[TowerTalk] Chicken Wire Ground Screen

[jimlux]

Jim Brown wrote:
> 
> 
> I generally agree about the "broadcast myth" part -- there was a lot of 
> "go for broke" mentality in it. Remember -- in the days where this was 
> developed and practiced, AM broadcasting was all there was, and it was 
> a very big deal!  HOWEVER -- one place ground screens were employed was 
> multi-tower directionals, where very small differences between towers 
> make a big difference in the depth of nulls, which is the whole point 
> of those patterns. 

This is an interesting point.  One function of the ground screen in this 
case would be that it makes the array pattern less sensitive to changes 
in the ground conductivity under the radiator.  I'd venture that the 
average ham isn't going to care much (or even notice) about a few dB 
change in a 20=30 dB null.  That's the kind of change you might get from 
the loss resistance changing a bit (say your radiation resistance is 30 
ohms and the loss resistance is 5 ohms.  Changing the loss resistance to 
6 ohms would change the current in the radiator by 3%, give or take)



This IS a very good reason for using them (and for
> making the radial system as good as practical).  This is the direct 
> equivalent of a ham 4-square vertical array, and the arrays at K9DX. 
> The only important difference is that AM BC arrays were optimized for 
> the nulls, and ham arrays are usually optimized for gain. If you want 
> the nulls to be wonderful, you need to pay lots of attention to the 
> symmetry of everything, including the radial system. 

Or use a non-trivial phasing system.


>