[600MRG] Distance from powerline question

[Edward R Cole]

Rudy, etal:

Reading the mail here.  I checked my 160m noise on my 80m inverted-V 
and its running S5/6 at mid-day (S4 on 3920-KHz where its 
resonant).  No overhead power lines within a 1/4 mile (subdivision is 
buried utilities).  On 495-KHz its lower at S1 but the antennas is 
grossly mismatched.  Can't test the 600m inverted-L because its on 
the ground with one leg broken (by a moose encounter in 
Sept.).  Typically 160m and 600m noise are comparable in my location.

Curious about the right-angle loop you describe.  Actually sounds 
like a 70Hx50L right-triangle with upper apex open with 
termination.  I don't have any supports that tall (only 50-foot 
towers) but maybe I could raise a shorter version of your antenna 
35Hx50L broadside to SE-NW?  This would keep it 15-foot off the 
ground so moose can walk under.

Do you use a tuner on Rx?

73, Ed

At 09:55 AM 12/9/2014, Rudy Severns wrote:
>Hi Jim,
>
>Let me give you some background on my earlier comments about noise, 
>near-field, etc.  When I moved to my new QTH I could just look 
>around at the power lines, the wood products plant nearby and the 
>neighboring homes to see that from a 600m noise point of view I was 
>in deep doo-doo.  A remote rx via the internet was certainly 
>considered but before going that route I've been busy trying 
>different rx antennas, phasing schemes, grounding, etc.
>
>The experiment which prompted my near-field comments began when  I 
>purchased a DXEngineering NCC1 phasing box and a pair of their 
>voltage probe verticals with amplifiers.  The whips which came with 
>the package were only 108", ok for higher bands but not so good at 
>600m so I replaced them with 30' Al tubing whips.  I installed the 
>two probes/whips 200' apart, which is about the useful minimum at 
>475 kHz, oriented E-W.  The first probe was 200' from the power 
>lines and the second 400'.  200'-400' is well within the near-field 
>at 475 kHz.  On each probe I installed a 400' length of RG6 back to 
>the shack.  The cables were carefully matched.  I fired up the box 
>and  found it worked very well for distant signals but was almost 
>useless for noise within a 2000' (roughly a wavelength at 475 kHz) 
>or so.  That was discouraging so I slapped my spectrum analyzer on 
>each of the probes and saw immediately that, for local noise, the 
>signal levels were grossly different, about 15 dB.  Using the 
>analyzer I carefully adjusted the gains in the NCC1 to equalize the 
>signals and then I could get some nulling but only over a very small 
>angle.  Ok for a point source, like a neighbors TV, but no help for 
>the majority of my noise.
>
>At this point I modeled this array using EZNEC and AutoEZ to graph 
>the intensity of the E-fields within 200'-1000'.  The first thing 
>that jumped out was the near-field pattern is grossly different from 
>the far-field patterns which are what we usually use when discussing 
>this kind of antenna.  The reason for the gross difference was the 
>very rapid change in field intensity and phase near the 
>antenna.  Over the 200' separation distance the fields change by 
>15-18 dB.  The root of the problem was that the size of the rx array 
>was comparable to the distance from the noise source and both were 
>a  fraction of a wavelength at 600m.
>
>I then extended the modeling to other rx antennas to see if I could 
>do better.  What seems to work best are antennas which are 
>dimensionally small compared to the distance to the noise source.  I 
>found that the terminated loop (or flag) antennas would retain their 
>nulls in the near-field.  The optimum termination resistance and 
>location were a bit different for the best null for near-field 
>versus far-field but I could get reasonable nulls.  Right now I'm 
>using a right-angle loop, 70' high by 50' on the bottom with the 
>termination near the top of the diagonal wire.  It seems to work 
>well, as good or better than the 500' BOG I have.
>
>Besides the unbelievable number and variety of noise sources, near 
>and far,  there are also the issues of common conduction of noise 
>into the rx.  For my rx antennas I've begun to use shielded 
>isolation transformers at the feedpoints, very aggressive grounding 
>schemes in the shack with 12" wide copper strips, common mode 
>chokes, filters, etc.  At the point where my feedlines reach the 
>building I've installed four 8' ground stakes separated several feet 
>which connect to a common grounding bar.  Every feedline is grounded 
>at this point before entry to the building.  I'm not claiming 
>victory, this is very much an ongoing effort.  I need to do better 
>but at least as last months QSO's showed I can hear some of you.  It 
>was not clear that would be possible when I moved in!
>
>I hope these comments are of some help.  73, Rudy N6LF, WD2XSH/20
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73, Ed - KL7UW
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