[TowerTalk] Field Strength comparison

[Wes]

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

On 9/4/2021 6:32 PM, Lux, Jim wrote:
> On 9/4/21 6:02 PM, Stan Stockton wrote:
>> I’m missing something.  How do I get 180 out of 2 * 900/80?
>>
>> Thanks, Jim.
>>
>> Stan
>>
>> Sent from my iPhone
>
> You're right.. brain freeze. But 22.5 meters seems awfully close.   2 
> D^2/lambda is the equation - 2 * 30*30/80 = 1800/80 = 22.5 meters
>
> Granted that's for "directive antennas" - Something weird (maybe it's invalid 
> for antennas that are "smaller than a wavelength") -
>
>
>>> On Sep 4, 2021, at 7:30 PM, Stan Stockton <wa5rtg at gmail.com> wrote:
>>> 
>>>
>>> Sent from my iPhone
>>>
>>> Begin forwarded message:
>>>
>>>> From: "Lux, Jim" <jim at luxfamily.com>
>>>> Date: September 4, 2021 at 5:58:44 PM CDT
>>>> To: TowerTalk at contesting.com
>>>> Subject: Re: [TowerTalk] Field Strength comparison
>>>>
>>>> On 9/4/21 3:36 PM, Stan Stockton wrote:
>>>>> How far away (minimum) should a receive antenna be located if you wanted 
>>>>> to accurately measure the difference between a couple of vertical arrays - 
>>>>> let’s say, for example a 4 Square and a single vertical.
>>>>>
>>>>> Thanks… Stan, K5GO
>>>> The traditional guide line for antenna ranges is 2 D^2/lambda, where D is 
>>>> the diameter of the antenna.  That has to do with the phase difference from 
>>>> a plane wave being small enough that the gain isn't significantly wrong (I 
>>>> can't remember if it's 0.1 dB or what, and I'm too lazy to calculate it now).
>>>>
>>>> For a 80 m foursquare, it's roughly 30 meters across (1.414 * 20m) so 2 * 
>>>> 900/80 = 180 meters away