What does "Far Field" mean when referring to vertical antennas?

[KD5NLV]

Hello,


I have heard the terms Near Field and Far Field being used when describing ways to evaluate vertical antennas. I know that "near field" is the ground at the base of your vertical under your radial system, but i'm trying to understand the meaning of "far field" and how to evaluate it for my location. Is far field a certain number of wavelengths from the base of my antenna? i.e. 140 feet or 1 wavelength on 40m, or is it much farther like miles away? How do you evaluate your far field as being poor, good, or excellent? Houses, buildings, hills, mountains that obstruct line of sight to the horizon?


I have a all-band vertical with 50 43' radials and a few more at 55' which should cover my near field as best that I can, i'm just trying to get a grasp on the "far field" term.


thank you

[W1JKA]

Type "far field radio frequency definition" into your browser.All the info is there.

[K2DC]

After 26+ years as a practicing Antenna Engineer, I can tell you that there is not a clear definition or complete agreement on what the "far field" for any antenna is (read this one: http://electronicdesign.com/article/communications/whats-difference-em-field-field-74017).

However, the most widely agreed upon definition is 2*D^2/Lambda where D is the longest linear dimension of the antenna, and Lamda is the free-space wavelength at the frequency of interest.  That distance is also significant in antenna measurements.  It is the distance at which the maximum phase error on an antenna under test induced by any residual spherical nature of an incoming wavefront (tested in receive case) is 22.5 degrees.  That's the point at which the accuracy of measurements of pattern features like beamwidth, sidelobe levels and pattern nulls improve significantly.  As you move inside that distance towards the antenna, measued performance gets a little mushy (the measured beamwidth broadens and pattern nulls fill in).

73,

Don, K2DC

[K3GM]

"Far Field" begins  about 1 wavelength from the antenna, and for 1/4 wave antennas extends out several wavelengths in an area known as the Fresnel zone.  For longer verticals, 1/2 wave for instance, the zone can extend out more than 100 wavelengths.  

Remember that what you do in the near field to some degree determines the radiated field strength in the far field, but soil conductivity "out there" is a major player.

.....I have a all-band vertical with 50 43' radials and a few more at 55'.....

How are you feeding this antenna?  Are you using your transceiver's built-in antenna tuner, or are you using a remote antenna coupler at the base of the antenna?

[G8HQP]

The D in 2D^2/lambda should really be the size of whatever is supporting radiating currents. For an antenna in free space it is just the antenna size, but for an antenna near other conducting objects it should include those objects. For a ground-mounted vertical the ground currents contribute very little radiation in the far field but near the antenna they are likely to enlarge the distance you need to go before you get to the far field region. At the least you should use twice the antenna length so you include the antenna image.

For some very small antennas mounted at low heights it is likely that ground currents are a significant source of radiation so their extent must be taken account of.

[W2ANZ]

Wikipedia has a nice discussion of near field vs far field.

The 2D**2/lambda term is usually applied to aperture antennas, like horns and dishes.   It is generally accepted by antenna engineers as the minimum distance required to make accurate gain and pattern measurements.  

EMC/EMI engineers have a completely different rule of thumb for far field. I forget what it is right now and never thought it made much sense.

I understand the 2d**2/lambda term in relation to apertures, but not so well in relation to wire antennas.  Wire antennas commonly have an effective electrical aperture that is larger than the antenna's physical aperture.

I think the bottom line is that hams really don't need to worry about the definition of far field.  Just lay out your radial field in accordance with the recommendations given in the ham radio literature.  Far field vs near field vs reactive near field are important to those who work in antenna ranges, but accurate antenna measurements at HF are darn near impossible anyway.

[N8CMQ]

It is something people use to try and beat up the performance of a vertical antenna with.
 
Like if you have a very large radial field, then the worse off you are, cause the far field
performance will nullify any benefits your radial system may show at the near field.
 
I go by how many contacts I get during contests running 100 watts VS all the KW stations
running ocf dipoles or whatever antenna was recommended by a friend of theirs.