Re: Bit OT: Why nulls improve with tilting?


Kevin Schanilec
 

I'll try and give it a shot. The antenna pattern is shaped like a donut, like the picture at:
http://www.fmx.dk/projects/wing_cam/119-1935_IMG.JPG

In this picture, imagine the ferrite loopstick sticking horizontally through the donut, with nulls at either end. The null zone is actually conical, meaning that you have to point the null properly in all three directions, including azimuth (elevation).

Now imagine a local pest that is coming in at, say, 45 degrees above the horizon. If you left your loopstick perfectly level, as in the picture above, signals coming from 45 degrees up will still be heard quite nicely. However, if you point it upward 45 degrees, the maximum null will also be 45 degrees up, matching the elevation of the pest's signal, and therefore the null is achieved.

For signals from far away, the angle of arrival is perhaps a few degrees at most above the horizon, and nulls are generally achieved with the loopstick essentially level, since the null of the loopstick is pointing more or less at the horizon

Hope this helps - Kevin S
Bainbridge Island, WA

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:

I have wondered the same thing. The only thing I could think of, is that by tilting a loop antenna, you are narrowing the beam. The figure 8 pattern is as viewed from above in 2 dimensions, but it is actually a cone in each direction and by tilting, you are using a narrower section of the cone. That is just my guess. I hope some one who really knows will chime in and tell us. - FARMERIK

--- In ultralightdx@..., "huelbe_garcia@" <huelbe_garcia@> wrote:

Hello all!

Stephen posted a few hours ago a note on something I always wondered the
reason: tilting the antenna does improve nulls of groundwaves. I can't
think on an good explanation why this happens :)

As all MF stations use vertical irradiators, I think no tilting would be
needed to null the station completely, specially when tuning groundwaves.

But this is not the case. Most of the time, the local stations need some
tilt to null it out. Only when I hear not-so-local stations (say some
20mi/30km away), tilting is not needed anymore.

Would you guys guide me why we need tilting on groudwave MF?

Thanks a lot!

Huelbe.

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