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Ho Hummmm

I just got done listening to last nights’ net and heard the discussions about the mysterious hum.

I have been dealing with this for years and we discussed it recently at a club meeting. The hum is the repeater’s PL tone. 2 meter rigs filter the tone so you don’t hear it. Some are better than others at doing this. My rig that I use to record the net is really bad at it so while processing the audio every Monday morning, I have to run a filter that removes it from the audio before I post it to the webpage. Normally, I run the filter once and the hum is gone. This week I had to filter it twice and there was still a bit of it left.

Paul mentioned that he even heard it on reverse. To access the repeater, you  have to send the PL tone on the input frequency, therefore, if your rig is not filtering PLs well enough, you will still hear it on reverse. Input to the repeater or output, the PL tone is still present.

To complicate things even more, the PL tone of 179.9 is almost exactly the 3rd harmonic of 60Hz, causing the hum at times to strengthen and weaken as the two frequencies mix.

As a test of my explanation, the next time you hear the hum, switch to a different rig and I’ll bet you that it goes away or at least changes. Like I said, some rigs are better at filtering it than others. Two of my 2 meter rigs completely remove the hum, another one doesn’t.

73, Chris – KC0TKS



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