Hi Steve.

Why did I not know about this site and especially the EZNEC tutorials??? I wish I had known. I will be trying out all the tutorials for sure.

I have already dabbled a bit especially with VDPA's on a single mast. I had a model for 17m sent to me and managed to upscale it for other bands to 10m. What I want to know in the first instance is how to see the difference it makes to the gain and takeoff angles from on land v next to the sea. I planning to be on a mini DX-pedition to the Shiants Island in July and we will be able to have out antenna's next to the water virtually. I want to be able to demonstrate to the guys how important it is to get the antenna's as close as possible. I have already demonstrated the difference in the take off angle between a yagi at 30 feet versus the VDPA's. That is a real eye opener.

Thanks for your time.


Gordon
MM0GPZ

Hi Gordon,

Select "Real>High Accuracy" for the Ground Type. In the "Ground Descrip" window right click over the existing entry and select the type you want from the pop-up window. If you save the elevation response when using one ground type you can then overlay it when using a different ground type and compare the two.

EZNEC allows you to have two different ground media, with the boundary between the two being radial or linear. For your situation you could use the linear option to place sea water to one side of the antenna and average ground to the other side.

Let me know if you need further explanation on that.

73,
Steve G3TXQ

Thanks Steve.

That creates a completely different plot for the same antenna on average ground and transforms a simple GP into a low angle DX animal.

My next question is how do you model salt water on one side and beach on the other? I presume you only get benefit from salt water when you are directing over it?

Obviously we will place the antennas as close to the water as possible but how close do they need to be to benefit. I recall they need to be within half a wavelength but not sure???

Thanks again.

Gordon

Hi Gordon,

Open up the Ground Descrip window and click the "Medium" menu option; then click Add. Then click "Medium" again and check that the boundary type is set to Linear.

You'll now have two ground medium entries which you can make any soil property you like with the right click technique. The X coordinate defines where the transition occurs between the two soil types - the boundary extends as a straight line to infinity in the +/-Y directions. You can even specify the two ground types to be at different heights.

You can move the antenna relative to the boundary - or better, simply move the boundary - to see what effect it has.

Hope that's clear.

73,
Steve G3TXQ

Thanks again Steve.

I've entered the data as you advised and want to check I understand.

I've set Ground medium 1 as Average Pastoral land, Height 0 m X Coord 0 m. These values can't be changed here.
Medium 2 is set to Salt Water Height 0m x Coord 0 m.

I presume that means the boundary is right on the antenna?

If I make X co-ord on medium 2 = 20m, does that mean I have average ground to infinity to my West and average ground 20m to the east when the salt water boundary appears to infinity in the other direction?

Should I also make boundary 1 linear type also?


Thanks

Gordon
MM0GPZ

Hi Gordon:

* The Linear/Radial option selection applies to [url]both[/url]mediums simultaneously - if you think about it you can't mix the two, so you couldn't apply radial to medium 1 and Linear to medium 2.

* If you set the Xcoord for Medium 2 to 20m, you would have Medium 1 everywhere from infinity in the West to a line 20m to the East of the origin, and Medium 2 from a line 20m to the East of the origin out to infinity in the East. Hope that's clear - if not you can quickly check by putting a vertical wire at the origin, making Medium 1 "very poor", making Medium 2 "salt water", and looking at the elevation patterns at different azimuths.

* Not sure what your antenna is, but if it needs a ground connection you must use Mininec Ground, otherwise use High Accuracy ground.

* And finally some warnings about using two mediums from the help file:

"The ground may be broken into two "media", each having its own conductivity and dielectric constant (relative permittivity). The second medium can be at a different height, but must be at the same level or below the first medium. The media can be arranged in parallel slices or concentric rings. One use of two media would be to model an antenna on a lake surrounded by land. A very important thing to understand is that the second medium is used only for far field pattern calculations, and is ignored for all other purposes. Be careful when using two media, and keep the following in mind

1. Even if you place the antenna over the second medium, EZNEC will always use the ground constants and height (z = 0) of the first medium for calculation of the impedances and currents.

2. The second medium is used only for far field calculations. Near Field and Ground Wave calculations assume that the first medium is of infinite extent, and ignores the second medium.

3. The effect of the second medium is taken into account only in a very simplified way. The vertical pattern is generated by tracing "rays" direct from the antenna and reflected from the ground. When a second medium is used, the ground reflection "ray" is determined by whichever medium it strikes the top of. The "ray" does not penetrate either medium, and diffraction or similar effects aren't considered. Because of this, a highly conductive inner medium and normally conductive outer medium is
not a good model of a ground radial system, and shouldn't be used for this purpose.

4. Buried wires in EZNEC Pro/4 will always be treated as though they're immersed wholly in the first medium

Hope that helps,
Steve G3TXQ

Hi Steve.

<< If you set the Xcoord for Medium 2 to 20m, you would have Medium 1 everywhere from infinity in the West to a line 20m to the East of the origin, and Medium 2 from a line 20m to the East of the origin out to infinity in the East. Hope that's clear - if not you can quickly check by putting a vertical wire at the origin, making Medium 1 "very poor", making Medium 2 "salt water", and looking at the elevation patterns at different azimuths.>>

Great, I got that bit then.

<<Not sure what your antenna is?>>
2 ele VDPA on a single support pole aka VP6DX style antenna so no radials required. 30/40/80 will as they will be GP's.

Re the heights etc, The far field will be lower as that is the sea and the beach will obviously be marginally higher. Probably not enough to matter. The QTH is interesting as we have a spit of land where the sea laps onshore on the west and the east and is only about 30 - 50m wide depending on tide. Pictures here http://www.madaboutmountains.com/shiants.htm

Thanks for all the help Steve and I'll get cracked into the EZNEC tutorials you created.

Many thanks.


Gordon
MM0GPZ

Looks to be a great place - I'm envious!

Steve G3TXQ