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Thread: Osage Ambulance Body

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    Default Osage Ambulance Body

    Has anyone ever dealt with RF interference in an Osage ambulance body? A neighboring agency contacted me yesterday afternoon about a problem with their new ambulance. It's an aluminum body with a standard NMO mount 1/4 wave antenna. Upon installation of the older VHF CDM1250, they noticed a loss of reception at about 10 miles away from their tower. TX was fine and RX was fine up until 10 miles out. After that, absolutely no RX at all. Not even static. They swapped the old radio for a new XPR model and had the same issue. After that, they completely replaced the antenna and coax with brand new. Still has the same issue. I've not personally looked at anything on this rig. It was a brief phone call and I did not ask too many questions. So, i'm wondering if anyone has experienced the same issue with Osage bodies. I did ask if they did an RF sweep of the rig and his response was "HUH??".
    "My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives".


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    RFI wouldnt affect TX. it could affect the handshake to the trunking controller but you should see the RTT incomming from a lot further out than 10 miles. I would suggest they do a quick spectrum analyzer look (noise floor) at whats coming in from the antenna. That may give a clue. The other test would be a portable on the freq outside the vehicle at 10 the failure point (and 10-20 feet away moving closer) to see if the portable suddenly dies near the vehicle. That will give a base set of parameters to help track the issue down.

    I am assuming th proper swr was done using a reflecting watt meter. I have seen bad re-installs or bad antenna whips cause similar issues

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    It's a VHF analog system. I did ask if a portable had the same symptoms while inside the vehicle. He says no, it doesn't. That the portable is fine. I'm thinking there has to be a problem somewhere along the coax run that's only affecting the mobile. But I don't understand how it's just an RX problem and not both RX and TX if it's a coax issue.
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    Are you certain it isn't affecting transmission as well. It may not be at the 10 mile mark like the receive issues, but it very well might still be at the 15 or 20 mile mark. I tend to think something is grounding the center pin or shorting the braid and pin together. That, or perhaps a bad ground on the radio power lead. Finally, there may be some sort of electronic doo dad that is producing a low power signal knocking out the receiver, and it is physically close to the NMO or radio or coax, but is sufficiently far from the portables that work fine (though, I would wonder where they work fine, everywhere, or only patient compartment or cab.)

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    Quote Originally Posted by wiredwrx View Post
    Are you certain it isn't affecting transmission as well. It may not be at the 10 mile mark like the receive issues, but it very well might still be at the 15 or 20 mile mark. I tend to think something is grounding the center pin or shorting the braid and pin together. That, or perhaps a bad ground on the radio power lead. Finally, there may be some sort of electronic doo dad that is producing a low power signal knocking out the receiver, and it is physically close to the NMO or radio or coax, but is sufficiently far from the portables that work fine (though, I would wonder where they work fine, everywhere, or only patient compartment or cab.)

    I'll try and get some more information from him. I'm curious as to what new equipment is in this rig than the old rig. The original radio came out of the old rig with no prior issues. I'm really leaning towards a grounding issue as well as additional electronic equipment in the new rig vs the old rig. Camera system, GPS, MDT, etc. causing some sort of issue.
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    Just got off the phone with him. The issue has been resolved. Upon further investigation it has been determined that a faulty installation was the problem. The jackwagon that installed the radio ran the power and ground directly to the power and ground for the multiplex system. Which is basically an isolated system in and of itself. After making a power and ground run to the appropriate place, the issue went away. Clear RX from well past 10 miles out. I would imagine the TX improved as well, even if it wasn't an apparent problem to begin with.
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    Wow, that was quicker and easier than I thought it would be.

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    LOL got to love a 'floating' isolated ground. Glad you found the issue and thanks for keeping us all in the loop (ground loop that is )

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    Always a good idea to include in an ambulance or engine spec that the manufacturer provide studs clearly labeled for B+, ground and ignition B+ specifically for radio operation. Makes life a LOT easier.

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    Is the NMO mounted in aluminum? Aluminum conducts different than metal maybe its some weird ground loop?

    Opps did not see the post about resolving the issue.

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    I would be nervous that the coax shield was subjected to TX current. An inspection of the antenna lead should be done.

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    I would be nervous that the coax shield was subjected to excess current. An inspection of the antenna lead should be done.