[mythtv-users] Attic Antenna: simple question.

Bert Haskins bhaskins at chartermi.net
Mon Sep 8 01:13:02 UTC 2014 

On 9/7/2014 8:36 PM, Tom Poe wrote:
> On 09/07/2014 04:40 PM, Daryl McDonald wrote:
>>
>>
>> On Sun, Sep 7, 2014 at 2:36 PM, Bert Haskins <bhaskins at chartermi.net 
>> <mailto: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
>>>     <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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>>
>>
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>>
>> Forgive my oversight  0 = infinity x 0, may I claim the senior moment 
>> too?
>>
>>
>>
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> Unfortunately, I know nothing that could contribute to this 
> discussion.  Is this antenna purposed to pull in digital television 
> signals?
Yes
> I live 72 miles from Minnesota tv farm.  Line of sight is minimal, so 
> signals are strong enough late at night, but reception is poor during 
> the day.  My attic antenna is a DIY Hoverman
I've built a Hoverman and got better results with a DIY quad bowtie.
I'm gathering parts to do the dual quad variation.
> with a pre-amp booster.  What is an antenna used for with Comcast cable?
So that I can keep using my HDHR and record more that one channel and 
edit the recordings.
Plus the sound is much!!! better with OTA.
Better than 90% of what I record is just for time-shifting.
>   Are you looking for digital over the air reception on your mythtv setup?
I am getting four out of the five channels that I care about.
Only two of these are easy to get.
> Tom

>
>
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