[RFI] [BULK] Re: Ambient Noise Levels
w8ji at contesting.com
Thu Aug 10 11:59:15 EDT 2006
> Brown wrote:
> "But for most sources, the transition from
> near to far field occurs at roughly 1/6 wavelength"
> I don't believe sources I cited not a month of two back
> are in agreement with that statement
Far field generally is at distance beyond 1/2 wl or 1 wl for
small antennas but can be further for very large antennas.
In a small loop the field is magnetically dominant within
about 1/10th wave, and becomes electrial field dominate at
larger distances until eventually at the far field distance
the field impedance settles at the value of freespace.
You can see a field impedance curve on this page:
All antennas are equal in the far field.
>NOR does it
> agree with the empirical results that those using
> shielded magnetic loops experience (which I had
> just alluded to in, I think, my previous post.)
There is a great deal of bad science surrounding small
loops. The shield for example does nothing at all except
improve loop balance or make it easier to balance the loop.
It doesn't filter the time-varying electric field at all. If
it did, the loop would go stone cold dead for all signals.
Anyone with a minimal amount of test equipment can prove
It's also in textbooks and has been in peer-reviewed
textbooks for many years. For example:
> As to your contention that a measureable amount of
> noise, from man-made incidental radiators, can create
> S4 levels of noise, say, 100 miles distant, even
The S unit has no definitive meaning in any signal level
discussion. It is whatever the manufacturer happened to
accept at that particular time. On a few dozen receivers I
have tested, it most often is 1 or 2 dB per S unit at the
lower end of the scale up to 5 or 6dB at S9, and S9 can be
Noise here is at least 10dB higher beaming towards
Barnesville or Forsyth during the daytime on my 160
transmitting verticals. Beaming south or north away from
both Barnesville (7 miles to the west) and Forsyth (7 miles
east), noise is 10dB less. That noise shows a steady
increase (except for driving past low level peaks from bad
powerline hardware) as the city is approached.
Anyone who has operated low band mobile from an electrically
quiet vehicle going from rural to urban areas has probably
noted this effect.
On bands where skywave propagation does not exist, several
dB of noise increase when beaming towards those two cities
using antennas with good low angle response.
> considering cumulative effects (that is, many sources
> adding together to create 'the noise') I cannot at this
> moment accept
Then you are not accepting something that has been very well
known in engineering circles for many years. It is even in
studies by Bell Labs, the FCC land mobile advisory
committee, and the Institute for Telecommunications
Sciences. Reference Data for Radio Engineers and other
textbooks have data that agrees with this.
Those bothersome little noise sources don't just go so far
and vanish. They propagate like any other signal. Even if
they are below ambient noise from other directions they
still contribute to overall noise floor. The accumulated
energy of many sources is hurting us already. It has already
been shown that the background noise is up several dB from
many years ago, and it isn't from lighting storms. It's a
blanket of noise from millions of very weak man-made
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