[600MRG] Distance from powerline question

Tue Dec 9 18:54:13 CST 2014

An easy noise meter is a hand held portable AM receiver tuned to bottom 
of dial where there is no station...
Terminate the audio out jack into a diode detector into a medium time 
constant R and across a DC milliammeter...
With speaker only is good for roaming around to find noisy field and 
with meter to find most quiet location...
Out back and into the woods most of the noise detected goes away at 
about 400 feet...
A portable selective receiver with a preamp and e-probe does a bit better...

The TX/RX antenna here for 630M to 10M is a 600' vertical loop NE/SW ( 
250' X 70' ) terminated directly at the house ( no feed line ) so I 
suffer some house noise, however, the power line runs NE from here so I 
feel that the AC lines which end up near the ocean over a mile away is 
like looking into a 'wave' antenna terminated into this loop... It picks 
up some noise but a lot more signal...

Going into the woods out back with an E-Probe antenna is good but not 
near what the loop is hearing...
Same for the 1000' Beverage...
Also I must be careful not to couple the house AC neutral noise out 
along the feed to the e-probe, but to use
transformers and balanced line along the ground out to there including 
one transformer break with a
ground rod on one side to keep the house noise down to minimum...
It then works well but not like the loop, especially at 17.2KC...
The antenna way out back in the woods and the loop then each feed a 
noise balance whizbang input and then you have a real directional RX 
antenna, or maybe a noise avoidance null... For instance I can tune 560 
in Portland and tweak the phaser for Portland or Springfield alternately 
nulling either station out completely and switching the polarity to hear 
only one of the stations on the same frequency at a time...
search: Dallas Lankford low noise antennas

TNX from Dave WD2XSH/17

On 12/9/2014 5:10 PM, Edward R Cole wrote:
> 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
> http://www.kl7uw.com
>     "Kits made by KL7UW"
> Dubus Mag business:
>     dubususa at gmail.com
>
>
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