Topband: K3 & some interesting noise lessons in the ARRL 160.

[Tom W8JI]

> Why would the NB null be frequency dependent?  Why does it change?  Why
> would a different antenna make any difference unless you pick up different
> noise sources with the two different antennas?  73, Greg-N4CC
>

This is an anomaly with the particular noise he has, and the particular 
noise blanker and all related settings. Unique things like this occasionally 
occur with noise blankers and noise reduction systems, or with any local 
signal we are trying to locate or get rid of.

It happens when the noise signal at the radio input is a mix of pulses 
arriving with various delays from multiple radiation points, such as when a 
single arcing source radiates from multiple points at greatly varying 
distances, all of which arrive at the receiver. As you change frequency the 
noise phase varies, because the distance in wavelengths for each path varies 
disproportionately. Say one radiation point is at a source, and another from 
the same source is 5 wavelengths further away (including velocity factor and 
the distance to and back from that point). If you change frequency 10%,  you 
have changed 10% more (or 36 degrees in phase) for each wavelength between 
the radiation points. Also, time is a factor (although generally nearly 
constant with frequency change). At 5 wavelengths you have changed phase 
between the radiation points (of the same noise source) 180 degrees.

This also happens when the noise source is not broad band, and does not have 
the same characteristics over a wide frequency range. You can occasionally 
find sweet and sour spots where the blanker system "likes" the particular 
waveshape or noise characteristics that appear at some frequency.

After all that, the blanker comes into play. It would take a few pages to 
describe how blankers work and why they are sensitive to noise 
characteristics, and why that sensitivity causes a blanker system to work 
entirely different on two different noises.

All we really need to remember is what happens at one place on one noise 
source and one receiving antenna for one blanker and one blanker setting 
isn't very likely to repeat at another station. It isn't going to repeat if 
one thing changes in the wrong way, let alone more than one thing changing. 
Since countless things can potentially change, what happens in one place 
with a blanker has little value elsewhere.

The only thing ever repeatable is a wide bandwidth impulse noise from one 
source.

73 Tom