[TriLUG] Re: OT: Old Thinnet coax & AM reception

[Scott G. Hall]

On 02-20-2005 20:51 EST, sholton wrote:
> 
> Brian Henning wrote:
>> Anyhow, since James asked, the intended use is relocating the
>> antennas for a wireless mic receiver at my church.  They're very
>> simple whip antennas that connect to the back of the receiver
>> with BNC connectors.
> 
> This isn't digital, this is analog. You've got to match the impedance
> or you'll wind up creating serious problems in the system.
> 
> Thinnet is 50 ohms impedance. A very similar co-ax often used for
> wiring cable TV is 75 ohm impedance. You can intermix the two without
> blowing any equipment, but [you may end up with poor performance].

Most "Thinnet" (10base2) is RG-58 cable, the same kind and impedance
as used for CB radios.  You'll find that most radios use 50-ohm antenna
connections, so using old thinnet wiring is ideal for them.  I did the
same for several of my radios, including a wireless intercom that I
used when I worked for a video production company doing remotes.  The
company I got laid off from threw out tons of the stuff.

I'll have to agree with Jim about extending the audio instead of the
RF.  Receiving RF signals from those little whips doesn't provide much
signal (about an order to 2 orders of magnitude compared to ethernet
levels), and any extension of wire will add to the noise levels being
picked up.  The longer the extension, the more like an antenna it too
becomes, picking up the signals from the equipment you are trying to
avoid.

Whereas the signal level of the line output for the audio is much
greater, and the frequencies of noise picked up are less common.  This
is why a lot of professionals put the wireless receiver in the dais
or under the alter.

On 02-21-2005 07:29 PST, Justin Skinner wrote:
> Rick DeNatale wrote:
>> Any suggestions on how to get better AM reception for the rare times
>> we want it?
> 
> I suggest about 6 feet of copper cable lay it in a corner out of site
> along the floor.  At the end make a loop.  You may need to move it
> around slightly.  That should give a decent horizontal ground plane
> for AM reception.
> 
> Or on occasion like during a heavy rain mount it vertically.  I usually
> put it along the blinds so it's near a window.

You're going to love what I did: I soldered a single wire to the window
screen material, and ran that to the AM antenna input.  Then I ran another
to a good house-ground.

Note that AM antenna inputs uses a single wire for the signal, and
another for the ground.  A good earth ground will usually clear up most
AM reception problems.  It is best to use a wire to the grounding rod
used by the power company (or one of your own) -- it is never a good
idea to connect to the 3rd prong of a power outlet, since any miswired
outlet or faulty equipment could create a shock hazard.

You'll want to use a small meter to check your equipment's antenna
inputs compared to its chassis -- one side is usually connected to the
chassis, and that is the one you'll want to connect to earth ground.
Shunting your AM pickup wire to the ground, and connecting ground to
other input will just clear out any AM reception you could have had.

-- 
Scott G. Hall
Raleigh, NC, USA
ScottGHall at BellSouth.Net