VHF RFI detector

FORANE

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FORANE
Question for the group...

Is there a more sophisticated quantitative method to track down RFI instead of a qualitative hunt hoping to notice a causal factor?

I know usb chargers frequently cause RFI trouble with our comms. Traditionally I have tried the old method of unplug one item at a time and see if there is a discernible change in squelch break. Is there a better way?

I see MFJ offers the MFJ-805: https://mfjenterprises.com/products/mfj-805
Anyone have experience with it?
 
You can use an old AM transistor radio...tune it off station to where you just hear noise, then move it close to source of rfi. Back in the day, I located a bad power transformer on a pole in my neighborhood using one.
 
sophisticated quantitative? A spectrum analyzer and one or more handheld directional antennas. But that's probably $3-5k at the low end. Those have at least the potential to find noise in a band of interest from around the neighborhood to components on a board. The critical part is the antenna.

Slightly better if you're just concerned about aircraft com? A handheld scanner or radio that has a receive power meter, and a handheld directional antenna. You probably need attenuators.

I don't think any of the MFJ referenced equipment above would help. Portable AM radio would help to find wideband AM noise, such as bad HV lines, bad DC motors, or other things that are arcing and causing lots of noise across the band, but I don't think it would help much to find noise caused by digital equipment leaking RF. It might, though.
 
MFJ doesn't make much of any use. You can neither take a high reading from that as problematic nor the absence of one as a sign of health. Typically, when interference is a concern (such as when mixing GPS and comm equipment), you test it with the in situ radios on various frequencies.
 
sophisticated quantitative? A spectrum analyzer and one or more handheld directional antennas. But that's probably $3-5k at the low end. Those have at least the potential to find noise in a band of interest from around the neighborhood to components on a board. The critical part is the antenna.

Slightly better if you're just concerned about aircraft com? A handheld scanner or radio that has a receive power meter, and a handheld directional antenna. You probably need attenuators.

I don't think any of the MFJ referenced equipment above would help. Portable AM radio would help to find wideband AM noise, such as bad HV lines, bad DC motors, or other things that are arcing and causing lots of noise across the band, but I don't think it would help much to find noise caused by digital equipment leaking RF. It might, though.
Yes, just concerned about aircraft com breaking squelch in cases of suspected RFI.
 
Is there a more sophisticated quantitative method to track down RFI instead of a qualitative hunt hoping to notice a causal factor?
In my experience, no. There can be a number of sources of RF interference and several modes of travel which can mask the culprit. So without isolating each potential source or each potential path of the interference you can be spending a lot of time (and money) on red herrings. But there are a number of tricks that help you get beyond simply "hoping" to find the problem. Unfortunately which trick/tip to use is usually dependent on what interference problem you have and where it manifests itself. But in general it is more than a simple "qualitative hunt."

For example with your squelch issue, instead of arbitrarily disconnecting systems start at the squelch control first. With engine off, turn on radios, set squelch break, and mark the squelch knob or setting. Now start engine and verify squelch setting is same as static. If not readjust squelch break and mark the new setting with engine running. What is the difference? Some aircraft change some don't. Excessive change usually points to the added RF of the engine ign/alternator. So look at those items first. Another trick is to start with the squelch completely off. This allows the weakest of signals to be amplified by the radio. So without engine running, turn everything on, then start switching things off and listen for any differences between the systems no matter how faint. And so on.

Regardless, a solid majority of RFI problems can be solved by simple proactive measures of ensuring the entire aircraft electrical ground/bond system is clean, secure, and connected to include all wire shields and antenna ground planes. While in some cases it can be time consuming even with a methodical plan in place, it's far from a needle in a haystack scenario.
 
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