[mythtv-users] Attic Antenna: simple question.

[blind Pete]

On Sun, 07 Sep 2014 15:13:20 +0100
John Pilkington <J.Pilk at tesco.net> wrote:

> On 07/09/14 14:21, Saul A. Peebsen 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.
> >>
> >> 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.
> >> 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).
> >
> I think commercial designs in the UK will usually have one side of
> the dipole, and all the other elements, 'grounded' and connected to
> the coax outer; the centre coax conductor connects to the other side
> of the dipole, which is insulated from the frame.  Often the dipole
> is 'folded' - ie a squashed loop - but the same applies.  'Balanced'
> dipoles are perhaps better in theory but rare, AFAIK, in receiving
> antennas feeding coax downleads.

There are cheap little devices called "baluns" that are basically
transformers.  They are used for impedance matching and connecting 
an unbalanced coax cable to a balanced antenna.  As a bonus they 
act as filters.  

-- 
testing
bP