[Towertalk] 20M Yagi ?

[Pete Smith]

At 02:36 PM 7/16/02 +0100, Bill Tippett wrote:
>WD4K wrote:
> >The qth here is on an 800' hilltop with 200-300' drop and horizon
>in the three main directions. Any suggestions before I start planning
>this next project?
>...
>         My personal guess is that you would be better off with two
>medium Yagi's (like 204BA's) at 40' and 90' with the ability to do
>either and BIP/BOP than you would with a single monster Yagi.  Given
>your effective height, you have to be careful to not get only low
>angles.  From here in NC, there are two predominant angles to Europe
>on 20 meters (1-3 degrees and 7-8 degrees) and a single antenna will
>probably not cover both well.


In his book Dave Leeson discusses the downward-sloping foreground in some 
detail (chapter 10).  For example, he shows a table of takeoff angle vs 
signal for a single 14-mhz yagi 100 feet over a 12 degree downward sloping 
foreground.  The level remains within 2 dB from 2 to 18 degrees, while the 
same antenna, at the same height over flat ground, is down 6 dB at the two 
extremes.  In other words, the sloping foreground has much the same effect 
as a stack, in terms of broadening the forward lobe.  He also cautions that 
in this situation it is quite easy to get your antenna too high.

Worth reading!

73, Pete N4ZR