[TowerTalk] Field Strength comparison

[Joe Subich, W4TV]

On 2021-09-04 11:19 PM, Lux, Jim wrote:
> D =30m (across the 4 square diagonally) is almost certainly bigger
> than the height of the 80m elements (20m?)
Even the distance across the diagonal doesn't include the capture
area of the individual verticals.  If one looks at the trends for
antenna stacking, the capture area of a single vertical/dipole
would be in the neighborhood of a half wave perpendicular to the
element and on the order of a full wave in the plane of a dipole,
half wave in the plane of a vertical.

I'd always understood the requirement for an antenna range to be
something like five or 10 wavelengths so that the measurements
were beyond the near field.

If one takes a lesson from the FCC/broadcast "proof of performance"
measurements, they measure field strength at one kilometer (one
mile?).  That (1 km) works out to be 2 wavelengths at the lowest
frequency in the broadcast band and 6 wavelengths at what used to
be the top of the band.

73,

    ... Joe, W4TV


On 2021-09-04 11:19 PM, Lux, Jim wrote:
> On 9/4/21 8:05 PM, Wes wrote:
>> For an illustration try this: 
>> http://www.cuminglehman.com/wp-content/uploads/Introduction_to_Antenna_Test_Ranges_Measurements_Instrumentation.pdf 
>>
>>
>> and look at the figure on  page 4.  This shows the usual antenna range 
>> situation where the test antenna is receiving a signal from a point, 
>> or small aperture source.  This is how I would run this comparison.  I 
>> think, but do not know for sure, that I would use the larger dimension 
>> of the vertical(s) as the "D" in the equation.  The idea is to have a 
>> plane, or near plane, wave over the whole aperture of the test antenna 
>> in both directions.  Note that some antennas, Yagis for instance, can 
>> have an effective aperture larger that the physical aperture.
>>
>> Wes  N7WS 
> 
> 
> But that's the 2D^2/lambda  - and that comes out strangely small.   And 
> it's not effective aperture (that's more about voltage/power at the 
> feed) - this is about the physical optics.
> 
> D =30m (across the 4 square diagonally) is almost certainly bigger than 
> the height of the 80m elements (20m?)
>