“Interference” What I Use To Find The Source

[KD9KVS]

Looking through posts I am seeing a lot of questions about interference and how to find the source. I’ll start with a story...

At our church we were having issues with volume levels on our recording and livestream. If the volume was low, preventing an air static noise, people could not here spoken words well. If the volume was up, there was the air/static noise and they did not like that. So I went through, checking grounds, making sure the power conditioners were operating properly, making sure there were no issues with amplifier oscillators causing an issue... all of which could very easily cause the issue. None of the above were causing any sort of problem. I decided to break out the ham gear and see if radio interference could possibly be the problem. Using a frequency counter, I found the transmit frequency of the wireless mics. Next I hooked up the S.D.R. And went to the frequency on the frequency counter with the mic transmitting. After turning the mic off, I saw 16db of radio activity in that area of the building and said, well that would be our noise!!! I moved to different locations checking the activity in the needed frequency range for the mic and found a location where the interference was significantly less and decided that was going to be the location for our receivers. We are now able to host the sound significantly, without having much of the background noise.

All of that said, putting a directional antenna on an sdr will make it so you can track down the source of the interference. Using an omnidirectional antenna will help you see if there is a better location with less interference to place radios, antennas and anything else that may receive and play that interference.

I hope this helps someone and is beneficial.

[brandon lind]

If you have access to a spectrum analyzer, you can see what areas on the spectrum are hot in the bad area and not so bad in the location you found to work better. Outside signals will, for the most part, remain at a constant level throughout a building, give or take a few dB. Once you identify the offending signal by its larger dB change around the building, you can have someone turn off breakers one at a time to see what circuit the offending device is on. Then narrow it down by pulling power cords. A higher noise floor at the intended frequency can be caused by much lower frequencies with harmonics, often something in the kHz range. If there is a 120Hz hum to the static, it is likely 60Hz grid related. If the noise is more white sounding, its likely a switch mode power supply somewhere (maybe a laptop charger). If you can take a look at 1kHz through about 100kHz while cutting the power to things, you might find the cause.

EDIT: most SDR's cant go that low but still show "noise". This noise is from the SDR and not from the antenna so be sure to check the useful lower range of the SDR if you use that as a spectrum analyzer for looking at <100kHz.

[5B4AJB]

I had a similar problem once, I used a simple crystal style radio with a sensitive meter on the output, biased with a battery. Basically a powered field strength meter.
Will have a rummage round and try to find the (extremely simple) circuit diagram, I still have the meter working daily to tell me about any local transmissions, lightning etc.
Going around touching the antenna probe on various metalwork eventually showed me the problem. Having a gain control is very useful for this!
You could go a step further and have a cheap frequency counter module (see ebay) to pinpoint the exact frequency, but interference is interference...

[edit] I tried to reverse-engineer it, but my eye's aren't what they used to be, maybe you can have a go...

parts list, best I can tell

diode - OA81
transistor - 2N549
regulator - 7808
potentiometer - 1k linear
resistor - 1M
inductor - 0.1uH
capacitor - 6.1uF

[KD9KVS]

I had a similar problem once, I used a simple crystal style radio with a sensitive meter on the output, biased with a battery. Basically a powered field strength meter.
Will have a rummage round and try to find the (extremely simple) circuit diagram, I still have the meter working daily to tell me about any local transmissions, lightning etc.
Going around touching the antenna probe on various metalwork eventually showed me the problem. Having a gain control is very useful for this!
You could go a step further and have a cheap frequency counter module (see ebay) to pinpoint the exact frequency, but interference is interference...

[edit] I tried to reverse-engineer it, but my eye's aren't what they used to be, maybe you can have a go...

parts list, best I can tell

diode - OA81
transistor - 2N549
regulator - 7808
potentiometer - 1k linear
resistor - 1M
inductor - 0.1uH
capacitor - 6.1uF

I like that a lot!! I used the frequency counter on the front end getting the frequency for the mics. Since the antenna for the mics are omnidirectional I took a general reading of the area. The bandwidth of wireless mics are much more wide than what I had expected! I guess that’s why they have a great signal and very little in the way of dropping signals. I think I will try and build one like what you have there to have more than one option for this type of thing.