[mythtv-users] Attic Antenna: simple question.

Daryl McDonald darylangela at gmail.com
Sun Sep 7 21:40:19 UTC 2014


On Sun, Sep 7, 2014 at 2:36 PM, Bert Haskins <bhaskins at chartermi.net> wrote:

>
> On 9/7/2014 1:45 PM, Daryl McDonald wrote:
>
>
> On Sep 7, 2014 9:23 AM, "Saul A. Peebsen" <jaglover at gmail.com> wrote:
> >
> > On Sun, 7 Sep 2014 12:24:45 +1000
> > blind Pete <0123peter at gmail.com> wrote:
> >
> > > On Sat, 06 Sep 2014 13:18:44 -0400
> > > Bert Haskins <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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Forgive my oversight  0 = infinity x 0, may I claim the senior moment too?
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