Explanation of far-field approximations

Hello,

it reads, the far field of an antenna starts in a distance

R= .... D / lambda

with D as the aperture dimension....

This means R is proportional to the frequency. The far fields starts in a greater distance for antennas at higher frequencies. Isnt it wrong.

Thanks

8 lambda far field?

May I know why it is wrong..?

frequency lambda 2.4ghz

ya, elektr0 I had the same confusion. If the physical dimension of your antenna is same for two different frequency, you have far-field far away for high frequency.

If you have an antenna which is one wavelength long at 2.4 GHz frequency your farfiled starts from around 240 mm away. However, farfield for the same antenna at 24 GHz starts from 2400 mm away. Important thing you need to observe is that, your antenna at 24 GHz frequency is 10 wavelengths long, that's why far-field goes farther away.

I assume your confusion came from this kind of experience: you have seen for low frequency (e.g, 900 MHz) large distance is needed for pattern measuement (large anechoic chamber), however, for high frequency (e.g, 24 GHz) you need smaller distance (small anechoic chamber). The fact is, the antennas which are used in high frequency are physically smaller than those which are used for low frequency.

I guess this explanation helps.

far field distance

See first of all, from length point of view there is a electrical length of antenna and other is physical length of antenna.
It is this electical legnth,that we all the time talk about and not the physical length.SO when we go from 24MHz to 24GHz,the length(electrical ) of 1 lambda will remina same,though physical length is changing.So that is why we talk about lambda/2 dipoles, or full wave antennas and not in terms of physical dimension .
Even though u change freq,the directional pattern will remain same as electrical lenght is same.
There is a concept called CATR i.e called Compact Antenna Test Range which generates the same planer wavefront in a very short distances(15m) as compared to 2D*D/lambda and in fact they do us some kind of collimating devies for that purose like lens,reflectors etc,but there are some drawbacks of that menthod also like blockage.
And as per my knowledge Anechoic chembers are used in microwave regions mostly and they are .So of course u can't test a 7MHz diople for example in that.
Hope this helps.

far field distance high frequency

There are several far-field approximations (see Stutzman and Thiele pg 24):

Rff>2*D^2/lambda
Rff>>D
Rff>>lambda

D=maximum dimension of antenna

You pick the longest of the three.

Rff>>lambda is used for electrically small antennas, such as an HF ferrite rod, or can also be used for dipoles, patches, monopoles, etc.. Although I don't think you would measure a 1 MHz ferrite antenna at a distance of 3 km.

Rff>>D is used when the antenna D is on the order of 1*lambda. Maybe for low directivity arrays such as a couple of dipoles.

Rff>2*D^2/lambda is used for arrays where D>>lambda. If you had a 30 GHz planar array that was 1m^2 you would not encounter the far field at 10*lambda=10cm. Make sense? 10m may be too close. 200m as given by the last equation is good, but do you have a 200m chamber? This is why near-field measurements are made on arrays.

To summarize, the old rule of thumb (Rff=>10*lambda) is no good for large arrays, hence the last equation.

Re: antenna far field

@atul_microwaves_antenna
@madengr

Thank you very much for your helpful answers.

To summarize D/lambda is right, but D= proportional to lambda ... therefore it is an antenna - characteristic.

elektr0