[600MRG] Sweet Spot for E & H Field Antennas

[Frank Lotito]

I am having trouble understanding what is going on in my suburban back yard when it comes to finding the "sweet spot" to locate my "E" and "H" field receiving antennas.  As I understand, at the "sweet spot" (if one or more exist) these antennas intercept less man-made noise than they would if they were located in another area of my back yard. At 50 MHz and higher frequencies I would say yes, one of more sweet spots may indeed exist. I have no trouble understanding that at these "short" wave lengths (emphasize "short"), multipath signals may reinforce, or partially cancel each other as the "E" or "H" field antenna is moved a few feet one way or another.


However, at frequencies below the Standard AM Broadcast Band, for example at 500 KHz, the wavelength is 600 meters (almost 2000 feet!)  How can moving a few meters in one direction or another make the adjacent and co-channel noise so different at MF, LF, VLF and even ELF frequencies?  Can someone explain what is happening over short distances when the offending and desired signal's wave length measures in hundreds to tens of 1000's of meters, and then some?


One more related question having to do with the effect of distance from sources of man-made adjacent and co-channel noise.  Is there a "rule of thumb" to estimate how fast the "near field" signal strength falls off as one backs away from the offending source?  Is this effect wavelength dependent?  This fall I constructed my version of the McGreevy BBB-4 VLF receiver.   I did not make quantitative measurements, but qualitatively, I was surprised how fast the 60 Hz and "N times 60 Hz" noise dropped off as I backed away from the 13.6 KV power lines bordering my front yard. (The wavelength at 60 Hz is 5 million meters!)  I suspected that the power line noise would decrease, but not as fast as it did!


73 Frank Lotito  K3DZ / WH2XHA