Saturday, September 5, 2009

Signal takeoff angles for long-distance DX, revisited

Some more simulations from EZNEC, again pursuing maximum radiated signal at 10 degrees WITHOUT regard for the impact at higher angles.

Our goal is to outperform the half wavelength dipole at a half wavelength in heigh for a takeoff angle of 10 degrees.

Here's the azimuth pattern for that dipole over real ground.

The number to beat at 10 degrees take-off angle is 1.49 dBi. Remember this number, we will revisit it.

20 meters

Let's look at some plots for 20 meters. First, we see for the Buckmaster off-center-fed dipole oriented N/S and flat at 64 feet, we have a good radiation pattern at 10 degrees takeoff NW and NE (SE Asia and SW Asia for low angles of arrival, which is what we want) COMPARED TO THE DIPOLE. In fact, we have multiple lobes in directions all around the compass at 10 degrees takeoff angle. 7.31 dBi at 55 degrees azimuth for 10 degree takeoff angle. NO WAY will we get that from a half wavelength dipole at 32 feet! (Remember 1.49 dBi) So it looks like this is our answer for the low angles of radiation.

When combined with the hex beam at 32 feet, which covers the patters around 30 degrees (thanks to W1GQL - looking at the leftmost pattern for the hex beam at 30 feet...

40 meters

Let's take a look at a simple elevated vertical with two radials, a quarter wave in length and placed with the base a quarter wave in height.

Slightly worse than an isotropic radiator at 10 degrees takeoff angle, and it does not exceed the 1.49 dBi threshold of the dipole. So we're still better with the dipole on 40 at a have wavelength high. (NOTE: On 80 meters, this would NOT be the case at all! No way can I get a dipole that high...128 feet. So the elevated vertical WOULD be better).

Back to 40 meters. What about the Buckmaster flat at 64 feet?

Looks like 2.34 dBi, which just beats out the half wave dipole (and this makes sense, since the off center fed dipole/Buckmaster is at the same height as the half wave but is twice as long).

Let's revisit the Buckmaster at 32 feet with the ends vertical to make like phased vertical pair.

The result? 3.02 dBi, 1.5 dB better than the dipole reference. Imagine that! We get better results at the low angle with an antenna that is raised at HALF the height! Pretty neat.


Back to the previous post on this blog, my plans now look like 20 meters will include the Buckmaster at 64 feet and the hex beam at 32 feet.

40 meters will have a second Buckmaster at 32 feet with the ends vertical.

I will also put up the 43 foot vertical as a second antenna for both bands. Always good to have options!

The remaining question here is: IS THIS GOOD ENOUGH FOR THE PATH? With the sunspots still in the doldrums, and me with 100 watts, can I get through with these antennas? I have increased my chances, but I don't want to have a situation where I double the chance and the original chance was ZERO, so TWO multiplied by ZERO is still ZERO.

Only one way to find out...put 'em up and get on the air. I can hardly wait for the end of October when we move!!

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