[mythtv-users] Attic Antenna: simple question.

[John Pilkington]

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.