[TowerTalk] Field Strength comparison

[Lux, Jim]

On 9/5/21 6:31 AM, Joe Subich, W4TV wrote:
> 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.


Effective area (or capture area, as you describe it) is not really the 
relevant thing here - that relates more to the radiation resistance, and 
in any case, the effective aperture of an infinitely small dipole is 
3/(8 pi) * lambda^2.

But here, you're more concerned about whether the wavefronts from 
element 1 and element 2 are "sufficiently close" that the represent the 
far field (also, often given as Fraunhofer condition or where a 
spherical wavefront is the same as "plane" for the antenna under test).




>
> 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.

Near field has multiple meanings - one is "inside the line where the 
energy stored in the magnetic and electric fields is equal to the energy 
being radiated away" - the other is "where the illumination is 
effectively planar, so a gain measurement is accurate"

The 2 d^2/lambda is the latter, and I think you hit on the other 
constraint - you don't want to be so close to the antenna(s) under test 
that you are in the reactive near field.  The magnetic field drops off 
as 1/r^3, the electric field as 1/r^2, and the radiated field (what you 
want to measure) as 1/r, so you need to be far enough away that the 
inverse r cubed and inverse r squared terms are "small" relative to the 
radiated field.

https://www.antenna-theory.com/basics/fieldRegions.php has a nice 
discussion




>
> 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.

1 mile, and that's chosen for historical reasons, probably (i.e. a round 
number), rather than a detailed consideration of the fields.

>
> 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?)
>>
>
>
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