[ARC5] Aerial noise [Was BC-230 mic]

Mon Nov 10 15:59:04 EST 2014

  Hello Dennis/all,
  The subject of noise is interesting.   I followed the discourse
  between you and Neil with close attention.
  At the same time I'm interested Lloyd Butler's ideas about noise, and
  noise reduction published on his web-site.
  As further incentive to understand this topic Tony Langdon published
  published a circuit in Technical Topics (with the now-gone Pat Hawker)
  about a 'neat' little regen superhet, but when I wrote to him some
  years ago he responded by saying local noise precluded operating on HF
  from his QTH.  My friend Ron Scifleet, VK2YQJ,  suffers terrible local
  noise at his QTH, and it's very local noise.

  So, I repeat, the topic is of great interest.  The difficulty in
  understanding goes beyond the fact that there are several concepts
  involved in this discussion.
  We have "common mode" v "differential mode" - a topic that I only saw
  when I first sought to understand op-amps.
  I don't remember that idea in the ARRL or RSGB handbook, to be
  specific.
  Then there is the idea of a Faraday shield, and I presume this
  operates because of Gauss' Law, that no electrical field can exist
  inside a closed conductor.
  This is the basis behind co-ax cable, but people tell me noise flows
  in the cable as common mode noise.

  Then there is the question of ground current, caused by the common
  mode noise in the receiver ground circuit.
   I came across the problem of ground noise for the first time in my
   early 20's when a friend built an (audio) amplifier with significant
   hum.
  I had to think VERY carefully about ohm's law, and understand that a
  copper conductor has resistance, and that the flow of current induces
  voltage before I could correctly connect the ground cables in Ken's
  'you-beaut' stereo amplifier and get rid of the hum.  I learned a good
  deal in that exercise, and (later) found that knowledge very useful.  

  Now, if all that isn't enough, you add the phenomena of  skin effect. 
  Yegads!   
  I know about skin effect, of course, but never considered it may
  contribute to receiver noise.
  Understanding this is like walking into a minefield carrying a metal
  shovel in one hand while depending on a magnetic compass to get my
  bearing.  
  Thinking about these ideas may cost me an arm or a leg!  

  However, the real problem in trying to understand all this is that IN
  THE END ONLY REALITY COUNTS!
  False understandings result in false conclusions.
  In other words, unless I get gear "on the bench" and measure, and form
  an understanding BASED ON MEASUREMENT, any theory in my head may be
  wrong.
  Simply wrong!

  Still, I appreciate the time you spend explaining "the theory", and I
  hope some will "stick".
  Meanwhile, I have a frame aerial for 160m half finished.  How much
  quieter will my frame aerial be than a random wire?

  Only experiment will tell.  That's the interesting thing about
  experimental radio.

  73 de Les Smith
  vk2bcu at operamail.com
  PS I note the subject of discussion has "morphed" a good deal.

On Tue, Nov 11, 2014, at 06:43, Dennis Monticelli wrote:
> With all respect, I don't think that is true.  If the skin depth is well
> below the thickness of the shield, then the inner currents and outer
> currents do not mingle nor do their fields couple.  If it were true then
> wrapping the coax around and through a torroid would wreak havoc upon the
> normal differential currents carried with that transmission line.  And we
> know that doesn't happen.
> 
> I like to think of coax as a three wire cable.  What goes on inside is
> not
> effected by what goes on outside and vice versa....if the shield is thick
> enough and conductive enough to do its intended job.
> 
> Balanced two-wire transmission line is easier to understand conceptually
> and can work well (I use it) but keeping everything in balance so its
> common-mode rejection is decent is a real challenge.  The antenna, the
> line, it's immediate environment and the tuner or matching network must
> all
> be in balance.  Not easy in a real world environment.  If one cannot
> create
> that balance, one may be better off with well-shielded coax, despite the
> higher loss.
> 
> Dennis AE6C
> 
> On Mon, Nov 10, 2014 at 10:37 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:
> 
> > Hi Dennis
> >
> > Well yes, but an even simpler explanation is that because radiated hash is
> > picked up only by the braid, the opposite polarity of that hash voltage
> > appears
> > on the coax inner wrt the braid. So in effect, the inner carries into the
> > receiver
> > whatever the braid picked up.
> >
> > 73 de Neil ZL1ANM
> >
> >
> >
> > On Tue, Nov 11, 2014 at 3:49 AM, Dennis Monticelli <
> > dennis.monticelli at gmail.com> wrote:
> >
> >> Another way of putting it is the differential signal is protected by the
> >> coax shield from the intrusion of noise but any common mode noise flows
> >> right down the outside of the braid into the receiver and makes the circuit
> >> common (usually the chassis) hot with noise.  If the receiver has very good
> >> common mode rejection (i.e. an excellent RF ground) then no problem but in
> >> most cases that is not true.  This is why common mode chokes are so popular.
> >>
> >> Dennis AE6C
> >>
> >>
> >>
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