[mythtv-users] Attic Antenna: simple question.

[Bert Haskins]

On 9/7/2014 1:45 PM, Daryl McDonald wrote:
>
>
> On Sep 7, 2014 9:23 AM, "Saul A. Peebsen" <jaglover at gmail.com 
> <mailto:jaglover at gmail.com>> wrote:
> >
> > On Sun, 7 Sep 2014 12:24:45 +1000
> > blind Pete <0123peter at gmail.com <mailto:0123peter at gmail.com>> wrote:
> >
> > > On Sat, 06 Sep 2014 13:18:44 -0400
> > > Bert Haskins <bhaskins at chartermi.net 
> <mailto:bhaskins at chartermi.net>> wrote:
> > >
> > > >
> > > > I have been trying to recover at least a few channels after charter
> > > > effectively  bricked my HDHR. One, CH8 is marginal and in an attempt
> > > > to improve it I want to modify a existing antenna. The antenna has
> > > > all of the elements including the center point of the dipole
> > > > connected (grounded) to the main beam which then is bolted to the
> > > > grounded mast. This just doesn't seem right to me, i.e. it seems
> > > > like the dipole should be insulated from all the other parts(?).
> > > > I've searched the net and found lots of other information but
> > > > nothing on this.
> > >
> > > Antenna design is a bit of a black art.
>
You are preaching to the choir on that point!
Sixty-five years in electronics and I still many many more questions
than answers where antenna are concerned.
>
> > >
> > > With a Yagi-Uda design all of the directors and the reflector are
> > > simple conductors insulated from the boom.  There might be a bunch
> > > of plastic washers and sleeves where they are not obvious to the
> > > casual glance.
> > >
> > > If you doubled the length of the passive elements there
> > > would be a node at the centre, so electrical connection
> > > would be OK.  But that would make the antenna twice as
> > > large as necessary.  Such a design would surprise me.
> > >
> > > With a log-periodic antenna there are two parallel booms.  half
> > > of the elements are electrically connected to one and the other
> > > half are connected to the other.
> > >
> > > Many, many, weird and wonderful designs are possible.
> > >
> > > If you like mucking about with this stuff, go for it.
> > >
> > > If you just want a marginal signal to be a bit better
> > > consider buying a new (corrosion free), slightly larger antenna.
>
Q. Does mild surface corrosion affect the signal?

> > > Make sure that it is pointing in the right direction.
> > > Check that there are no obstructions, hills, neighbour's
> > > new attic, wet trees, etc.   Yes, at some frequencies a mess
> > > of leaf sized conductors will block a signal.  Dry leaves
> > > don't conduct enough to matter.  Then all of the usual stuff,
> > > check the cables and connectors.  Is anything generating
> > > electronic noise nearby?
> >
> > I've used Yagi antennas where all elements are grounded in the middle.
> > Dipole is not insulated. As much I remember of this type of design
> > insulation is not necessary because there is never any voltage in
> > center point (although it is the point of max current).
> >
> > --
> > Cheers, Saul
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>
> Since E = I x R, how can there be no voltage (E) and max current (I) 
> at any point?
>
Impedance.
E and I are shifted in respect to each other so at a given point in time 
you can have zero voltage and max current at a given point.
There are many good explanations on the net and this is but one of the 
many things that puts antennas in the "black art" category.

Please replace "dipole" in my original question with "folded dipole"... 
yet another senior moment.

>
>
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