Here is a good RFI problem for everyone. The other dept I work has a UHF conventional repeater system. Its wideband single site using DPL. Our new 4 channal narrow band system will be going up soon.
Anyway - over the week portable radios have been suffering from desense in a particular area of the park. This area has several park owned buildings of various ages on it, 2 pump houses a 200 car parking lot and 2 temp office type trailers.
The desense extended for several hundred yards emmitting a distinct constant carrier with loud AC buzzing. Signal was not strong enough to break squelch with DPL but would desense the portables to point of missing calls entirely. THe signal ran from about 2 Mhz either side of our frequency.
Additionally very low in the signal just above the noise floor you could barely make out voices as if someone had a open mic.
Quite puzzling. So before we broke out the fancy tools I decided to do some quick and dirty RFI hunting.
I rolled out my Suburban and drove a large portion of the park and the two residential areas that border it. Using the Spectra RSSI feature I found the RFI signal to have a fairly wide footprint of at least 1/4 mile radius. Once I eliminated the possiblity of outside problems I concentrated on our buildings. Ultimately the RSSI peaked at what we suspected would be the source - our visitor information center (one of the temp buildings)
Possible thoughts for the source were the RF linked fire alarm system or possibly a new and very fancy point of sale equipment for the gift shop.
The overheard voice comms were still barely audible with the occational words or sentence able to be understood.
We sent in one of our officers hoping we could detect his voice above the others and sure enough we clearly heard someone say good night to him!
BINGO
So we went in and started turning off a plethora of electronic equipment, PC's fax machines, copiers, cash registers, CC charge pad et etc
THe noise could still be heard along with the background voice traffic until we pulled a power strip that had a GP300 portable and two 2-unit drop in chargers for Motorola FRS radios.
Obviously it wasn't our radio but would you believe the noise went away when we unplugged the FRS radio chargers!!!
Better still, they had become microphonic and would pickup any voice nearby!
I attribued this to poor shielding on the 110ac to 7.5v dc transformer at least for the awful AC noise.
But microphonic drop chargers?????? Who'd have expected that one.
Not bad for a quick and dirty RFI hunt. Spent about 2 hours tracking this down. I had expected to have to come back with RDF gear and or a spectrum analyzer later.
strange RFI problem (solved)
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apco25
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strange RFI problem (solved)
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