all, a decent VHF roamer of some repute. 
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N3OX
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« Reply #13 on: January 29, 2010, 09:30:15 PM »

Here's what EZNEC+ says regarding a vertically polarized antenna backing away from the ocean.  
The antenna is a full size 20m self resonant vertical dipole, for simplicity's sake.  The bottom tip of the antenna is 5 feet off the ground, and the top is at 38 feet.  Conductor diameter is 1 inch and there are 31 segments.

Two ground media are defined, the "land" is 0.005 S/m dielectric constant 13 "Average" ground, the "water" is 4 S/m dielectric constant 80 seawater.

All this detail is for people who want to play with the multiple ground media feature in EZNEC.  Here are the results:

http://n3ox.net/files/20m_ocean.jpg

All azimuth plots are for an elevation angle of 5 degrees.  An enhancement of around 10dB is noted when close to the water.  At eight wavelengths away from the water, the 5 degree elevation pattern doesn't know the ocean is there.  At four wavelengths, there is some enhancement in the direction of the ocean.

From a ray optics standpoint, I would expect there to be zero effect on the azimuth pattern at five degrees elevation once a ray from the very tip of the antenna going downward and meeting the ground at a five degree angle hits the land instead of the ocean.   In EZNEC flatland, this happens at about 6.25 wavelengths from the antenna, betwen the plotted four wavelength and eight wavelength curves.

I went back and checked, and so this appears to be the EZNEC breakpoint for this tall antenna.

To take advantage of enhancement at modest takeoff angles (i.e. not zero degrees), a shorter, lower antenna, like a mobile antenna, will have to be closer to the water  than my very tall dipole (making it perhaps not the best example, but I thought this through after I ran the model and edited the plots and exported to JPEG :-)  )Very low angle enhancement will remain further back from the ocean.

In some sense, your antenna does need a clear view to the saltwater to pick up an enhancement.  But it is NOT just an issue of clear view with vertically polarized antennas.  The effect of saltwater on verticals is pretty profound.  

At the land-water boundary, you're basically going to get an "over average ground" pattern for high angles and angles headed away from the land-water boundary, and a saltwater pattern for low angles toward the ocean.  The zone of enhancement will depend on antenna height above ground and distance.  

Here's a 3D pattern of a 10 foot tall short dipole 3 feet off the ground 1/2 wavelength from the "ocean:"

http://n3ox.net/files/20m_ocean_3d.jpg

The real land-water boundary is not a perfectly straight, infinitely wide flush transition from one ground medium to another, but these plots are suggestive of roughly what should happen at the oceanside.  You should be able to get very substantial, measurable enhancement if you drive right up to the water's edge.

This doesn't work at all for horizontal antennas, by the way.  The reflection of horizontally polarized waves is about the same for ordinary dirt and saltwater.  The patterns would be nearly unchanged with distance from the ocean.  But vertically polarized antennas, even mobile antennas, benefit a lot.

This would be an interesting and pleasant summer experiment... ideally you'd find a site where you could get a signal source up at a few degrees elevation well off in the distance beyond a beyond a very large expanse of saltwater and also have a site on the antenna-under-test side where you could back away from the water a great distance.  It should all work out roughly close to what you might predict just using geometry.

- - - - -

Long story short, the "low loss ground plane" effect really requires that the ground reflection zone for a given takeoff angle has to lie on the salt water.  The following picture tells the story if you imagine that the beam is a vertical, ES1TU's house is the land and the ground is the water.... green takeoff angle gets saltwater enhancement, red doesn't.

http://n3ox.net/files/groundref.png

That would be bad (maybe?) for Aadu for his yard to turn to water and his house to land , but I figure I should recycle figures where I can :-)

A mile or whatever away, your ground reflections for practically useful HF takeoffs are guaranteed to hit land.  You drive right up to the beach, and that's not necessarily true.

73
Dan
« Last Edit: January 29, 2010, 09:37:51 PM by Dan » Logged

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

Monkey/silicon cyborg, beeping at rocks since 1995.
ZENKI
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« Reply #14 on: January 29, 2010, 09:57:14 PM »

You should read NCJ July August  2005.

K3LC analyzed this subject too death.

Basically at  1.5 wavelengths you lose the "enhancement" of seawater. The ideal distance is less than 0.25 wavelengths.

The articles are also published in the ARRL's "More vertical antenna classics. Its a good read.

His recommendation was 2 gull wing radials close to the seawater, AKA Team Vertical etc etc.

Its just  a shame islands cost so much, and any waterfront property these days is full of noise and way too expensive.

I have dreams of finding a small atoll and installing   long boom vertical polarized log periodic antennas in all directions for 40 through too 10 meters.  Why? I took  my kilowatt with a full size quarter wave vertical down to a saltwater location. I was working dx that a good friend on a 5 over 5 over 5  on 20 meters could not hear! When we compared signals he was 5db louder on a calibrated Perseus receiver!
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