[mythtv-users] Attic Antenna: simple question.

Saul A. Peebsen jaglover at gmail.com
Sun Sep 7 13:21:39 UTC 2014


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).

-- 
Cheers, Saul


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