How does Ground Topography affect radiation pattern ??

[unit_399]

Our farm is at the top of a mountain (5600' ele.) Except for a small level portion where the farm house is, our property heads downhill til it reaches the river. I recently added an attached room to our garage for a new shop and tool storage. I was considering moving my station there too, but that would require a new tower (or moving the current one) because the coax run from the present tower location will be over 200' and too lossy. Before spending the bucks on this, I want to see if it's gonna be worth the hassle.
The new antenna tower would be located as shown in the sketch below. From the tower location on the level part of the property, the terrain goes up at a steep angle until it reaches the top. The tower top and mountaintop would be approx the same height. The quad will be mounted on a 10' mast, and will look over the top of the mountain.
I assume that the steep terrain to the South of the antenna will help boost my signal, and give it a lower takeoff angle in a Northerly direction, but I have no clue how it will affect signals going South. Since the antenna will clear the mountaintop , I hope the signal (to the South) will be unaffected, but I'm not sure.

Any of you antenna gurus out there help me out on this ?? Signals to the South gonna be OK ?? Or. . . are there problems I'm not seeing ??

Gracias para su ayuda, Amigos. 73

J.J. 399

 

[BC Coyote]

You want as low an angle as possible to the horizon. The lower the angle the further your signal will go before bouncing off the ionosphere and returning to Earth. DX is possible with higher angles of takeoff from the antenna, but you won't reach as far due to the takeoff angle and bounce angle being a mirror image of each other.
Your sketch shows the actual antenna just barely clearing the mountain top so I would think that communication would be somewhat more limited in that direction.
Just my two pesos worth.

 

[Cable Guy]

I'm no antenna guru, never claimed it, but given the distance from Tower to mountain (150ft.) I would think the proximity of the mountain peak, regardless of far-side steepness and depth, would give a higher angle. I feel the angle is affected by the area immediately surrounding the radiator and you would be talking very close (per hop) to the south, and as you swing northward, you would talk (hop) farther away due to the proximity of ground to radiator in that direction.

 

[Crawdad]

JJ,
I understand and firmly believe the Height vs. Take-off Angle theory BUT, having said that, I have learned thru experience that it's not carved in stone for every situation. My HexBeam is only 25ft AGL to the 20m elements, maybe 1.5ft lower for the 17m elements and another 1.5 lower for the 15m. All three are less than 1/2 wavelength above earth and yet I have made 8000, 9000 and 10,000mi contacts on all three bands. Rotating right from 0 degrees (North) to 180 degrees (South) the ground drops away (nothing as much as yours JJ) so that at one or two wavelengths out my radiated field is actually higher than at the tower base. When bands are open I pretty much have a pipeline to Europe, western Asia, South America, most of Africa and not bad to the Indian Ocean. Usually if I can hear them, I can work them.

Rotating left from 0 degrees to 230 degrees the ground is closer to level or slightly higher and as a result I sometimes struggle trying to work eastern Asia, Indonesia, and especially islands in the Pacific. Not only is the ground higher but my signal also has houses, etc. to reflect off of. I still make my fair share of contacts but no pipelines or easy pile-up busting. One of my 10,000mi contacts on 20m was Indonesia @ 330 degrees, so it still works, but not every time. I can't always work them if I can hear them.

Not just the height matters but also the conductivity of the soil or what ever is under your signal in both the near and far field. There is a ton of variables to this stuff that are difficult or impossible to analyze ahead of time. My guess is when you're looking South you'll be able to work all the way to Cape Horn but it won't be as easy as looking North.

Could you rig up something temporary as a test before moving your tower, etc. ??

7 3

 

[Alexis Mercado]

Capo:

No te preocupes demasiado. Instala la antena y a disfrutar el DX. A veces la teoría no es concluyente, la práctica si. Localmente si podrías tener problemas. En cuanto a DX no creo.

Por cierto, ¿ cuál es tu 10-28 , en que frecuencias se te puede encontrar ?

Yo soy de Puerto Rico.

 

[928bolo]

Run a good quality coax from the current tower to the new shop. A 200 foot run of LMR-400 attenuation is only 1.4 dB @ 30Mhz. No one will notice that very small drop in your signal. DX Engineering Comparison chart.

If you need to bury the coax to the new shop, Bury-Flex from Davis RF has similar attenuation characteristics to LMR-400.

 

[Crawdad]

https://kv5r.com/ham-radio/coax-loss-calculator/