Hello,

Not too sharp with antenna and noise theory, so please bear with me a bit.

If I have a run of coax from a Balun right at my outside receiving only
antenna (well grounded at that point) right to my receiver, is it still
possible to have common-mode noise induced into the coax ?

Say from a PC, Modem, Router near the receiver.

Wouldn't the coax shied prevent this ?
If not, why not ?

Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

Thanks,
Bob

On Dec 16, 6:14*am, Bob wrote:
Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

It doesn't have to. The inner shield and outer shield are shorted
together at the chassis. If noise exists on the chassis, i.e. the
chassis is not really "ground", the result is differential noise on an
unbalanced input. Essentially the same thing happens when SWR meter
readings are affected by common-mode signals during transmit or when
an unbalanced antenna analyzer is connected to balanced feedline
without a choke. A good choke on the coax near the chassis often
reduces the magnitude of the problem in all of the above cases.
--
73, Cecil, w5dxp.com

Hi,

Thanks for help; appreciate it.

I wasn't too clear.

By the "inner shield," I mean the signal carrying innermost (solid)
conductor of the coax.

Guess I am wondering how can emi noise get thru the outer shield and to
the signal carrying conductor. Unless, of course, it's a really poor
outer and leaky outer shield.

Regards, and thanks again,
Bob
------------------

On 12/16/2010 7:49 AM, Cecil Moore wrote:
On Dec 16, 6:14 am, wrote:
Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

It doesn't have to. The inner shield and outer shield are shorted
together at the chassis. If noise exists on the chassis, i.e. the
chassis is not really "ground", the result is differential noise on an
unbalanced input. Essentially the same thing happens when SWR meter
readings are affected by common-mode signals during transmit or when
an unbalanced antenna analyzer is connected to balanced feedline
without a choke. A good choke on the coax near the chassis often
reduces the magnitude of the problem in all of the above cases.
--
73, Cecil, w5dxp.com

"Bob" wrote in message
...
Hello,

Not too sharp with antenna and noise theory, so please bear with me a
bit.

If I have a run of coax from a Balun right at my outside receiving only
antenna (well grounded at that point) right to my receiver, is it still
possible to have common-mode noise induced into the coax ?

Say from a PC, Modem, Router near the receiver.

Wouldn't the coax shied prevent this ?
If not, why not ?

Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

Thanks,
Bob


One key point here is that the coaxial cable shield is grounded at two
points, at the receiver and at the antenna. You might have what is known
as a ground-loop. The best thing to do is to ground the receiver, the
computer, the modem, and the router with leads as short as possible, AND
TO THE SAME GROUND POINT. Since you have a balanced to unbalanced
conversion (Balun) at the antenna, there is no need to ground the coax
shield at this point.

Years ago, I had three grounds in my ham shack: the power line ground at
the service entrance to the house, the tower ground where I had mounted a
rotary coax relay, and a copper cold water pipe running beneath the shack
which connected to the buried galvanized water pipe running from the
street to the house. I measured from 3 to 15 volts AC between the three
grounds. When connecting them together, I measured as much as an amp of
current flowing. I bonded everything together at the water pipe and used
this as the single point ground to which everything else was grounded.

I agree with Cecil that the noise is probably common mode noise. But the
quality of the coax certainly does affect how much signal leaks in or out
of a coaxial cable. I haven't bought Radio Shack coax in years, but it
used to be very poorly shielded. The braid shield looked more like
window screen than it did a solid sheath. This is the reason most CATV
providers use flexible coax with two shields: a foil or metalized plastic
shield covered by the braid shield. Special coaxial cables with high
leakage are often used in long tunnels to provide FM reception in the
tunnel.

73, Barry WA4VZQ

On Dec 16, 7:42*am, Bob wrote:
Guess I am wondering how can emi noise get thru the outer shield and to
the signal carrying conductor.

Common mode RF signals cannot exist on the inner conductor of an
*ideal* coax feedline and is forced to the outside of the braid by the
laws of physics. The coax has to be less than ideal for common mode to
exist on the inner conductor. But common mode signals don't have to
make it to the coax inner conductor to cause noise problems in a
single-ended unbalanced receiver circuit. A differential amplifier is
often used in control circuits to minimize common-mode problems. A
link-coupled transformer will accomplish much the same function for
amateur circuits.
--
73, Cecil, w5dxp.com

On Dec 16, 9:42*am, Bob wrote:
Hi,

Thanks for help; appreciate it.

I wasn't too clear.

By the "inner shield," I mean the signal carrying innermost (solid)
conductor of the coax.

Guess I am wondering how can emi noise get thru the outer shield and to
the signal carrying conductor. Unless, of course, it's a really poor
outer and leaky outer shield.

Regards, and thanks again,
Bob
------------------

On 12/16/2010 7:49 AM, Cecil Moore wrote:



On Dec 16, 6:14 am, *wrote:
Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

It doesn't have to. The inner shield and outer shield are shorted
together at the chassis. If noise exists on the chassis, i.e. the
chassis is not really "ground", the result is differential noise on an
unbalanced input. Essentially the same thing happens when SWR meter
readings are affected by common-mode signals during transmit or when
an unbalanced antenna analyzer is connected to balanced feedline
without a choke. A good choke on the coax near the chassis often
reduces the magnitude of the problem in all of the above cases.
--
73, Cecil, w5dxp.com- Hide quoted text -

- Show quoted text -

Lets try the simplest expalnation first. If you are familar with RF
current flowing on the outdside of a coax cable during transmit(often
discussed here) it is the same thing but in reverse.

Jimmie