Antenna Analyzer sensitivity to near-field effects

[jferg977]

Hoping this isn't too far afield, I think someone mentioned that the
anaylzer he/she uses was
capable of seeing near field effects and indicating the cumulative
results in its display.  Does this
mean that an analyzer could be used to position a vertical in a complex
environment (such as
the one on my boat) for optimum efficiency?
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[Mark J. Schreiner]

There is a specific formula to determine if you are in the near field or far field and the distance is based on the wavelength to give you a specific distance from the source antenna.  It has to do with a changing phase front (in the near field) or a constant phase front (in the far field).  BTW, the amount of power that is being transmitted is not a factor in the near field vs far field determination.

However, I think the comment was that if you are near (not necessarily in the near field) a high power transmitter, for example an AM broadcast station and the field density is quite high it can give erroneous readings of SWR on an antenna you are testing.  The reason for this is that there is no filter on the front end of the cheaper analyzers to look only at the  power incident from the antenna to the analyzer.  This power is the sum of the refelected power due to the real SWR at the test frequency plus any power that is being picked up locally by the analyzer.  So, in such a case, if it is due to an AM broadcast station for example I think the suggestion was to use a BCB filter, or actually a high pass filter with a cutoff below where you are trying to measure.  This gets more difficult when trying to measure an antenna on 160m since it is so close to the high end of the AM Broadcast Band.  Anyway, I hope you get the idea.  I'm not sure I answered your question directly, and probably didn't, but hope I shed some light on the topic anyway.

BTW, nearfield antenna ranges need to accurately measure magnitude and phase over the measurement area and then do some fancy Fourier Transforms to predict what the antenna patter would look like in the far field.  It is an indirect but proven method of measuring antennas.  Far field ranges of course are much larger and often end up including a sensing antenna that is located outdoors, oh and only have to measure magnitude (not phase).  Things like vehicles driving past, weather and changes in ground conductivity influence some of those range results, so in many cases even though fancier (more expensive) test equipment is needed to do near field measurements it often is a benefit by saving time and improving repeatability of measurements due to changing environments.

73,

Mark, NK8Q


>From: [hidden email]
>Date: Wed Aug 03 11:03:34 CDT 2005
>To: [hidden email]
>Subject: [Elecraft] Antenna Analyzer sensitivity to near-field effects

>Hoping this isn't too far afield, I think someone mentioned that the
>anaylzer he/she uses was
>capable of seeing near field effects and indicating the cumulative
>results in its display.  Does this
>mean that an analyzer could be used to position a vertical in a complex
>environment (such as
>the one on my boat) for optimum efficiency?
>_______________________________________________
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>
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>Elecraft web page: http://www.elecraft.com

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[Stuart Rohre]

It is very true that antenna analyzers are affected adversely, without
filtering, in the near field of transmitters such as AM stations.

Here at the research lab, we are about a mile from a directional AM station.
One of the popular antenna analyzers gave swinging SWR meter readings
following the AM station modulation.  It was necessary to "eye ball average"
the swings to a value for what we were measuring at 400 MHz!

Now there are filters sold for that AM interference.
The other way of dealing with the directional station is to wait and do
measurements after sundown when they reduce power in our direction.  Many
stations drop power in all directions at sundown to avoid skip interference
to others.

Near field can be distinguished from far field by knowing that the energy is
leaving the antenna in all directions in the near field, and has not reached
a planar directional wave front appearance.  In the far field, you can find
the wave has its plane wave orientation, and the distinguishing character of
the field by horizontal or vertical polarization of the electric component.

Near field can be out to a couple of wavelengths, and far field beyond that,
as a rough generalization.



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[Stuart Rohre]

Hmm, I am not sure what that would mean, an analyzer that can see near field
effects.

The in line analyzers indirectly might show near field effects simply by
being interfered with by strong RF nearby.

Almost anywhere on the boat that is away from structure and conductors will
have the same HF efficiency, if a full size HF antenna, or nearly so.

To really have differing efficiency, you would have to be able to move
things at least a quarter to half wave to see an effect.  I am thinking you
mean a pleasure boat and not a larger yacht.   Most small boats just don't
give you many options.   You probably need to place the vertical well aft,
out of the way of sail apparatus if a sail boat.  On a motor boat, you could
be anywhere along the center line.  Probably the most important efficiency
aid is to have wide straps going to a keel ground plate or plates.  The more
metal in contact with the water the better.

Most sail boats have a very effective/ efficient antenna in loading the back
stay guy of the mast as a sloper, rather than using a true vertical.  Such
an antenna will have some vertical and thus low angle take off component off
the beam of the boat.

Stuart
K5KVH



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