Interference

Interference or noise is the effect of unwanted radio signals in the same frequency that one is listening to or wishes to transmit on. Interference is generally classified as being either man-made or natural.

Man-made
Main article: QRM

Man made interference (also called QRM) is created by man-made electrical devices. Broadband over Power Lines (BPL) is a good example of this. Links to another page of this wiki can be found below.

Natural
Main article: QRN

Natural interference (also called QRN) is created by natural phenomenons. However, the abbreviation (QRN) is ofent used to designate any source of noise. QRN can be terristrial or cosmic.

Terrestrial sources include lightning, which is generally worse in summer on the lower HF bands, due to increased numbers of storms. Rain and snow storms can also produce significant interference.

Cosmic interference results from the emission of high energy radio waves from stars including our own sun. The sun has by far the greatest effect in this category, due to its proximity to Earth.

Reducing interference
There are various ways of reducing QRM or QRN from reaching your station.

QRM on the outer coax shell
One of the important things is to make sure the QRM around your station doesn't propagate to your antenna. Even though a coax cable is "shielded", the outer shell can still carry interference all the way back up the feedline to the antenna and then back in the inner shell and into your station.

A good way to work around that is to add 2 RF chokes separated by a 1/4 wavelength distance from the antenna. Adding another one close to your station is also a good thing.

There is a very detailed article about "common-mode chokes" (PDF) that workaround this phenomemon.