Re: 150Khz Antenna question

"Joerg" <notthisjoergsch@[EMAIL PROTECTED]
> wrote in message 
news:PCRak.9391$89.2693@[EMAIL PROTECTED]
> christofire wrote:
>> "christofire" <christofire@[EMAIL PROTECTED]
> wrote in message 
>> news:5sydnRVLb6_dU_bVnZ2dnUVZ8vSdnZ2d@[EMAIL PROTECTED]
>>> "Joerg" <notthisjoergsch@[EMAIL PROTECTED]
> wrote in message 
>>> news:r7Pak.8821$89.4911@[EMAIL PROTECTED]
>>>> Tom Bruhns wrote:
>>>>> On Jul 2, 7:29 am, "Jacques St-Pierre" <multe...@[EMAIL PROTECTED]
>
>>>>> wrote:
>>>>>> Anyone can help me with the design of a compact 150Khz antenna?
>>>>>>
>>> . . . snippity snip . . .
>>>
>>>> Back at the university I always wondered about that: Immature fields 
>>>> (near field), juvenile fields, adolescent fields? So where does an EM

>>>> field reach puberty?
>>>>
>>>> :-)
>>>>
>>>> -- 
>>>> SCNR, Joerg
>>>
>>> John Kraus gave a workable definition in his book 'Antennas', that the

>>> [appropriately] normalised magnitudes of the different field
components 
>>> (reactive and radiation) are all equal to pi at a distance lambda/2.pi

>>> away from the 'antenna' so, closer than that, the 1/r^2 and 1/r^3 
>>> reactive terms dominate, and further away the 1/r radiation terms 
>>> dominate.
>>>
>>> So the larger value of lambda/2.pi can be taken as the bare minimum 
>>> separation of two simple antennas intended to operate independently. 
Of 
>>> course, there will still be major coupling between them on account of 
>>> radiation (TX) or re-radiation (RX) and gainful antennas demand 
>>> pro****tionately more separation in their line(s) of fire.
>>>
>>> Chris
>>
>> PS: That final statement is 'in practice' in respect of coupling, of 
>> course. The mechanisms that give rise to directivity for the radiation 
>> field are likely to affect the reactive fields in a similar way at
small 
>> distances.
>
> In EMC the rule of thumb is usuallt (2*D^2)/wavelength, with D being the

> antenna width. That's where you can begin to measure real radiation 
> patterns.
>
> Of course Jacques seems to be well below that distance.
>
> -- 
> Regards, Joerg


Could've mentioned that, but it's more about arrays than fundamental 
elements.  I guess with a stated frequency of 150 kHz, a wavelength of 2
km, 
and a 'plastic case of 2" x 4" inches' there's not much chance of an
array!

2D^2/lambda, as you say, is a useful criterion for minimum measuring
radius 
for dishes, for example.  I did the expt once at 11 GHz and the depth and 
shape of the first null was wrecked closer than 2D^2/lambda.  Beyond it,
the 
pattern was essentially independent of the distance away.

Chris