FM interference on ship's PA system

By ShipsEngineer
Jan 29, 2010
  1. First off, brief introduction: I'm a Computer Engineering student currently taking time off of school, working on a boat, started working with the boat's electrical systems because hey, I understand DC power!

    Now the issue: I wired in a PA system to our VHF radio unit. It works great, except the system was designed to be 2-way. When you hook the system up to a bullhorn, it works as a microphone and can transmit back to the VHF unit.

    We had a bullhorn, but it got corroded and we replaced it. Now we have waterproof speakers that are fine except they pick up FM radio and send that back to the VHF unit.

    You don't hear the FM radio over the speakers, just back at the VHF unit, and transmission from the VHF to the speakers seems to be fine. This isn't a problem, per se, but it's annoying to hear.

    The problem is with the speakers, and I tried putting a ferrite ring on, to no avail, although a different one with a different stopband could work. What I'm unsure about, is technically the system is working the way it should, we just hooked up something other than what the system is designed for!

    I do know what frequency radio it's picking up (100.3 MHz) but I don't know what frequency it outputs when the PA is in use. Keep in mind I only have access to a multimeter onboard.

    Any suggestions?
  2. jobeard

    jobeard TS Ambassador Posts: 9,147   +597

    you need a LOW-PASS filter between the connection to the VHF. This will allow A/F but kill R/F (which is being 'detected' and converted to the A/F content :)

    A low pass filter is an inductor in series with the signal and a capacitor in parallel to ground
     this should be a shielded pair (eg coax)
                  --------- inductor ---+---
    signal source                       =   connection to VHF
                  ----------------------+ ---                 
    The capacitor is ~ 0.001 mmf and the inductor doesn't matter too much
  3. ShipsEngineer

    ShipsEngineer TS Rookie Topic Starter

    Thanks! Just to make sure, when you say mmf, you mean micro-micro-Farads, right?

    I'll give that a shot, though I have no idea where I'll pick up the parts I'll need here... before you make any suggestions, there are literally no electronics stores around here. There's a Radioshack and a computer repair place, both of which I'm sure will be no help whatsoever.
  4. jobeard

    jobeard TS Ambassador Posts: 9,147   +597

    That's funny as RS started as a SCRAP store -- I used to pick thru piles of components. I think that a good size store STILL carries some components, however.

    The 0.001 mmf will be printed on the device -- find some dead radio and get out the clippers / sodering iron :) Likewise, some old equipment may be lying about with 445kc tank style intermediate transformers(IF) (ie between 1st and 2nd RF amps). Use an ohm meter to find two wires that give a reading (that will be one side of the IF and use that as the inductor to your low-pass.
  5. ShipsEngineer

    ShipsEngineer TS Rookie Topic Starter

    Yeah, this isn't a good-sized store, mostly just sells consumer electronics with a few audio patch cables thrown in for good measure. I went in and started asking about one of their wireless cards and the guy had no clue what he was talking about...
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