Posted by
Mark J. Schreiner on
Aug 03, 2005; 6:35pm
URL: http://elecraft.85.s1.nabble.com/Antenna-Analyzer-sensitivity-to-near-field-effects-tp380959p380961.html
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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