[RFI] Ambient Noise Levels

Tom Rauch w8ji at contesting.com
Thu Aug 10 06:16:22 EDT 2006

> It was 'the sun'.

The sun certainly can cause noise that we hear above 10-15 
MHz or so, as we can also hear accumulated noise from other 
galactic sources. All it takes is a low terrestrial noise 
level and very low ionospheric attenuation of galactic 

> Fred, I don't but wonder if your noise is due to, by
> chance, to "static build-up" on your antenna.

Charge build-up does not cause noise. Charge movement does. 
Corona discharge from an antenna in inclement weather gives 
rise to a hissing whining noise that increases in pitch, 
often abruptly disappearing for an instant when a distant 
lightning flash reduces cloud to earth potential. If that 
same charge difference is present without the corona, say 
the antenna has rounded points or does not protrude into the 
air, you hear no noise.

> you have even a high-valued resistor to ground to
> preclude any charge accumulation (provide a simple
> leakage path to earth)?

That only protects from a charge difference between the 
antenna and earth that might cause something to arc. It has 
no effect on corona noise as the element discharges into 
space. The resistor will primarily cure, if there are no 
other dc paths, the "tic tic"  or "snap snap" of something 
in the antenna arcing to ground. Nothing else.

It is a good idea to have a dc path of some form to prevent 
equipment damage as some dielectric might punch through. An 
example is antenna tuners. Charges can accumulate in a 
T-network antenna tuner's output capacitor to the point it 
flashes over. That discharge "rings" the coil, and that 
energy  can pass through and ruin the detector diodes in the 
directional coupler or even the receiver!

> I take it you were able to 'kill' the noise by removing
> the antenna from the radio at which time the the
> noise level dropped back significantly. (I notice
> today's rigs employ two levels of 'preamp' as well
> as several levels of attenuator; I find the preamps
> generally just contribute additional white noise
> with little real improvement in S/N ratio

The noise they "contribute" is very minor. You can test it 
by terminating the receiver input and turning the preamp on 
and off. I think what you are trying to say is the preamp 
increases the level of signals and noise from the antenna. 
At my location in winter using narrow selectivity, I have to 
use preamps. Otherwise I am limited by receiver noise floor. 
This is especially true on 20 meters and higher with 
directional transmit antennas used on receive, or on lower 
bands with dipoles or beverages.

*2. I have
> an old Yaesu FT-101EE that has no preamp
> function and therefore never sounds like a 'rushbox'
> quite like some of the later products do on the
> bands.)

If you can't clearly hear "antenna noise", you don't have 
the best S/N ratio you could have.

By the way, shielded loops can't filter noise by virtue of 
the shield. That is a popular but very wrong technical myth. 
They also can't sort good signals from bad noise by virtue 
of "magnetic" or "electric"  field response unless the noise 
source is very close to the antenna and the conditions are 
just right. Time-varying fields behave the same whether we 
like to hear them or not. As a matter of fact at a distance 
of about 1/8th wave, a magnetic loop is electric field 
response dominant! It's only right next to the antenna it is 
magnetic field dominant, and a noise source can have any 
field impedance.

In reasonably quiet suburban or rural locations on low bands 
the noise we hear is indeed an accumulation of all the 
little drill motors, power line arcs, thunderstorms, and so 
on that propagate and accumulate to make what sounds like a 
"white noise". On upper HF galactic sources come into play.

This is the real danger of BPL. The accumulated energy of 
hundreds or thousands of individual sources that by 
themselves are too weak to hear will add and raise the noise 
floor of everyone, even people thousands of miles from the 
sources. We have polluted the radio spectrum with noise from 
incidental radiators just like we are destroying every other 
resource we have.

73 Tom 

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