Noise when using ANY CAT interface.
Although rare, from time to time I get questions regarding noise being picked up when using HRD and CAT interfaces. Typically, something along the lines of; "No noise on 10m 15m and 20m, but noise on 80m" What is it and where is it coming from?
There are of course many forms of "noise", by far the most common when using HRD and CAT interfaces is the noise that's generated from your very own PC (or some peripheral attached to it). This noise can vary from, no noise, to a permanent S9+. It's most likely the noise from the PC simply being picked up by your antenna "system". A good antenna system will pick up very little noise, whilst an antenna system with one or more problems may pick up so much as to make CAT unusable. If you have "noise" being picked up, you can easily determine if the noise is being picked up by your antenna system… [As there are other factors including separation between PC and antenna, not everyone with a poor antenna / radio match will suffer]. Simply disconnect the antenna from the radio. If the noise disappears then you are on the right track and you have confirmed that the noise problem is actually an antenna problem. Your antenna should not pick up vast amounts of noise from your PC. (That's assuming that your PC is a reasonable distance from your antenna). You can then try unplugging everything on your PC in order to determine if you have one or more "noisy" peripherals. The first course of action is to look at your antenna system and ask some simple questions…

[1] Is the antenna a good match to the coax that's feeding it? If your using any form of tuner at the side of your radio then the answer will probably be no. Inevitably your antenna will be prone to pick up any noise - the match between your radio and your coax will in most cases be good 50ohm to 50ohm and you should put the tuner between coax (50 ohm) and antenna (who knows what impedance?) where it will function correctly. As well as removing any PC noise, this action will also vastly improve your stations general performance.

[2] Is the antenna being used on the band(s) it's designed for? If not then you probably have a bad match and may be prone to pick up noise.

So, how do I get rid of the noise? - Simple, use an antenna that has a good match to your coax. (or use a tuner in the correct place). And use it on the bands where it functions correctly and doesn't pick up all the PC noise. If you want to use a good antenna on other bands where its not designed to be used you can expect to pick up some noise, generally speaking, the worse the match, the more the noise.
You may be able to reduce some of the noise to a lesser or greater extent from radiating from your PC unplugging any "noisy peripherals" that may be contributing. But as always, you should first pay attention to the antenna where the majority of problems seem to occur.

Want to experiment? Noise can be picked up by simply making a small random length wire antenna (3ft of equipment wire will suffice), plug it directly into your radios antenna socket. Connect the CAT and see the noise rise. Then plug in a decent antenna and watch the noise drop to zero.