[600MRG] Distance from powerline question

[Rudy Severns]

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