Topband: Minimum distance of noise generator

[K9AY]

> what is the minimum distance (in wavelengths) for reliable (far field)
> results???
>
> Uli, DJ2YA
>

Uli and all,

The distance must be sufficient so your measuring instrument does not "see" 
the elements of the array separately. It also must see a single element as 
one source, since near-field radiation varies from bottom to top.

One common rule of thumb is 10 times the maximum array spacing (or antenna 
height). This implies a "viewing angle" of 5.7 degrees, which should be OK 
for amateur use, but precision measurements probably need 20x 
spacing/height.

Another recommendation comes from AM broadcasting, where FCC rules require 
all measurements within 2 miles to be corrected for near-field effects. 
Assuming this applies to the worst case, which is the low end of the AM 
band, the corresponding distance at 160M is around 0.6 mile (~1 km).

In the VHF/UHF/microwave world, 10 wavelengths is a common distance for "far 
field" measurements. This is more than either of the above rules, but 
antennas at these frequencies are also typically much larger (in 
wavelengths) than at lower frequencies.

In some cases, useful (but not precise) gain and F/B measurements can be 
made quite close to the antenna, especially if you have elements in line 
like Tree's array. I'd use such measurements for tune-up only, then confirm 
them with more distant data.

73, Gary
K9AY