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

[Brian Pease]

Your AM radio is probably picking up near field radiation from the power 
lines next to the road, which are carrying the noise.  It can be very 
difficult to find the source because the strength varies so rapidly with 
your distance from the power line as well as your position along it.  An 
ordinary radio with AGC is not much help. You need a radio with at least 
an S meter, or better yet a real selective level meter.

On 12/26/2016 8:58 PM, lstoskopf at cox.net wrote:
> Probably a different problem.  From my rural location...at times a loud roar shows up...not sure whether it is a wet insulator, cow water heater or what.  As I drive along the rural road listening to the AM radio the signal almost nulls and then peaks.....for several miles.  Obviously I'm listening to standing waves.  Is this the same?
>
> N0UU
>
> ---- Brian Pease <bpease2 at myfairpoint.net> wrote:
>> Your backyard is well within the near field region.
>> Assuming that all signal pickup is on the antennas and not the
>> feedlines, and that the direction of the fields is constant, the H field
>> strength (received voltage) will drop off as the third power of
>> distance, so if distance is doubled the noise will drop off to 1/8 or
>> -18dB.  If you move from 10 ft to 100 ft distance, the noise will drop
>> to 1/1000 or -60dB.
>> The E field strength drops as the square of distance, so moving from 10
>> ft to 100 ft only drops strength to -40dB.  Because the magnetic field
>> forms circles around the wire, you should be able to rotate and tilt a
>> small loop sideways off vertical to null out most of the H field.
>> Buried metal pipes or wires, metal fences, etc will have currents
>> induced by the power lines and can become noise sources themselves.
>>
>> On 12/26/2016 2:21 PM, Frank Lotito wrote:
>>> 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
>>>
>>>
>>>
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>>
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